B.Sc. First Year�Semester-II�Paper Name:– Comparative Anatomy of Vertebrates �Paper No.-III

SWAMI RAMANAND TIRTH MARATHWADA UNIVARSITY , NANDED

Gramin (ACS)Mahavidyalaya vasantnagar, Kotgyal Tq. Mukhed Dist. Nanded

Dr. S. K. Pawar

Head and professor

Department of Zoology

Syllabus

Subject:- Zoology Semester:- II Paper no.:-III

Paper Name:- CCZ-I:- Comparative Anatomy of Vertebrates

UNIT – I

1. General characters, Origin and Ancestry of Vertebrates.
2. Integumentary System:- Development, General Structure and Function of integument;

Derivatives of integument- Epidermal and Dermal derivatives;

3. Skeletal System:- Evolution of visceral arches; Comparative account of Limbs and girdles.

UNIT – II

1. Digestive System:- Brief account of Alimentary canal and Digestive glands.
2. Respiratory System:- Brief account of different Respiratory organs in vertebrates-

Gills, Lungs, Skin, Air sacs and Accessory respiratory organs.

UNIT – III

1. Circulatory System:- Brief account of Evolution of heart in vertebrates. Modifications of aortic arches in vertebrates; Blood circulation in various vertebrate groups- Single and Double circulation
2. Urinogenital System:- Developmental Succession of kidney,

Evolution of Urinogenital system in vertebrates.

UNIT – IV

1. :- Structure of Neuron; Comparative account of Brain of Vertebrates.
2. Sense Organs: - Types of receptors- Mechanoreceptors; Photoreceptors; Phonoreceptors.

UNIT–I

1. General characters, of Vertebrates:-

Animals in the phylum Chordata share four key features that appear at some stage during their development.

In chordates, four common features appear at some point during development:-

1. Notochord.
2. Dorsal hollow nerve cord.
3. Pharyngeal slits.
4. Post-anal tail.

1. Notochord:-

• The chordates are named for the notochord:- A flexible, rod-shaped structure that is found in the embryonic stage of all chordates and also in the adult stage of some chordate species.
• It is located between the digestive tube and the nerve cord, providing skeletal support through the length of the body.
• In some chordates, the notochord acts as the primary axial support of the body throughout the animal’s lifetime.
• In vertebrates, the notochord is present during embryonic development, at which time it induces the development of the neural tube which serves as a support for the developing embryonic body.
• The notochord, however, is replaced by the vertebral column in most adult vertebrates.

2. Dorsal Hollow Nerve Cord:-

• The dorsal hollow nerve cord derives from ectoderm that rolls into a hollow tube during development.
• In chordates, it is located dorsally to the notochord.
• In contrast to the chordates, other animal phyla are characterized by solid nerve cords that are located either ventrally or laterally.
• The nerve cord found in most chordate embryos develops into the brain and spinal cord, which comprise the central nervous system.

3. Pharyngeal Slits:-

• Pharyngeal slits are openings in the pharynx that extend to the outside environment.
• In organisms that live in aquatic environments, pharyngeal slits allow for the exit of water that enters the mouth during feeding.
• Some invertebrate chordates use the pharyngeal slits to filter food out of the water that enters the mouth.
• In vertebrate fishes, the pharyngeal slits develop into gill arches, the bony or cartilaginous gill supports.
• In most terrestrial animals, including mammals and birds, pharyngeal slits are present only during embryonic development.
• In these animals, the pharyngeal slits develop into the jaw and inner ear bones.

4. Post-anal Tail:-

• The post-anal tail is a posterior elongation of the body, extending beyond the anus.
• The tail contains skeletal elements and muscles, which provide a source of locomotion in aquatic species.
• In some terrestrial vertebrates, the tail also helps with balance, courting, and signaling when danger is near.
• In humans and other apes, the post-anal tail is present during embryonic development, but is vestigial as an adult.

Origin Of Chordates:-

Origin Of Chordates:-

Introduction:-

It proposes that the origin of chordates, which took place about 570 million years ago during the Precambrian, must be from some deuterostomes (animals in which the mouth is not formed from the blastopore of the early stages of development and that includes echinoderms, hemichordates and chordates).

• Here we shall discuss the origin of earlier chordate ancestors of the vertebrates.
• That chordates have originated from the invertebrates is not questioned by most zoologist now-a-days.
• Since the earlier chordate ancestors were soft-bodied forms, they left no fossil remains to us clues as to their origin.
• Therefore, only basis for finding out the origin of earlier chordates is available from the resemblance between the lower chordates and the invertebrates.
• There are some structural features shared by them, such as bilateral symmetry, antero-posterior body axis, triploblastic coelomate condition, metameric segmentation, etc., may be considered as basis of their common ancestry.

Ancestry of Chordates:-

Ancestry of Chordates:-

• Similarities existing between some invertebrates and the chordates have led to the enunciation of several theories on the origin of chordates.
• All these theories postulate that the chordates originated either directly from some invertebrates or through the intervention of some invertebrate chordates.
• Almost every invertebrate phylum—Coelenterata, Nemertean, Phoronida, Annelida, Arthropoda and Echinodermata—has been suggested.
• But these theories are far from being satisfactory and convincing and have only a historical value.
• Only the echinoderm theory has received some attention and acceptance and shall be considered and evaluated under deuterostome line of chordate ancestry.

2.Integumentary System:-

• The integumentary system is the set of organs that forms the external covering of the body and protects it from many threats such as infection, desiccation, abrasion, chemical assault and radiation damage.
• IN humans the integumentary system includes the skin – a thickened keratinized epithelium made of multiple layers of cells that is largely impervious to water.
• It also contains specialized cells that secrete melanin to protect the body from the carcinogenic effects of UV rays and cells that have an immune function.
• Sweat glands that excrete wastes and regulate body temperature are also part of the integumentary system.
• Somatosensory receptors and nociceptors are important components of this organ system that serve as warning sensors, allowing the body to move away from noxious stimuli.

Definition:-

Development of the Integumentary System:-

• The integumentary system develops from all embryonic layers (ectoderm, mesoderm, and neural crest cells).

• The integumentary system is the largest organ system in the human body, responsible for protection from physical and environmental factors.
• The integumentary system is both a barrier and a sensory organ, and includes the skin (the largest bodily organ), as well as appendages, sweat and sebaceous glands, hair, nails and arrectores pullorum (tiny muscles at the root of each hair that cause goose bumps).

Fetal Skin Formation:-

• Fetal skin forms from three layers: ectoderm, mesoderm, and neural crest cells.
• At 4 weeks gestation, simple ectoderm epithelium forms.
• Between 4 and 12 weeks, the basal cells divide repeatedly to form the stratified epithelium while the mesoderm forms the blood vessels and connective tissues.
• Epidermal ridges (e.g. fingerprints) begin to develop around 10 weeks gestation and are completed by 17 weeks gestation.
• Sensory nerves also develop.
• At 16 weeks gestation, the basement membrane folds.
• Melan oblasts that form melanocytes migrate with neural crests cells to the epithelium and begin producing melanin prior to birth.
• The connective tissue differentiates into the various layers of the dermis.
• Ectoderm thickens into fingernails and toenails.
• Other regions of the ectoderm form into epithelial columns called cords which become hair follicles and sebaceous and sweat glands.
• At 20 weeks gestation, hair begins to grow from sebaceous glands, while sweat glands are formed from coiled cords.
• Other cords begin to form mammary glands.

Fetal Skin Formation:-

General structure of integument:-

• The skin is made up of two mutually dependent layers that are distinguished based on their structure and location.
• These layers – the epidermis and the dermis – contain a variety of structures, including blood vessels, hair follicles, and sweat glands.
• Beneath the dermis lies the hypodermis (subcutis).
• It is composed mainly of fatty tissue.

• The most superficial layer, the epidermis, is composed of stratified squamous epithelia that are keratinized at the outermost surface, melanocytes, immune cells (Langerhans that modulate immune response) and sensory receptors (Merkel cells that detect light touch).
• The function of the epidermis layer is “protection.”
• The keratinocytes and immune cells help protect the skin.
• The dermis lies beneath the epidermis and is composed of two layers of connective tissue: a loose layer (papillary) and a dense irregular layer (reticular).
• Both layers of the dermis contain connective tissue components (collagen, elastin, fibroblasts), plus blood vessels, sensory receptors and lymphatics.
• The dermis is a “functional” layer.
• The dermis is connective tissue that can stretch and retract because of the strong and elastic extracellular matrix.
• The dermis also contains nerves.
• Beneath these two layers lies the hypodermis, composed of loose connective tissue (adipose and areolar).
• The hypodermis is the “connection” layer.
• It connects the integument (epidermis and dermis) to organs and muscles in the body.
• This layer contains adipose tissue and connective tissue as well as blood vessels, nerves and immune cells.

Functions Of Integumentary System:-

The integumentary system has many functions, most of which are involved in protecting you and regulating your body’s internal functions in a variety of ways:-

• Protects the body's internal living tissues and organs
• Protects against invasion by infectious organisms
• Protects the body from dehydration
• Protects the body against abrupt changes in temperature
• Helps dispose of waste materials
• Acts as a receptor for touch, pressure, pain, heat, and cold
• Stores water and fat

Derivatives of integument- Epidermal and Dermal derivatives:-

Derivatives of integument- Epidermal and Dermal derivatives:-

A. Keratin Structures:-

• New epidermal cells are formed continuously in the lower layers of the epidermis.
• In terrestrial vertebrates, new epidermal cells push more superficial ones to the stratum corneum, the outer-most epithelial layer.
• In the process of self-destruction, these exterior epidermal cells accumulate protein products called keratin.
• Keratinized or cornified skin serves to prevent water escape and to protect against friction and direct mechanical stimulation (e.g. calluses in humans).
• The production of all of the following structures involves keratinization:

Epidermal Scales:- 

• A continuous layer of repetitious thickenings of the stratum corneum; you cannot dissect an individual epidermal scale out of the skin!
• These scales may be shed entirely or in small flakes.
• Examine preserved specimen of snake and dried specimens of bird legs and feet.

Claws and Talons:-

• Curved, laterally compressed keratinized projections from the tips of digits.
• See dried specimen of cat claws and bird talons.

What are the possible functions of these structures?

Hooves:- Enlarged keratinized plates found on the ends of ungulate digits. Examine the hooves of pig and horse.

Nails:- keratinized epithelial cells produced at the name base by pushing the existing nail forward. Protect from mechanical injury and stabilize skin for better grasping. Found only in primates.

Horns:- A tough, cornified layer of the integument covers horns. Their core, however, is bone, of dermal origin. Horns are found in bovines (cattle, antelope, sheep, goats, bison, wildebeest). They are retained year-round and grow throughout the animal’s lifetime.

Baleen:- Found in some whales, is a series of keratinized plates that arise from oral epithelium. These sheets hang from the palate along its length. See the display. Of what use would the sieve-like action of these plates be?

Beaks:- Epidermal structures, jaws are covered by keratinized sheaths in birds.

Feathers:- Are believed to have evolved from reptilian scales. Columns of epidermal cells project into the skin initially to form an invagination called the feather follicle.

• Continued growth results in their projection out of the skin to form the feather shaft.
• These columns then separate from each other, become keratinized and develop into barbs.
• Feather growth is initiated by mesodermal papillae, which die in the grown feather to form feather pulp.
• Examine the dried specimens.
• Note the quill, which attaches to the body and extends as a rachis.
• From the rachis project many veins with barbs and barbules to hold them together.

Hair:-

•  Just as in feathers, there is an initial ingrowth of epidermal cells to form the hair follicle, followed by an outward growth of keratinized cells to form the hair shaft.
• Dermal papillae die to form the core substance of hair follicles.
• Note the similarities between hair and feathers both in development and in general anatomy.
• They both possess dermal papillae, shafts, an inner pulp and columns of specialized keratinized cells.
• Hair is characteristic of mammals.

B) Glands:-

• Specialized to secrete specific products (oil, sweat, milk, etc.), these cells are derived by an infoldings of the epidermis.

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• In many cases they retain a connection to the stratum corneum whereby their secretions can be released at the skin surface.

THE DERMIS AND ITS DERIVATIVES:-

• The dermis is generally much thicker than the epidermis and lies more deeply.
• It is made of a fibrous mass of connective tissue and is of mesodermal origin.
• It may directly produce dermal bone.
• The dermis is important in defense against injury and in the maintenance of body heat.
• Deeper regions of the dermis often contain fatty deposits, smooth muscle, blood vessels and nerves. 
• Chromatophore cells are sometimes epidermal, but usually dermal in origin.
• They secrete melanin, which can be passed to the stratum corneum of skin and to hair shafts to produce colour and block harmful sunlight.

Dermal Bone:-

• Once present in some extinct fish - Ostracoderms with a complete head shield, Placoderms with a broken head shield and body armor.
• Now demonstrated in Turtle dermal bone, antlers, and in the dermal armor of Armadillo.
• In antlers the velvet is epidermal in origin and shapes and provides blood to the dermal bone.
• Once grown, the velvet is shed and only the bone remains.
• Antlers are found in deer, elk, moose and their relatives, often only in males.
• They are shed annually.
• In most modern vertebrates, dermal bone is formed from embryonic mesenchyme by extra membranous ossification, and contributes to the skull and skeleton, rather than being manifested externally.
• An exception is teeth, which are partly derived from dermal bone.

Fish Scales:-

• Fish scales are also called dermal scales since they are derived mainly from the dermis.

Cosmoid Scales:- 

• Found in Placoderms as plates, and also typical of the Lobe Finned Fishes or Sarcopterygii, Extinct fish had scales of enamel, cosine and bone with pulp cavities.
• Modern ones, like Coelacanth and the lung fish have calcified fibers so this type of scale is almost extinct.

Placoid Scales: See bioplastic mounts and dogfish slides. Made of enamel (epidermal) and the dermal derivatives, dentine and bone with a pulp core. Typical of cartilaginous fishes. Placoid scales are responsible for the rough feeling of dogfish skin.

Ganoid Scales: See bioplastic mounts, slides, the plates of Sturgeon, called scuttles, and the scales of the Gar Pike on display. Made of multi-layered enamel called ganoin over lamellar bone. Primitive (now extinct) species also had a cosine layer and vascular bone with pulp, but these were lost in modern day examples.

Teleost (bony fish) scales

These are thin scales of dermal bone. They have a thin covering of epidermal tissue over them. It is derived by reduction (loss) of parts of a ganoid scale. There are two types depending on their shape.

Cycloid Scales: See bioplastic mounts and slides. A round ended scale.

Ctenoid Scales: See bioplastic mounts and slides. A comb shaped end is characteristic of this scale type.

Referring to the bioplastic mount and slides, make a sketch of the placoid, ganoid, cycloid and ctenoid scales.

Skeletal System:-

Skeletal System:-

Introduction to the Skeletal System:-

• Humans are vertebrates, animals having a vertebral column or backbone.
• They rely on a sturdy internal frame that is centered on a prominent spine.
• The human skeletal system consists of bones, cartilage, ligaments and tendons and accounts for about 20 percent of the body weight.
• The living bones in our bodies use oxygen and give off waste products in metabolism.
• They contain active tissues that consume nutrients, require a blood supply and change shape or remodel in response to variations in mechanical stress.
• Bones provide a rigid framework, known as the skeleton, that support and protect the soft organs of the body.
• The skeleton supports the body against the pull of gravity.
• The large bones of the lower limbs support the trunk when standing.
• The skeleton also protects the soft body parts.
• The fused bones of the cranium surround the brain to make it less vulnerable to injury.
• Vertebrae surround and protect the spinal cord and bones of the rib cage help protect the heart and lungs of the thorax.

• Bones work together with muscles as simple mechanical lever systems to produce body movement.
• Bones contain more calcium than any other organ.
• The intercellular matrix of bone contains large amounts of calcium salts, the most important being calcium phosphate.
• When blood calcium levels decrease below normal, calcium is released from the bones so that there will be an adequate supply for metabolic needs.
• When blood calcium levels are increased, the excess calcium is stored in the bone matrix.
• The dynamic process of releasing and storing calcium goes on almost continuously.
• Hematopoiesis, the formation of blood cells, mostly takes place in the red marrow of the bones.
• In infants, red marrow is found in the bone cavities.
• With age, it is largely replaced by yellow marrow for fat storage.
• In adults, red marrow is limited to the spongy bone in the skull, ribs, sternum, clavicles, vertebrae and pelvis.
• Red marrow functions in the formation of red blood cells, white blood cells and blood platelets.

Evolution of visceral arches:-

Evolution of visceral arches:-

• Visceral arches are pieces of cartilages or bones that support the pharyngeal region of vertebrates and also help attach the jaws with the skull.
• The visceral arches are also known as pharyngeal arches.
• These structures are seen in the embryonic development of vertebrates which are recognizable precursors for many structures.
• There are typically 7 pairs of visceral arches in vertebrates which modify in different groups depending upon the presence or absence of gills and type of jaw suspension.
• The splanchnocranium is the name given to the gill arches and their derivatives.
• These include the jaws.
• The splanchnocranium is formed from the splanchnic mesoderm in the wall of the pharynx between gill-clefts for their support.
• It consists of series of a paired visceral bars of cartilage which become united with one another ventrally by an unpaired cartilage to form visceral arches, the visceral arches are horse-shoe-shaped and encircle the pharynx all round except dorsally.
• Those visceral arches which contribute to the formation of the skull constitute the splanchnocranium.

Evolution:-

Evolution:-

Evolution:-

• The placoderm is an ancient chordate characteristic.
• Its forerunner was associated with supporting the filter feeding structures of cephalochordates and urochordates.
• Typically there are 7 visceral arches in fishes, though this number varies from 4 to 9 in different groups.
• The first visceral arch is known as mandibular arch having two cartilaginous pieces called pterygoquadrate and Meckel's cartilage.
• In fish the pharyngeal arch is called gill arch.
• It is the first of six pharyngeal arches that develops during the fourth week of development.
• It is located between the stomodeum and the first pharyngeal groove.
• This arch divides into a maxillary process and a mandibular process, giving rise to structures including the bones of the lower two-thirds of the face and the jaw.
• The maxillary process becomes the maxilla and palate while the mandibular process becomes the mandible or lower jaw.
• This arch also gives rise to the muscles of mastication.
• Meckel's cartilage forms in the mesoderm of the mandibular process and eventually regresses to form the incus and malleus of the middle ear, the anterior ligament of the malleus and the sphenomandibular ligament.

• The mandible or lower jaw forms by perichondral ossification using Meckel's cartilage as a 'template', but the maxillary does not arise from direct ossification of Meckel's cartilage.
• The skeletal elements and muscles are derived from mesoderm of the pharyngeal arches. Second visceral arch, called hyoid arch, consists of hyomandibular, ceratohyal and basial.
• The hyomandibular cartilage articulates with the chondrocranium.
• It is the second of six pharyngeal arches that develops in fetal life during the fourth week of development and assists in forming the side and front of the neck.
• Cartilage in the second pharyngeal arch is referred to as Reichert's cartilage and contributes to many structures in the fully developed adult.
• It is composed of two distinct cartilaginous segments joined by a faint layer of mesenchyme.
• Dorsal ends of Reichert's cartilage ossify during development to form the stapes of the middle ear before being incorporated into the middle ear cavity, while the ventral portion ossifies to form the lesser corn and upper part of the body of the hyoid bone.
• Caudal to what will eventually become the stapes, Reichert's cartilage also forms the styloid process of the temporal bone.
• The cartilage between the hyoid bone and styloid process will not remain as development continues, but its perichondrium will eventually form the stylohyoid ligament.

• From 3rd to 7th visceral arches are called branchial arches, since they support gills and typically consist of 4 pieces of cartilages, namely, pharyngobranchial, epibranchial, ectobronchium and hypobranchial.
• These branchial arches do not contribute to the formation of skull.

Cyclostomes:-

• There is no resemblance with the typical pattern but cyclostomes possess a splanchnocranium in which usual cartilages are not identifiable.
• The whole pharyngeal skeleton fuses to form a branchial basket to support gills.

Elasmobranchs:-

• They contain full set of visceral arches and three unpaired branchial cartilages called basibranchial.
• The arrangement is close to basic pattern as they have 5 pairs of functional gills and skeleton is all cartilaginous.
• Bony fishes Meckel’s cartilage forms articular and becomes part of the lower jaw.
• Hyoid arch is modified for the movement of operculum and functioning of the lower jaw.
• Symplicit helps in jaw suspension.
• Last branchial arch shows sign of degeneration as the number of gills is reduced to 4 pairs.
• Amphibia Larval frogs have 6 visceral arches and the last 3 bear gills.

• In urodeles having gills third, fourth and fifth epibranchials support gills while their basibranchial and coracobrachialis are reduced to two pairs.
• Hyomandibular modifies as columella of the middle ear cavity in frogs and toads.
• The air breathing hyobranchial apparatus of frogs and toads is made by the fusion of 2nd, 3rd and 4th visceral arches.
• Reptiles Quadrate and pterygoid bones of the skull are modifications of pterygoquadrate, and articular of the lower jaw is a modified Meckel's cartilage.
• Hyoid arch forms a small hyoid plate that also extends forward to support the tongue.
• One of two coracobrachialis may form the posterior corn of the hyoid plate.
• Birds Modification is similar to reptiles except that there is only one corn of the hyoid plate that is modified from the third visceral arch.
• Mammals Pterygoquadrate breaks into alisphenoid and incus, the former becomes part of the skull and the latter joins the ear ossicles.
• Meckel’s cartilage modifies into malleus and hyomandibular into stapes of the middle ear cavity.
• Larynx of mammals evolved from the fourth and fifth visceral arches.
• Thyroid cartilage is a modification of 4th and 5th visceral arches while arytenoid and cricoid cartilages are modified fifth visceral arch.

• These arches work together in different ways to create jaw movement, based on the ways in which these arches articulate with the chondrocranium these are divided into the following three types –
• Amphistylic (primitive cartilaginous fishes) - jaw is supported both by the hyomandibular and by a direct connection between the jaw and the chondrocranium.
• Hypostyle (elasmobranchs and most bony fishes) - upper jaw loses any major direct connection with the chondrocranium and the upper and lower jaws are supported solely by the hyomandibular.
• Autostylic (lungfishes and in tetrapod ancestors) - upper jaw (pterygoquadrate cartilage) articulates or is fused with the chondrocranium, lower jaw forms from the mandibular cartilage, and the jaw remains unsupported by the hyomandibular.

Comparative account of Limbs and girdles:-

The pectoral girdles are to the upper limbs as the pelvic girdle is to the lower limbs; the girdles are the parts of the appendicular skeleton that anchor the appendages to the axial skeleton.

The difference between the pectoral girdle and pelvic girdle are listed below.

Pectoral Girdle

• The pectoral girdle is a part of the appendicular skeleton, which connects the upper limbs to the axial skeleton.
• The human skeleton is divided into two sections, the axial skeleton and the appendicular skeleton.
• Axial skeleton is made up of 80 bones and includes the skull, the vertebral column, ribs and the sternum.
• Appendicular skeleton is made up of 126 bones and includes the pectoral girdle, the pelvic girdle and limbs.
• The pectoral girdle is also known as shoulder girdle.
• It constitutes the shoulder part and also the attachment site for the humerus.

Pectoral Girdle Bones

• Pectoral girdle can be divided into two equal halves.
• Each half of the pectoral girdle consists of two bones, namely scapula and clavicle.
• It helps in the articulation of upper limbs to the axial skeleton.
• It aids in the movement of arms and shoulders.
• The right and left parts are not joined together and it allows independent movement.
• The main parts of the pectoral girdle are:

Scapula (Shoulder blade)

• It is a triangular-shaped flat bone, which forms the posterior side of the shoulder.
• It is present dorsally in the thorax region between the second and the seventh ribs.

Spine of scapula- 

• It is an elevated ridge present on the posterior side of the scapula.
• It divides the convex posterior portion of the scapula into the smaller supraspinous fossa and the bigger infraspinous fossa.
• The ventral surface of the scapula faces ribs and there is a concave depression, which is known as subscapular fossa.

Acromion- 

• It is a flat and expanded process.
• The spine extends laterally to form acromion.
• It articulates with the clavicle by acromioclavicular joint.
• It forms an arch over the glenohumeral joint.

Glenoid cavity- 

• It is present below the acromion. It is in the form of depression at the lateral surface of the scapula.
• The head of the humerus articulates at the glenoid cavity by glenohumeral joint.

Clavicle (Collar bone)-

• It is a long s-shaped bone, which forms the anterior part of the pectoral girdle.
• It is present horizontally and is also known as the collar bone.
• The main function of the clavicle is that it attaches the upper limb to the sternum and transmits forces from the upper limb to the axial skeleton.
• It also protects the underlying nerves and vasculature, which connects the upper limb to the trunk.
• On one side, it articulates with the sternum and on the other side, it articulates with the acromion.

Coracoid process- 

• It is a hook-like projection present above the glenoid cavity and below the clavicle.
• It is attached to the clavicle by a ligament.
• Muscles of arms and chest attach here.
• The large and triangular sternal end is also known as the medial part, which articulates with the manubrium of the sternum by sternoclavicular joint.
• It is supported by the costoclavicular ligament or rhomboid ligament present between the clavicle and the first rib.
• The shaft portion is the attachment site of many muscles, e.g. deltoid. trapezius, sternohyoid.
• The acromial end is also known as the lateral part.
• It articulates with the acromion of the scapula by acromioclavicular joint.
• It forms the bony tip of the shoulder laterally.
• Conoid and trapezoid ligaments are also attached to it.

UNIT–II

Digestive System:-

Introduction:-

• The digestive system includes the digestive tract and its accessory organs, which process food into molecules that can be absorbed and utilized by the cells of the body.
• Food is broken down, bit by bit, until the molecules are small enough to be absorbed and the waste products are eliminated.
• The digestive tract, also called the alimentary canal or gastrointestinal (GI) tract, consists of a long continuous tube that extends from the mouth to the anus.
• It includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. The tongue and teeth are accessory structures located in the mouth.
• The salivary glands, liver, gallbladder, and pancreas are major accessory organs that have a role in digestion.
• These organs secrete fluids into the digestive tract.

Food undergoes three types of processes in the body:-

1. Digestion
2. Absorption
3. Elimination

Digestive System:-

• Digestion and absorption occur in the digestive tract.
• After the nutrients are absorbed, they are available to all cells in the body and are utilized by the body cells in metabolism.
• The digestive system prepares nutrients for utilization by body cells through six activities, or functions.

Ingestion:-

• The first activity of the digestive system is to take in food through the mouth.
• This process, called ingestion, has to take place before anything else can happen.

Mechanical Digestion:-

• The large pieces of food that are ingested have to be broken into smaller particles that can be acted upon by various enzymes.
• This is mechanical digestion, which begins in the mouth with chewing or mastication and continues with churning and mixing actions in the stomach.

Chemical Digestion:-

• The complex molecules of carbohydrates, proteins, and fats are transformed by chemical digestion into smaller molecules that can be absorbed and utilized by the cells.
• Chemical digestion, through a process called hydrolysis, uses water and digestive enzymes to break down the complex molecules.
• Digestive enzymes speed up the hydrolysis process, which is otherwise very slow.

Movements:-

• After ingestion and mastication, the food particles move from the mouth into the pharynx, then into the esophagus.
• This movement is deglutition, or swallowing.
• Mixing movements occur in the stomach as a result of smooth muscle contraction.
• These repetitive contractions usually occur in small segments of the digestive tract and mix the food particles with enzymes and other fluids.
• The movements that propel the food particles through the digestive tract are called peristalsis.
• These are rhythmic waves of contractions that move the food particles through the various regions in which mechanical and chemical digestion takes place.

Absorption:-

• The simple molecules that result from chemical digestion pass through cell membranes of the lining in the small intestine into the blood or lymph capillaries.
• This process is called absorption.

Elimination:-

• The food molecules that cannot be digested or absorbed need to be eliminated from the body.
• The removal of indigestible wastes through the anus, in the form of feces, is defecation or elimination.

Alimentary canal and digestive glands:-

• The food passes through a continuous canal called alimentary canal.
• The canal can be divided into various compartments:- (1) buccal cavity, (2) oesophagus, (3) stomach, (4) small intestine, (5) large intestine ending in the rectum and (6) anus.
• The activities of the gastro-intestinal tract [alimentary canal] are under neural and hormonal control for proper coordination of different parts.
• The sight, smell and/or the presence of food in the oral cavity can stimulate the secretion of saliva.
• Gastric and intestinal secretions are also, similarly, stimulated by neural signals.
• The muscular activities of different parts of the alimentary canal can also be moderated by neural mechanisms.

1. The Alimentary canal :- is consist of

​

1. Mouth :-

2. Pharynx :-

3. Oesophagus :-

4. Stomach :-

5. Small intestine :-

6. Large intestine :-

7. Rectum :-

2. Digestive glands:-

1. Liver:-

3. Salivary gland :-

2. Pancreas:-

Alimentary canal and digestive glands:-

Mouth:-

• Food is chewed into smaller pieces.
• Adults have 32 specialized teeth—teeth that can grind, chew, and tear different kinds of food.
• The tongue is an organ consisting of skeletal muscles that move the food around the mouth to allow for efficient mechanical digestion.
• Salivary glands beneath and in back of the tongue secrete the saliva that allows for easier swallowing of food and the beginning of chemical digestion. 

Pharynx:-

• Swallowing forces the chewed food through a tubular entrance to the oesophagus.
•  As food is swallowed a flap-like valve, the epiglottis, closes over the trachea to prevent food entering the windpipe and causing choking.  

Oesophagus:-

• The oesophagus connects the pharynx with the stomach. 
• Contractions of the oesophagus push the food through a sphincter  and into the stomach

1. The Alimentary canal :-

Stomach:-

• The stomach is a muscular and stretchable sac with three important functions:-
• 1) It mixes and stores food until it can be further digested.
• 2) It secretes chemicals that help break the food into more digestible forms.
• 3) It controls the passage of food into the small intestine.
• The stomach starts chemical digestion of protein. 
• Secretions from the stomach lining consist of about two liters of hydro chloric acid (HCl), pepsin, and other fluids that make up gastric fluids each day.
• The fluid is extremely acidic and it helps kill bacteria and other pathogens that may have been ingested.
• The thick mucus also produced by the stomach lining usually keeps the acids from damaging the lining.
• If not enough mucus is produced or if too much acid is produced, peptic ulcers form. 
• Heredity, stress, smoking, and excessive alcohol intake can make the ulcers worse.
• The condition can worsen and bleeding ulcers can result. 
• Food stays in the stomach for approximately 3-4 hours and moves through another sphincter muscle to pass into the small intestine.

Small intestine:-

• Nearly 7 meters in length, the small intestine is folded and curled around a small area in the abdominal cavity.
• The inside surfaces of the intestine are covered with projections called villi.
• These finger-like structures are covered in smaller projections called microvilli and work to absorb food molecules that have been broken down by the processes of chemical digestion.
• The small intestine has three distinct parts:  the duodenum, the jejunum, and the ileum. 
• Each day, about 9 liters of fluid enters the duodenum. 
• Most chemical digestion takes place in the duodenum by chemicals secreted by the liver, pancreas and small intestine.
• The other two sections of the small intestine, the jejunum and the ileum, absorb food molecules by way of the villi directly into the blood stream.

Large intestine:-

• The large intestine receives the material “left-over” from chemical digestion that is basically nutrient free.
• Only water, cellulose, and undigestible materials are left.
• The main job of the large intestine is to remove water from the undigested material.
• Water is quickly removed from the material through villi and returns to the blood stream.  

Rectum:-

• The last part of the digestive tract is the rectum, a “holding area” for the undigested material.
• Waste leaves the body from this area.

Liver:-

• The liver is a large organ located just above the stomach. 
• The liver produces bile which helps digest lipids. 
• Bile is stored in the gallbladder and flows from the gallbladder to the duodenum where it helps digest fats.
• The picture at the left shows a human liver.

Pancreas:-

• The pancreas has three important functions that help the digestive system change food into a form that can be used by the cells.

1) It produces enzymes which help break down proteins, lipids, and carbohydrates.

2) It produces the hormone, insulin, which helps regulate blood glucose levels.

3) It produces sodium bicarbonate which helps to neutralize stomach acids.

2. Digestive glands:-

Salivary glands:-

• The major salivary glands, the parotid, submandibular and sublingual, are found as paired sets.
• Their ducts empty into the oral cavity.
• Their main function is to secrete saliva, which contains mucin, water and ions, as well as a few digestive enzymes, such as amylase and RNAs.
• Salivary glands are distinguished by the morphology of their secretory elements and duct structure, as well as by the predominant type of product they synthesize.
• They are called serous, protein secreting, or mucous, glycoprotein secreting.
• Although serous cells do secrete glycosylated proteins and, therefore, can be considered to be seromucous cells, this morphological terminology is still used because of the much greater amount of carbohydrate in the mucous secretions synthesized by the mucous cells.

3. Respiratory system:-

• Respiration is the process in which organisms exchange gases between their body cells and the environment.
• From prokaryotic bacteria and archaea's to eukaryotic protists, fungi, plants, and animals, all living organisms undergo respiration.
• Respiration may refer to any of the three elements of the process.
• First, respiration may refer to external respiration or the process of breathing also called ventilation. 
• Secondly, respiration may refer to internal respiration, which is the diffusion of gases between body fluids and tissues. 
• Finally, respiration may refer to the metabolic processes of converting the energy stored in biological molecules to usable energy in the form of ATP.
• This process may involve the consumption of oxygen and production of carbon dioxide, as seen in aerobic cellular respiration, or may not involve the consumption of oxygen, as in the case of anaerobic respiration.

3. Respiratory system:-

• The respiratory system is the network of organs and tissues that help you breathe.
• It includes your airways, lungs, and blood vessels.
• The muscles that power your lungs are also part of the respiratory system.
• These parts work together to move oxygen throughout the body and clean out waste gases like carbon dioxide.

Introduction:-

• Respiration, process by which an organism exchanges gases with its environment.
• The term now refers to the overall process by which oxygen is abstracted from air and is transported to the cells for the oxidation of organic molecules while carbon dioxide (CO₂) and water, the products of oxidation, are returned to the environment.
• In single-celled organisms, gas exchange occurs directly between cell and environment, at the cell membrane.
• In plants, gas exchange with the environment occurs in special organs, the stomates, found mostly in the leaves.
• Organisms that utilize respiration to obtain energy are aerobic, or oxygen-dependent.
• Some organisms can live in the absence of oxygen and obtain energy from fuel molecules solely by glycolysis  these anaerobic processes are much less efficient, since the fuel molecules are merely converted to end products such as lactic acid and ethanol, with relatively little energy-rich ATP produced during these conversions.

Different respiratory organs in vertebrates:-

1. Gills:-

• Respiration in fish takes place with the help of gills.
• Most fish possess gills on either side of their head.
• Gills are tissues made up of feathery structures called gill filaments providing a large surface area for exchange of gases.
• A large surface area is crucial for gas exchange in aquatic organisms as water contains very little amount of dissolved oxygen.
• The filaments in fish gills are organized in rows in the gill arch. 
• Each filament comprises lamellae, which are discs supplied with capillaries.
• Blood moves in and out of the gills through these small blood vessels.
• Though gills in fish occupy only a small section of their body, the extensive respiratory surface produced by the filaments renders the whole organism with efficient gas exchange.

• Fish take in oxygen-rich water via their mouths and pump it over their gills.
• When water moves over the gill filaments, the blood within the capillary network takes up the dissolved oxygen.
• Then, the circulatory system supplies oxygen to all tissues of the body and finally to the cells while taking up carbon dioxide that is eliminated through the gills from the body.
• It exits the body of the fish once the water moves past the gills through the openings provided in the sides of the throat or through the operculum, a flap, usually found in bony fish, that covers and protects the fish gills.

2. Lungs:-

Lungs:-

• Their role is to take oxygen into the body, which we need for our cells to live and function properly, and to help us get rid of carbon dioxide, which is a waste product.

Structure:-� The lungs are paired, cone-shaped organs which take up most of the space in our chests, along with the heart.

• We each have two lungs, a left lung and a right lung.
• These are divided up into ‘lobes’, or big sections of tissue separated by ‘dividers.
• The right lung has three lobes but the left lung has only two, because the heart takes up some of the space in the left side of our chest.
• The lungs can also be divided up into even smaller portions, called ‘bronchopulmonary segments .These are pyramidal-shaped areas which are also separated from each other by membranes.
• There are about 10 of them in each lung.
• Each segment receives its own blood supply and air supply.

3. Skin:-

• The skin is made up of two mutually dependent layers that are distinguished based on their structure and location.
• These layers – the epidermis and the dermis – contain a variety of structures, including blood vessels, hair follicles, and sweat glands.
• Beneath the dermis lies the hypodermis (subcutis).
• It is composed mainly of fatty tissue.

• The most superficial layer, the epidermis, is composed of stratified squamous epithelia that are keratinized at the outermost surface, melanocytes, immune cells (Langerhans that modulate immune response) and sensory receptors (Merkel cells that detect light touch).
• The function of the epidermis layer is “protection.”
• The keratinocytes and immune cells help protect the skin.
• The dermis lies beneath the epidermis and is composed of two layers of connective tissue: a loose layer (papillary) and a dense irregular layer (reticular).
• Both layers of the dermis contain connective tissue components (collagen, elastin, fibroblasts), plus blood vessels, sensory receptors and lymphatics.
• The dermis is a “functional” layer.
• The dermis is connective tissue that can stretch and retract because of the strong and elastic extracellular matrix.
• The dermis also contains nerves.
• Beneath these two layers lies the hypodermis, composed of loose connective tissue (adipose and areolar).
• The hypodermis is the “connection” layer.
• It connects the integument (epidermis and dermis) to organs and muscles in the body.
• This layer contains adipose tissue and connective tissue as well as blood vessels, nerves and immune cells.

4. Air sacs:-

• Air sac, any of the air-filled extensions of the breathing apparatus of many animals.
• Air sacs are found as tiny sacs off the larger breathing tubes (tracheae) of insects, as extensions of the lungs in birds, and as end organs in the lungs of certain other vertebrates.
• They serve to increase respiratory efficiency by providing a large surface area for gas exchange.

• Air sacs are spaces within an organism where there is the constant presence of air.
• Among modern animals, birds possess the most air sacs , witht heir extinct dinosaurian relatives showing a great increase in the pneumatization (presence of air) in their bones.
• Theropods, like Aerosteon, have many air sacs in the body that are not just in bones, and they can be identified as the more primitive form of modern bird airways.
• Sauropods are well known for the number of air pockets in their bones (especially vertebra), although one theropod, Deinocheirus, shows a rivalling number of air pockets.

5. Accessory respiratory organs:-

• Accessory respiratory organs in fishes are generally found in the fishes which live in shallow stagnant freshwater of tropical regions where deoxygenation of water is a main feature.
• Such structures may also develop in fishes inhabiting torrential streams of the hills, which are liable to dry up during summer.
• Buccopharyngeal epithelium:-
• This is the most primitive type of adaptation and has been found in Monopterus javanensis, Electrophorus electricus, Periopthalamus sp and Bleopthalamus sp.
• In these fishes, the epithelial lining of the mouth cavity is highly vascular and forms vascular network and thus work as efficient respiratory site.

2. Skin:-

• The extent to which fishes use cutaneous respiration is poorly known.
• It is best documented in eels (Anguilla anguilla), which leave the water and migrate from one place to another through damp vegetation.
• During this time, the moist skin (richly supplied with blood) work as an important organ for aerial respiration. 
• In some fishes like Monopterus and Mastacembelus, which live in oxygen deficient stagnant waters, the skin is of little use for respiration. 
• The glandular secretions of the skin protect skin from desiccation in water deficient condition.

3. External gills:- 

• These are highly vascular and filamentous structure of the ectoderm covering the outer surface of the visceral arches or visceral skeleton.
• They are in direct contact with water, so they help in exchange of gases. 
• To aid respiration, the embryos of certain elasmobranchs possess long filamentous structures present as outgrowth from spiracles. (External gills - develop from the skin ectoderm of the branchial area but are not directly related to the visceral skeleton or branchial chambers are found most often in larval or paedomorphic amphibians).

4. Labyrinthiform organs:- 

• These organs have been present in few perches like Anabas testudineus and Trichogaster fasciatus.
• These are formed within the extrabranchial chambers enclosed between the gills and the operculum.
• These organs are originated from the 1^st gill arch.

5. Opercular lungs:- 

• In advanced air breathing teleosts, the bag like diverticulae is called opercular lungs or air chambers.
• It develops from the dorsal surface of the branchial chamber or opercular chamber.

• The opercular lungs are present above the gills and contains specialized structures known as rosettes or arborescent organs (arbo = tree) which just increase the respiratory surface.
• Such accessory respiratory organs are found in Clarias, H. fossilis, A. testudineus, T. fasciatus etc.

6. Air bladder/ swim bladder:- 

• Swim bladder is present between the gut and kidney.
• This duct degenerates in the course of development or present throughout the life. 
• The fishes with air bladder have adapted to extensive drought conditions which may completely dry up.
• The teleost's like Chirocentrus, Cyprinoids come to the surface take air and pass it back to the air bladder which is highly vascular. (Swim bladder is a gas filled bag like structure which originates from the dorsal outgrowth from the alimentary canal in most of the ray finned fishes.
• It controls buoyancy at different depths in water bodies.
• The gases which are present in the air bladder are oxygen, carbon dioxide and nitrogen).

7. A special part of alimentary canal:-

•  In a small group of fishes, inhaled air is swallowed and forced back inside the alimentary canal, a part of which may be modified for aerial respiration.
• After respiratory exchange, the used up air is either passed outside by the anus or is expelled through the mouth.
• For helping the gaseous exchange some modifications have taken place in alimentary canal. 
• There is a great reduction in muscle layers of stomach and intestine making it thinner and transparent.
• Inner surface is lined by a single layer of epithelial cell which is highly supplied with blood.

UNIT–III

Circulatory System:-

• The circulatory system is made up of blood vessels that carry blood away from and towards the heart.
• Arteries carry blood away from the heart and veins carry blood back to the heart.
• The circulatory system carries oxygen, nutrients, and hormones to cells, and removes waste products, like carbon dioxide.

• The Blood Circulatory System is comprised of the heart and lungs, along with the arteries and veins which make up our circulatory system.
• These pieces work together, to ensure our blood flows properly throughout our body i.e. systemic circulation, our lungs i.e. pulmonary circulation and our heart i.e. coronary circulation.
• The combination of blood circulatory and heart make up our cardiovascular and pulmonary circulation system.
• A human heart works like a pump to push nutrients rich blood to all our organs, cells and tissue in the body.
• Our Blood carries necessary nutrients and oxygen to every cell in the body while removing any waste or carbon dioxide generated by the cells.
• Our circulatory system is a one-way system and the blood flow process inside the body is known as circulation.
• The Blood circulatory system carries blood from our heart to the rest of our body through a very complex network of arteries, capillaries and arterioles.
• The blood then returns to our heart through veins and venules.
• In pulmonary circulation the roles are reversed.
• The pulmonary artery brings oxygen-lacking blood to our lungs while the pulmonary veins bring oxygen-rich blood back to our heart.

Evolution of heart in vertebrates:-

• The heart is an unpaired organ but its origin is bilateral.
• In an embryo the mesenchyme forms a group of endocardial cells below the pharynx.
• These cells become arranged to form a pair of thin endothelial tubes.
• The two endothelial tubes soon fuse to form a single endocardial tube lying longitudinally below the pharynx.
• The splanchnic mesoderm lying below the endoderm gets folded longitudinally around the endocardial tube.
• This two-layered tube will form the heart in which the splanchnic mesoderm thickens to form a myocardium or muscular wall of the heart and an outer thin epicardium or visceral pericardium.
• The endocardial tube becomes the lining of the heart known as endocardium.
• Folds of splanchnic mesoderm meet above to form a dorsal mesocardium which suspends the heart in the coelom.
• Soon a transverse septum is formed behind the heart which divides the coelom into two chambers, an anterior pericardial cavity enclosing the heart and a posterior abdominal cavity.
• The heart is a straight tube but it increases in length and becomes S-shaped because its ends are fixed.
• Appearance of valves, constriction, partitions in the heart, and differential thickenings of its walls form three or four chambers in the heart.

1. Single-Chambered Heart:-
1. In Amphioxus (primitive chordate), a true heart is not found.
2. A part of ventral aorta beneath the pharynx is muscular and contractile and acts as heart.

Two-Chambered Heart:-

• In cyclostomes, there are four chambers arranged in a linear order- a thin-walled sinus venosus, a slightly muscular atrium (auricle), a muscular ventricle and a muscular conus arteriosus or bulbus cordis.
• It lies in the body cavity in which other visceral organs are also present.
• Out of four chambers, only atrium and ventricle correspond to the four chambers (paired atria and paired ventricles) of the higher vertebrates.
• In the evolution of heart many changes have taken place.
• Elasmobranchs:
• Except Dipnoi, the circulatory system in fishes from cyclostomes to teleost's, only unoxygenated blood goes to the heart, from there it is pumped to the gills, aerated and then distributed to the body.
• The heart of cartilaginous dogfish is muscular and dorsoventrally bent S-shaped tube with four compartments in a linear series.
• They are sinus venosus and atrium for receiving venous blood, and a ventricle and conus arteriosus for pumping this blood.
• The heart is a branchial venous heart.
• The sinus venosus and conus arteriosus are accessory chambers.

• Atrium and ventricle are true chambers, thus, it is a 2-chambered heart.
• The sinus venosus opens anteriorly into atrium through sinus-atrial aperture guarded by a pair of valves.
• Atrium lies dorsal to ventricle and opens ventrally into ventricle through an atrio-ventricular aperture guarded by a pair of valves.
• The thick-walled, muscular ventricle opens into a narrow conus arteriosus containing valves in two series.

• The heart is enclosed within pericardial cavity separated from body cavity by a transverse septum.
• Conus pierces the pericardium and becomes continuous with the ventral aorta.
• Pericardial cavity communicates with the body cavity through two perforations in the transverse septum.

Teleost's:-

• Their heart resembles to that of elasmobranchs.
• In teleost's, the conus is reduced and has a single pair of valves.
• The proximal part of ventral aorta close to conus becomes greatly enlarged and thick-walled, called bulbus arteriosus.
• It is elastic and dilates at the time of ventricular contraction.
• The heart is, thus, 2-chambered with a single circulation of blood. 3. Three-Chambered Heart: In dipnoans a septum divides the atrium into a right and left chamber.
• This is correlated with the use of the swim-bladder as an organ of respiration and represents the first step toward the development of the double-type circulatory system whereby both oxygenated and unoxygenated blood enter the heart and are kept separate.
• Blood from right auricle of the lungfish passes into the right ventricle and is then pumped into the primitive lung-like gas bladder by pulmonary arteries which branch off from the sixth pair of aortic arches.
• The oxygenated blood returns to the left atrium by way of pulmonary veins like amphibians.

Amphibia:-

• In amphibians, the dorsal atrium shifts anterior to ventricle.
• The sinus venosus opens into right atrium dorsally and not posteriorly.
• The atrium is completely divided into right and left chambers and has no foramen oval in the inter-auricular septum, which remains open in dipnoans.
• Deep pockets develop in the ventricular cavity.
• The conus arteriosus divides into systemic and pulmonary vessels by a spiral valve.
• In lung less salamanders, the interatrial septum is incomplete and pulmonary veins are absent.

Reptilia:-

• In reptiles, the heart is further advanced.
• The atrium is always completely separated into a right and left chamber, and in many forms the sinus venosus is incorporated into the wall of the right atrium.
• The ventricle is also partly divided by a septum in most reptiles, and in the alligators and crocodiles is completely two-chambered.
• This means that oxygenated blood coming from the lungs to the left side of the heart is essentially separated from the non-oxygenated blood from the body to the right side.
• Thus, in crocodilians, the two types of blood is completely separated, and nearly complete in other reptiles, but some mixing does occur in other parts of the circulatory system.

The embryonic conus arteriosus splits into three instead of two vessels:-

1. Pulmonary arch carrying blood to the lungs from right side of the ventricle.
2. Right systemic aorta carrying blood from left side of the ventricle to the body by way of right fourth aortic arch.

(iii) Left systemic comes from the right ventricle to the left fourth aortic arch.

• At the point of contact with the systemic aorta from the left ventricle, even in crocodilians, an opening between the two is present, called the foramen of Panizza where there may be some mixing of the two types of blood.
• Thus, reptilian heart represents the transitional heart against amphibian heart-2 complete auricles and 2 incomplete ventricles with a little mixing of blood in right and left systemic.

4. Four-Chambered Heart:-

4. Four-Chambered Heart:-

Aves and Mammalia:-

• In birds, the ventricle is completely divided into two, so that the heart is four chambered (2 auricles and 2 ventricles).
• There is complete separation of venous and arterial blood.
• The systemic aorta leaves the left ventricle and carries blood to the head and body.
• While the pulmonary artery leaves the right ventricle and carries blood to the lungs for oxygenation.
• Thus, there is double circulation in which there is no mixing of blood at any place.
• The sinus venosus is completely incorporated into right auricle, which receives two precaval and a postcaval.
• The left auricle receives oxygenated blood through pulmonary veins, conus arteriosus is absent, the pulmonary aorta arises from the right ventricle, and single systemic aorta arises from the left ventricle, and both have valves at their bases.

Modifications of aortic arches in vertebrates:-

• The basic fundamental plan of the aortic arches is similar in different vertebrates during embryonic stages.
• But in adult the condition of the arrangement is changed either being lost considerably.
• The number of aortic arches is gradually reduced as the scale of evolution of vertebrates is ascended.

Cyclostomes:-

• In lampreys (Petromyzon) there are eight pairs of aortic arches and in hag fishes there are fifteen pairs.
• The aor­tic arch is divided into afferent branchial artery and efferent branchial artery.
• In lampreys each aortic arch divides and sends branches to the posterior hemi-branch and anterior hemi- branch of the adjacent gill pouch.
• In hagfishes each arch supplies to the hemi­-branch of a single gill-pouch.

Elasmobranchs:-

• Generally there are five pairs of aortic arches in elasmobranchs but in some cases there is a variation.
• In Hexarchies there are six pairs of aortic arches.
• In Heptarchies there are seven pairs.
• In elasmobranchs the first pair of aortic arches (mandibular) disappear.
• Second to sixth pair of aortic arches (II-VI) persist as branchial arteries.
• Each aortic arch is divided into afferent and efferent branchial arteries.

• Five pairs afferent arteries arise from ventral aorta and supply deoxygenated blood to the respective gills.
• The ventral aorta divides into two branches, called innominate arteries which again bifurcate into the first and second afferent branchial arteries.
• From the gills the oxygenated blood is collected by efferent branchial arteries.
• In elasmobranchs there are nine pair’s efferent branchial arteries of which the first eight arteries form a series of four complete loops but ninth efferent branchial artery collects blood from the demi- branch of the fifth gill pouch.

Teleost's:

• In teleost's there are four pairs of aortic arches. First pair (mandibular) and second pair (hyoidean) are lost, only four pairs (third to sixth) persist as branchial arteries.
• Four pairs afferent branchial arteries arise from the ventral aorta.
• They supply deoxygenated blood to the gills for aeration.
• The ventral aorta bifurcates anteriorly to form the first pair of afferent branchial arteries.
• In sturgeon and Amia each afferent branchial arch bifurcates as in elasmobranchs.

Dipnoans:

• Among dipnoans there are four or five aor­tic arches that develop from the ventral aorta which supply blood to the gills.

• In the Protopterus the aortic arches retain second, third, fourth, fifth and sixth.
• The arches like the fish have afferent and efferent divisions.
• They have two pulmonary arteries which develop from the efferent division of the sixth arch.
• There is no second efferent branch in Neoceratiids.

Amphibians:-

• In amphibians the aortic arches retain the bilateral symmetry.
• Of the six pairs of embryo­nic aortic arches, the fifth one is observed in adult Cryptobranchus (Urodele).
• The sixth arch becomes small.
• It gives rise to the pul­monary artery and continues as the ductus arteriosus to the dorsal stem.
• The pulmonary artery may give rise to musculocutaneous artery.
• In Nectarous the first external gill is supplied by the third afferent arch.
• The base of the sixth arch is absent in Nectarous.
• The ductus arteriosus connects pul­monary artery with the dorsal stem.

• In anurans (adult), only three pairs of aortic arches (III, IV and VI) are present.
• But in the tadpole the development of aortic arches cor­respond to the emergence of external and internal gills.

Reptiles:-

• Only three pairs of aortic arches persist such as third, fourth and sixth.
• The first, second and fifth pairs of aortic arch­es disappear.
• The fifth arch is present in reduced form in some reptiles.
• The remnant of the radix of aorta between third and fourth arches is present on each side in some snakes.
• The ill-defined conus arteriosus is splatted into three vessels.

• The fourth arch on the left side which becomes the left systemic arises from the right side of the partially divided ventricle.
• The fourth arch on the right side arises from the left side of the ventricle.
• It establishes a connection with a portion of the radix aorta of the right side and becomes the right aortic arch.
• The common carotid arch arises from the right aortic arch and becomes divided into external and internal carotid arteries.
• The sixth aortic arch loses all its connection with radices of aorta and becomes the pulmonary arteries.
• The radix of aorta between carotid and systemic arches is present as ductus Coroticus in many lizards. Similarly a part of the radix may remain connected with the sixth aortic arch.
• This connecting part is known as ductus arteriosus.
• Ductus Coroticus and ductus arteriosus are both present’ in Sphenodon.
• In crocodiles the right systemic arch develops from the left ventricle which gives rise to subclavian and innominate arte­ries.

Birds:-

• The birds retain three pairs of aortic arches.
• These are lllrd,IVth and VIth. Rest three pairs such as Ist, llnd, and Vth are lost.

• The third becomes carotid artery, the fourth becomes systemic aorta but retains only the systemic arch on the right side.
• The systemic arch on the left side disappears.
• The right systemic arch originates from the systemic aorta that develops from left ventri­cle.
• The sixth arch becomes the pulmonary aorta that divides into two pulmonary arteries and each artery goes to each lung.

Mammals:-

• The Ist, llnd and Vth aortic arches dis­appear.
• The lllrd, IVth and Vlth aortic arches persist of which the right aortic arch of IVth disappears.
• Only aortic arch on the left side persists.
• The IVth becomes the systemic aorta and Vlth arch becomes pulmo­nary aorta.
• The Ist and llnd aortic arches dis­appear but the basal stem becomes the external carotid and lllrd aortic arches become the internal carotid.
• The dorsal connection between the lllrd and IVth aortic arches disappears.
• The right side of IVth aortic arch becomes right subclavian artery.
• The left sub­clavian artery arises from the upper part of the left systemic arch.
• Right and left common carotid arteries with the right subclavian develop from a common aortic arch, called brachiocephalic artery.
• In the embryonic stages ductus arteriosus is seen on sides but ductus arteriosus in the left side remains after birth as fibrous band, called ligamentum arteriosum.

Blood circulation in various vertebrate groups:-

• All vertebrates have a muscular heart. Fishes possess a 2-chambered heart with an apterium and a ventricle.
• Lung fishes and amphibians have a 3-chambered heart with 2 atria and one ventricle.
• Reptiles except crocodiles have 3-chambered heart with 2 atria and partially divided single ventricle whereas crocodiles, birds and mammals have a 4-chambered heart with 2 atria and 2 ventricles

• In fishes the heart pumps out deoxygenated blood which is oxygenated by the gills and sent to the body parts from where deoxygenated blood is carried to the heart.
• It is called single circulation.
• In lung fishes, amphibians and reptiles, the left atrium gets oxygen­ated blood from the ills/lungs/skin/buccopharyngeal cavity and the right atrium receives the deoxygenated blood from other body parts.
• But both oxygenated and deoxygenated blood gets mixed up in single ventricle which pumps out mixed blood.

Single and double circulation systems:- 

• The circulatory system is a broad term that encompasses the cardiovascular and lymphatic systems.
• The lymphatic system will be discussed later in this chapter.
• The cardiovascular system consists of the heart (cardio) and the vessels required for transport of blood (vascular).
• The vascular system consists of arteries, veins and capillaries.
• Vertebrates (animals with backbones like fish, birds, reptiles, etc.), including most mammals, have closed cardiovascular systems.
• The two main circulation pathways in invertebrates are the single and double circulation pathways.

• Single circulation system as found in a typical fish species.
• The red represents oxygen-rich or oxygenated blood, the blue represents oxygen-deficient or deoxygenated blood.

Single circulatory pathways:-

• Single circulatory pathways as shown in the diagram below consist of a double chambered heart with an atrium and ventricle.
• Fish possess single circulation pathways.
• The heart pumps deoxygenated blood to the gills where it gets oxygenated.
• Oxygenated blood is then supplied to the entire fish body, with deoxygenated blood returned to the heart.

Double circulatory systems:-

• Double circulation pathways are found in birds and mammals
• Animals with this type of circulatory system have a four-chambered heart.
• The right atrium receives deoxygenated from the body and the right ventricle sends it to the lungs to be oxygenated.
• The left atrium receives oxygenated blood from the lungs and the left ventricle sends it to the rest of the body.
• Most mammals, including humans, have this type of circulatory system.
• These circulatory systems are called 'double' circulatory systems because they are made up of two circuits, referred to as the pulmonary and systemic circulatory systems.

Human circulatory systems:- 

• The human circulatory system involves the pulmonary and systemic circulatory systems.
• The pulmonary circulatory system consists of blood vessels that transport deoxygenated blood from the heart to the lungs and return oxygenated blood from the lungs to the heart.
• In the systemic circulatory system, blood vessels transport oxygenated blood from the heart to various organs in the body and return deoxygenated blood to the heart.

Pulmonary circulation system:-

• In the pulmonary circulation system, deoxygenated blood leaves the heart through the right ventricle and is transported to the lungs via the pulmonary artery.
• The pulmonary artery is the only artery that carries deoxygenated blood.
• It carries blood to the capillaries where carbon dioxide diffuses out of the blood into the alveoli (lung cells) and then into the lungs, where it is exhaled.
• At the same time, oxygen diffuses into the alveoli, and then enters the blood and is returned to the left atrium of the heart via the pulmonary vein.

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Systemic circulation:-

Systemic circulation:-

• Systemic circulation refers to the part of the circulation system that leaves the heart, carrying oxygenated blood to the body's cells, and returning deoxygenated blood to the heart.
• Blood leaves through the left ventricle into the aorta, the body's largest artery.
• The aorta leads to smaller arteries that supply all organs of the body.
• These arteries finally branch into capillaries.
• In the capillaries, oxygen diffuses from the blood into the cells, and waste and carbon dioxide diffuse out of cells and into blood.
• Deoxygenated blood in capillaries then moves into venules that merge into veins, and the blood is transported back to the heart.
• These veins merge into two major veins, namely the superior vena cava and the inferior vena cava.
• The movement of blood is indicated by arrows on the diagram.
• The deoxygenated blood enters the right atrium via the superior vena cava.
• Major arteries supply blood to the brain, small intestine, liver and kidneys.
• However, systemic circulation also reaches the other organs, including the muscles and skin.

Urinogenital System:-

• The urogenital system includes both the reproductive organs and the excretory organs.
• They are considered together because they share some common ducts.
• We will begin by exploring the excretory system, which is comprised of the kidneys, ureters, urinary bladder, and urethra.
• The kidneys function to eliminate nitrogenous wastes produced during the breakdown of proteins, regulate water balance, pH and the ionic composition of the body fluids.
• These bean-shaped organs are located in the abdominal region adjacent to the dorsal body wall.

Developmental Succession of kidney:-

Basic Structure Kidneys:-

Developmental Succession of kidney:-

Basic Structure Kidneys:-

• In vertebrates, there is a pair of compact kidneys, lying dorsal to the coelom in trunk region, one on either side of vertebral column.
• A kidney is made of a large number of uriniferous tubules or nephrons.
• Their number, complexity and arrangement is different in different groups of vertebrates.
• The uriniferous tubules arise in an embryo from a special part of the mesoderm, called mesomere or nephrotomy extending on each side along the entire trunk in between upper segmental mesoderm and lower lateral plate mesoderm.

• Primitively the uriniferous tubules develop from the nephrotomy in a sequence commencing from the anterior end.
• They are segmental in arrangement with one pair of uriniferous tubules for each trunk segment.

A uriniferous tubule has the following parts:-

(i) A ciliated peritoneal funnel near the proximal end of the tubule which opens into the splanchnic by a nephrostomy.

(ii) A convoluted ciliated tubule opening into a longitudinal collecting duct, and a Malpighian body or renal corpuscle.

• The Malpighian body has a double-walled Bowman’s capsule, enclosing a network of intraarterial blood capillaries, called glomerulus where filtration of blood takes place.
• Bowman’s capsule and glomerulus together form the Malpighian body or renal corpuscle.
• An afferent arteriole brings blood into the glomerulus and an efferent arteriole takes blood away from it.
• Then the efferent arteriole breaks up into capillaries along the entire course of the uriniferous tubule and finally the blood goes to a renal vein.
• Encapsulated glomerulus is internal glomerulus which is common.
• The glomerulus without a capsule suspended freely in the coelomic cavity is called external glomerulus which is found in embryos and larvae.
• Malpighian bodies with glomeruli are lacking in some fishes, embryos and larvae and their kidneys are called a glomerular.
• In an adult the uriniferous tubules are elongated and coiled, so that their segmental arrangement is lost and they become enclosed in a connective tissue capsule to form a kidney.

Archespores:-

Archespores:-

• The ancestral vertebrates had a pair of kidneys running through the entire length of the coelom.
• Each had segmentally arranged tubules; one pair per body segment.
• Each tubule opened separately into the coelom by a peritoneal funnel and nephrostomy.
• Near each funnel was an external glomerulus (without capsule) suspended in coelom.
• All the tubules of each kidney opened into a common longitudinal duct which joined the cloaca.
• This kidney is called an archespores and its duct is an arch nephric duct.
• Among the living vertebrates an archinephros is found only in the larval Myxine and some apo dan amphibians.
• In present-day vertebrates the uriniferous tubules develop antero-posteriorly in two or three stages in succession, these stages are pronephros, mesonephros and metanephros.
• These stages have evolved from the original archinephros.
• Thus, in present vertebrates, kidneys are of three types: 1. Pronephros, 2. Mesonephros and 3. Metanephros.
• All three are fundamentally alike, differing principally in their relationship to the blood system, in degree of complexity, and in efficiency.

1. Pronephros:-

1. Pronephros:-
1. Pronephros is the most primitive and while present in the embryonic development of all vertebrates, is functional in the adult of none.
2. Some larval cyclostomes, however, have a kidney, part of which appears to be homologous to the embryonic pronephros of higher vertebrates.
3. Pronephros develops in the anterior-most part of the nephrotomy.
4. There are only 3 to 15 uriniferous tubules in each, one pair to each segment.
5. Near each tubule is a glomerulus projecting into the coelom, which is not connected with the tubule is the external glomerulus.
6. Each tubule opens into coelom by a funnel or nephrostomy.

• The uriniferous tubules of each pronephros open into a common pronephric duct which grows back to enter the embryonic cloaca.
• In some, there is a large pronephric chamber which surrounds the glomus and tubules.
• All glomeruli project into the pronephric chamber where they may unite to form a single compound glomerulus called glomus.
• Pronephric chamber is derived from pericardial or pleuroperitoneal cavity.
• All the tubules of a pronephros open into a common pronephric duct opening posteriorly into the embryonic cloaca.
• A pair of pronephroi appear in all vertebrate embryos but they become functional kidneys only in some cyclostomes and embryos of all anamniotes.
• In others they degenerate during development but the pronephric ducts persist.
• Pronephros is replaced by mesonephros.
• In those vertebrates in which pronephroi become adult kidneys, they are called head kidneys due to its anterior position behind the head.

2. Mesonephros:-

2. Mesonephros:-

• Mesonephros develops from the middle part of the nephrotomy behind the pronephros which degenerates.
• At first it consists of paired segmental uriniferous or mesonephric tubules, each with a peritoneal funnel opening into the coelom and internal glomerulus enclosed in a Bowman’s capsule both are collectively called renal corpuscle.
• These mesonephric uriniferous tubules join the existing pronephric duct or arch nephric duct on each side, which with the disappearance of pronephros is called a mesonephric duct or Wolffian duct.

• Later the mesonephric tubules undergo budding to form hundreds of tubules, so that their segmental arrangement is lost.
• The mesonephric tubules are coiled or convoluted both proximally and distally and lead into a common longitudinal collecting duct the arch nephric duct.
• This in turn leads to the outside, usually by way of the cloaca.
• Water, salts and waste products from the blood stream pass into the capsule and then may flow through the tubule to the mesonephric duct and ultimately out of the body.
• In sharks it functions as a gonadial duct and the kidneys have developed new accessory urinary ducts.
• The mesonephric tubules have no peritoneal funnels.

• Mesonephro form the adult functional kidneys in some cyclostomes, fishes, amphibians and the embryos of amniotes in which they degenerate in the adult.
• The mesonephro of amniote embryos lack peritoneal funnels, except in monotremes.
• The mesonephros of anamniotes is not exactly equivalent to that amniote embryos. In anamniotes the mesonephros extends throughout the length of the coelom behind the pronephros and is formed from the entire nephrotomy behind the pronephros and is functional both in embryos as well as in adults.
• While in adults anurans, urodeles and amniote embryos the mesonephros is formed only from the middle part of the nephrotomy and it does not extend throughout the length of the coelom.
• In sharks and caecilians, the kidney is opisthonephros, i.e., mesonephric tubules extend posteriorly throughout the length of the coelom.
• The amphibian kidneys, like those of fishes, are of the opisthonephric type.
• In tailed amphibians the kidneys are rather elongate structures as in elasmobranchs, but in anurans there is a tendency for these structures to be short and compact Renal corpuscles are large to assist in the elimination of water and, thus, prevent excessive dilution of the body fluids.
• In some amphibians the arch nephric duct is both genital and excretory in nature in the male, whereas in others the arch nephric duct serves only for the transport of sperms, and the kidney is drained by a new duct, somewhat comparable to the ureter of higher vertebrates.

• The kidney of marine fishes, play a major part in maintaining the proper balance with in the body.
• Salt water tends to dehydrate the body, which is only slightly saline, and it also increases the salt concentration in the body.
• Freshwater has the opposite effect.
• Some marine bony fishes have salt-excreting glands on the gills which help to eliminate excessive salt.
• Most marine fishes, however, other than elasmobranchs, have the renal corpuscles very small so as to reduce water loss.
• Since the corpuscle is the filter, the smaller it is the less filtrate passes through.
• The renal corpuscles are much larger in freshwater fishes than in marine species.
• This means more liquid output, which is necessary to prevent overdilation of the body fluids.
• Elasmobranchs, unlike most marine fishes, have large renal corpuscles.

3. Metanephros:-

• Metanephros is the functional kidney, develops only in amniotes.
• It is formed from the posterior most part of the nephrotomy behind the embryonic mesonephros which is displaced somewhat anterior and lateral.
• During embryonic life, however, both pronephros and mesonephros make their appearance.

• The metanephros fundamentally resembles the mesonephros, but arises more posteriorly in the body, is more compact and contains a few greater number of renal units.
• Furthermore, the renal tubules, instead of draining into the arch nephric duct open into larger collecting tubules which ultimately lead to a new excretory duct called the ureter.
• Metanephric has a double origin, a tubular outgrowth arises from the base of mesonephric duct near the cloaca and it grows anteriorly and dorsally and eventually the metanephric tubules open into it.
• Its distal end dilates to form the pelvis which divides several times to form collecting tubules, while its proximal part becomes the ureter or metanephric duct.
• The nephrotomy gives rise to metanephric uriniferous tubules of which there are thousands with no segmental arrangement.
• The metanephric tubules become long and much coiled and have glomeruli enclosed in Bowman’s capsules but they lack peritoneal funnels so that all connection with the coelom is lost.
• Metanephros are the functional kidneys of adult amniotes and they have achieved the separation of the urinary function from the genital function which appears to be the trend in the evolution of the urinogenital system.
• In metanephros, the metanephric tubules are much convoluted, and a thin U-shaped loop of Henle is formed in between proximal and distal convolutions of the tubule.

• It is absent in reptiles and rudimentary in birds.
• Kidney is differentiated into outer cortex having renal corpuscles, and inner medulla possessing collecting tubules and loops of Henle.
• These two aggregated into one or more pyramids projecting into pelvis.
• In reptiles, the renal portal system has begun to lose its importance and some of the blood from the caudal region goes directly through the kidneys instead of filtering slowly through the capillary network.
• In birds, renal portals do not break up into capillaries and are, therefore, not comparable to the renal portals of lower vertebrates.
• Mammals also lack renal portal system.
• In excretion by vertebrate kidneys three processes are involved- a filtration of blood in the glomerulus, secretion of certain waste substances by the cells of uriniferous tubules into the lumen of the tubules, and a selective reabsorption by uriniferous tubules of useful substances from the glomerular filtrate.
• In aquatic vertebrates the kidneys are concerned chiefly with elimination of excess water absorbed in the body, and they excrete ammonia well diluted with water, if ammonia is not diluted, it is highly toxic, whereas in terrestrial vertebrates one of the chief functions of the kidneys is to conserve water for maintaining the water balance of the body, and they excrete urea or uric acid.

• The functioning of kidneys is not under the nervous system but is under the control of several hormones.

Urinary Bladder:-

Urinary Bladder:-

• In most vertebrates there is a bag-like urinary bladder serving as a reservoir for urine.
• In some fishes the bladder is formed by an enlargement of the terminal parts of mesonephric ducts and is used for temporary storage of urine.
• In amphibians the urinary bladder is derived as a diverticulum from the ventral wall of the cloaca, and not from the protoderm, it is lined with endoderm and is called a cloacal bladder.
• Urine, thus, first pass from the ducts of the kidneys into the cloacal chamber. From here it is then forced into the bladder for storage.
• Many reptiles also possess urinary bladder, like that of amphibians, is an outgrowth from the ventral wall of cloaca.
• It is lacking in crocodilians, snakes and some lizards.
• The only bird known to possess a bladder is the ostrich.
• In embryos of amniotes a large bag-like allantois arises from the hindgut.
• It serves as an excretory and respiratory organ, in hatching or at birth the allantois is lost but its basal part persists and along with a portion of the cloacal wall it becomes the adult urinary bladder which is endodermal, and is called an allantois bladder.

• Generally the kidney ducts (ureters) do not open into the urinary bladder but open dorsally into the cloaca, except in mammals.
• In mammals, except monotremes, the ureters open directly into the bladder that opens outside through a short, narrow tube the urethra.
• Cloaca is not found in mammals except monotremes.
• Phylogenetic Trends The most primitive vertebrate kidney was the holonephric kidney and was believed to have consisted of a series of nephrotomies arranged segmentally along the entire length of the trunk.
• Each gave rise to a tubule, which all joined together to form the arch nephric (wolffian) duct on each side, which drained the kidney.
• The nephrotomy in each segment also opened directly into the coelom via a peritoneal funnel.
• This is still found in hagfish larvae. In adult hagfish, the most anterior tubules (the pronephros) remain while the more posterior tubules have regressed.
• This is a pronephric kidney.
• The more cranial portion may still open into the coelom via peritoneal funnels but the more posterior portions drain only by the arch nephric duct.
• In adult lamprey, the most anterior tubules (the pronephros) have disappeared.

• The remaining more posterior portion is the opisthonephric kidney.
• It does not open into the coelom at all but drains only by the arch nephric duct.
• This kidney may have more than one tubule per segment. In adult female sharks, the arrangement is just as we have described it in the adult lamprey.
• In adult male sharks, the more posterior portion of the kidney losses its segmentation and the number of tubules increases dramatically.
• These tubules now drain through a new structure, the accessory arch nephric duct (urinary duct).
• Thus another of the phylogenetic trends that we see is an increase in the number of nephrons in each body segment.
• As the number increases, it disrupts the primitive segmentation of the kidney.
• In some fish, a pronephric kidney persists.
• It is usually replaced during development with a mesonephric kidney.
• Most often, more tubules are added posterior to this to form an opisthonephric kidney.
• This is also true of most amphibians.
• In all amniotes, during late development, a metanephric kidney develops, drained by ureters. The tubules in each segment arising from each glomerulus tend to be long with three distinct segments.

• These are the proximal, intermediate and distal tubules.
• In many mammals particularly, the intermediate tubule can become very elongate and form a long loop called the loop of Henle.
• The longer the loops, the greater the ability to concentrate urine and conserve water.
• The best mammalian kidneys can concentrate urine up to 25X plasma levels.
• In some birds we also see convergent evolution in the form of kidneys with short loops analogous to the loops of Henle in mammals.
• These loops give these species limited ability to concentrate urine.

UNIT–IV

Nervous System:-

Nervous System:-

Introduction:-

• The nervous system is the major controlling, regulatory, and communicating system in the body.
• It is the center of all mental activity including thought, learning, and memory.
• Together with the endocrine system, the nervous system is responsible for regulating and maintaining homeostasis.
• Through its receptors, the nervous system keeps us in touch with our environment, both external and internal.
• Like other systems in the body, the nervous system is composed of organs, principally the brain, spinal cord, nerves, and ganglia.
• These, in turn, consist of various tissues, including nerve, blood, and connective tissue. Together these carry out the complex activities of the nervous system.
• The various activities of the nervous system can be grouped together as three general, overlapping functions:-
• Sensory
• Integrative
• Motor

• Millions of sensory receptors detect changes, called stimuli, which occur inside and outside the body.
• They monitor such things as temperature, light, and sound from the external environment.
• Inside the body, the internal environment, receptors detect variations in pressure, pH, carbon dioxide concentration, and the levels of various electrolytes.
• All of this gathered information is called sensory input.
• Sensory input is converted into electrical signals called nerve impulses that are transmitted to the brain.
• There the signals are brought together to create sensations, to produce thoughts, or to add to memory; Decisions are made each moment based on the sensory input.
• This is integration.
• Based on the sensory input and integration, the nervous system responds by sending signals to muscles, causing them to contract, or to glands, causing them to produce secretions.
• Muscles and glands are called effectors because they cause an effect in response to directions from the nervous system.
• This is the motor output or motor function.

Structure of Neuron:-

Neurons have four specialized structures that allow for the sending and receiving of information:- the cell body (soma), dendrites, axon and axon terminals.

1. Cell body or soma:-

• The cell body is the portion of the cell that surrounds the nucleus and plays a major role in synthesizing proteins.

2. Dendrites:-

• Dendrites are short, branched processes that extend from the cell body.
• Dendrites function to receive information, and do so through numerous receptors located in their membranes that bind to chemicals, called neurotransmitters.

3. Axon:-

• An axon is a large process that extends from the cell body at a point of origin-called the axon hillock-and functions to send information.
• In contrast to the shorter dendrites, the axon can extend for more than a meter.
• Because of this length, the axon contains microtubules and is surrounded by myelin.
• Microtubules are arranged inside the axon as parallel arrays of long strands that act as highways for the movement of materials to and from the soma.
• Specialized motor proteins "walk" along the microtubules, carrying material away from the soma (anterograde transport) or back to the soma (retrograde transport).

• This system can move materials down the axon at rates of 400mm/day (see lowest figure).
• Myelin consists of totally separate cells that coil and wrap their membranes around the outside of the axon.
• These are essential for electrical insulation and to speed up action potential propagation.

4. Axon terminals:-

• Once an axon reaches a target, it terminates into multiple endings, called axon terminals.
• The axon terminal is designed to convert the electrical signal into a chemical signal in a process called synaptic transmission (further explained in the section "Physiology of the Neuron").
• Most neurons are amitotic or lose their ability to divide.
• Exceptions to this rule are found in olfactory neurons (those associated with smell) and hippocampal regions of the brain.
• Fortunately, lifespans of amitotic neurons is near 100 years.
• Still, if a neuron is damaged or lost, it is not easily replaced.
• For this reason, there is usually limited recovery from serious brain or spinal cord injuries.
• Perhaps the slow recovery rate or lack of regeneration is to ensure that learned behavior and memories are preserved throughout life.
• Neurons also have exceptionally high metabolic rates and subsequently require high levels of glucose and oxygen.

• The body will go to great lengths to ensure that neurons are adequately fed; in fact, if for some reason the brain detects that it is not receiving adequate amounts of nutrition, the body will shut down immediately.

Comparative account of Brain of Vertebrates:-

Comparative account of Brain of Vertebrates:-

• The brain is an organ that serves as the Centre of Nervous System in all Vertebrate animals.
• It is located in the head, usually close to the sensory organs for senses such as vision.
• All vertebrate brains share a common underlying form which appears most clearly during early stages of development.
• In its earliest form the brain appears as three swellings at the front end of the neural tube; these swellings eventually become the forebrain, midbrain and hindbrain (prosencephalon, mesencephalon and rhombencephalon resp).
• In the initial stages of brain development, the three areas are roughly equal in size.
• In many classes of vertebrates, such as Fish and Amphibians, the three parts remain similar in size in the adult, but in Mammals the forebrain becomes much larger than the other parts, and the midbrain becomes very small.

Functions of Brain:-

1. Olfactory lobes:- Sense of smell.
2. Cerebral hemispheres:- Seat of intelligence and memory.
3. Diencephalon:- Controls the general metabolic functions of the body.
4. Optic lobes:- Sense of vision.
5. Cerebellum:- Co-ordinates the movements of voluntary muscles.
6. Medulla oblongata:- Controls the involuntary functions of the body.

Brain of all vertebrates, from Fish to Man, is built in series in different vertebrates in accordance with the habits and behavior of the animals.

Cyclostomes:-

• The brain exhibits the typical pattern that is found throughout the Vertebrate series.
• Some of the structures that are well developed in the higher forms occur in a relatively primitive condition.
• The brain is divisible into three primary parts viz; Forebrain, Midbrain and Hindbrain.
• The forebrain includes large paired olfactory lobes.
• The Cerebral hemispheres are small attached to the diencephalon.
• The midbrain possesses a pair of large optic lobes rather dorsally placed.
• The hind brain is differentiated into a small transverse dorsal band, the Cerebellum and much larger ventral Medulla Oblongata.
• The ventricles within the brain are four as in other vertebrates.
• In fundibulum bears a hypophysis and pituitary body.

Fishes:-

• Brain of Fishes is more advanced than Cyclostomes.
• However, subdivisions of brain are seen in their primitive relations.

Elasmobranchs (Cartilaginous Fishes):-

• In Cartilaginous Fishes, the olfactory organs are enormous so that olfactory lobes of brain are correspondingly large, attached to Cerebrum by short but stout olfactory tracts.
• Optic lobes and pallium are relatively moderate in size.
• Midbrain cavity is quite large and extends into optic lobes.
• Pineal apparatus is well developed.
• Features of Hindbrain are less pronounced.
• Cerebellum is especially large due to active swimming habit.

Osteichthyes (Bony Fishes):-

• In bony Fishes brain is more specialized than in Elasmobranchs.
• Olfactory lobes, Cerebral hemispheres and Diencephalon are smaller while optic lobes and cerebellum larger than in a shark.
• The anterior part of the Cerebellum forms valvula cerebella which extend under the optic lobes; it is characteristic of bony Fishes and controls active movements.
• The medulla oblongata is well developed with special lobes for entry of lateral line nerves.

Amphibians:-

• The brain of Amphibians is remarkably unspecialized and is scarcely more advanced than that of cartilaginous fishes and lung fishes The cerebral hemispheres are more separate from one another than in fishes, so they share little common ventricle.

• Smaller olfactory lobes and larger optic lobes indicate a greater reliance on sight rather than smell.
• Corpus striatum (floor of cerebrum) receives greater number of sensory fibers projected forward from thalamus than in fishes.
• The walls of midbrain are thickened and reduce the lumen into a narrow passage called aqueduct.
• Medulla is small and cerebellum is poorly developed.
• A small pineal body is present in all the modern Amphibians.

Reptilians:-

• Reptilian brain shows advancement in size and proportions over that of Amphibians because of complete terrestrial mode of life.
• The brain is a narrow elongate, and nearly straight.
• Olfactory bulbs tend to be smaller than for fishes.
• Olfactory tracts are long.
• A fine vomeronasal nerve from the organ of Jacobson goes to the olfactory bulbs.
• A pair of auditory lobes is found posterior to optic lobes which are not hollow.
• The Cerebrum is large because of the expansion of the corpus striatum and associated neocortex.
• Cerebellum is somewhat pear shaped and larger than in Amphibians.

Birds:-

• Avian brain is proportionately larger than that of a Reptile, and is short and broad.
• Olfactory lobes are small due to poor sense of smell.
• Two cerebral hemispheres are larger, smooth and project posterior over the diencephalon to meet the cerebellum.
• The enlargement of cerebral hemispheres is due to very large and complex corpora striata which are characteristic of birds.
• The cerebral hemispheres are responsible for an intelligent behavior in birds, and they control the reflex behavior governing the lives of birds.
• The dorsal thalamus is even more developed than in Reptiles.
• Optic nerves, chiasma and tracts are large.
• Optic lobes are particularly large and are layered within.
• They have connections from all sense organs and with the cerebrum.
• Squeezed between the cerebrum and cerebellum, the optic lobes have uniquely lateral position.
• The Cerebellum is larger than in other vertebrates except some mammals.
• It is highly convoluted, and the organ is high and narrow.
• Related to the marked development of the cerebellum are the appearance of the pons under the brainstem and enlargement of the olivary nuclei within the broad medulla.

Mammals:-

• The brain reaches its highest development in mammals with better integration and mastery over the environment, the cerebral hemispheres reaching the status of a dominant integrating part of the brain and acting as coordinating centers of the brain.
• The cerebral hemispheres are smaller and smooth in Prototheria and larger in Metatheria and become greatly enlarged and divided into lobes in Eutheria.
• In mammals such as man and sheep, surface of cerebral hemispheres is immensely convoluted with a number of elevations separated by furrows.
• This folding increases the surface area or gray matter containing nerve cells, resulting in greater intelligence without adding to the size of the brain.
• Olfactory lobes are relatively small but clearly defined and covered by the hemispheres.
• Diencephalon and midbrain are also completely covered by the cerebral hemispheres.
• Characteristic of mammals are 4 solid optic lobes, called corpora quadrigemina, on the roof of the midbrain.
• The third ventricle of midbrain is a laterally compressed vertical passage, called cerebral aqueduct.
• Cerebellum is also large, conspicuously folded and may overlie both midbrain and medulla. Usual folds are a median vermis, two lateral floccule and their mushroom like projections, the parafollicular.

• The other chief topographical features of mammalian hindbrain include the pyramids carrying voluntary motor impulses from higher centers, the pons variola with crossing fibers connecting opposite sides of cerebrum and cerebellum, and the trapezoid body of transverse fibers relaying impulses for sound.
• The medulla oblongata lies ventrally and is much thickened.
• It has centers which control respiration, heart beat and blood vessels; it also has conduction pathways for impulses passing from the cerebral hemispheres to the spinal cord and again in the opposite direction.
• The hindbrain contains centers for the regulation of digestion, respiration and circulation.

Sense Organs: -

• The sense organs are the body organs by which humans are able to see, smell, hear, taste, and touch or feel.
• The five sense organs are the eyes (for seeing), nose (for smelling), ears (for hearing), tongue (for tasting), and skin (for touching ).
• Key Facts & Information:-

The Five Senses:-

• Senses are important because they allow us to perceive the world in which we live.
• Our five senses are our sense of sight (also known as vision), smell (olfaction), hearing (audition), taste (gustation), and touch (somatosensorial).
• A sense is our ability to detect stimuli which are then interpreted and responded to accordingly.
• Humans are not the only creatures with sensory capacity – animals have senses too.
• The degrees of sensory capability vary among species.
• Some animals have a weaker sense of smell than others.

• Some have a sharper sense of sight, etc.
• Sensory organs are organs of the body that access those sensory capabilities and help us become conscious and respond to our surroundings.
• There are two types of receptors depending on the sensory organ:-
• general receptors and special receptors.
• General receptors are present in the skin and muscles.
• Special receptors are in the form of photoreceptors (in the eyes), chemoreceptors (in the mouth and nose), and mechanoreceptors (in the ears).

Eyes:-

• Sight, also referred to as vision, is our ability to see.
• Eyes are the visual sensory organs of the human body.
• Other animals, birds, and fish also see through their eyes.
• Human eyes vary in color depending on the amount of melanin in the body.
• Eye colors can be brown, blue, gray, green, and even combinations.
• Our eyes are sensitive to images of light.
• Seeing occurs when eyes detect and focus on these images.
• The scientific study of sight is called optics.
• Photoreceptors present in the eye’s retina are what translates light into images.

• Rods and cones are the two types of photoreceptors.
• Rods are more sensitive to light and motion while cones are more sensitive to colors and details.
• The optic nerve is what carries the impulses to the brain.
• Blindness is the inability to see.
• Blindness can be temporary or permanent.
• Causes of blindness include, but are not limited to, injury to the eyeball, damage to the optic nerve, and trauma to the brain.

Ears:-

• Hearing, also referred to as auditory perception or audition, is our ability to perceive sounds.
• We have our auditory system by which we detect vibrations and hear sounds.
• Our ears are auditory organs.
• Vibrations are transmitted through a medium such as air.
• These vibrations are mechanically carried on from the eardrum through the tiny bones named the malleus, incus, and stapes.
• Mechanoreceptors in the inner ear turn vibrations into electrical nerve pulses.
• An impulse is then sent through the cochlea to the eighth cranial nerve then to the brain.
• Humans may experience hearing loss when the ability to hear is lost partially or completely.
• Deafness is the inability to hear.

Nose:-

• The sense of smell is also referred to as olfaction.
• We have our olfactory system by which we smell and perceive different odors and scents.
• The nose is an olfactory organ.
• The nose can also be an organ to aid in our sense of taste.
• Humans breathe through two holes called nostrils.
• There are hundreds of olfactory receptors that interpret the smells around us.
• When you smell a substance, the chemicals present bind to the cilia in your nasal cavity.
• Afterwards, it produces a nerve impulse which is transported through the olfactory cell, then to the olfactory nerve fiber, then to the olfactory bulb and ultimately, to the brain.
• Olfactory receptor neurons in the nose have the unique ability to regularly die and regenerate.
• Olfactory neurons in the nose can also detect pheromones, which is a chemical substance released by humans which could affect how they relate with each other.
• Animals generally have a sharper sense of smell than humans.
• Anosmia is the inability to smell.

Tongue:-

• We have one tongue by which we perceive various tastes and flavors like sweet, salty, sour, and bitter.
• The small bumps on the tongue are the papillae.
• In between the papillae are the taste buds.
• Taste buds, also called gustatory caliculi, are the sensory organs on the tongue’s upper surface.
• The different parts of the tongue detect distinct flavors: front for salty and sweet, back for bitter, and sides for sour.
• The fifth basic taste is called umami.
• Taste, also referred to as gustation, is the sense we use to detect the taste of food and other substances.
• Ageusia is the inability to taste.

Skin:-

• Our skin is the largest organ as it covers our whole body.
• The receptors on our skin allow us to perceive texture, pain, temperature, pressure, and pain.
• Touch is also referred to as tactician, somatosensorial, or mechanoreception.
• The sense of touch is activated by neural receptors found in the skin, and other surfaces like the tongue and hair follicles.
• Skin receptors generate an impulse which is carried to the spinal cord then to the brain.
• Pressure receptors in the skin are sensitive to changes in pressure.
• Itch-specific neurons in the skin give us the touch sense of itching.
• Tactile anesthesia is the inability to feel anything physical.

Types of receptors:-

1. Mechanoreceptors:-

• Primary mechanoreceptors:- Four of the primary mechanoreceptors in human skin are shown.
• Merkel’s disks, which are unencapsulated, respond to light touch.
• Meissner’s corpuscles, Ruffini endings, Pacinian corpuscles, and Krause end bulbs are all encapsulated.
• Meissner’s corpuscles respond to touch and low-frequency vibration.
• Ruffini endings detect stretch, deformation within joints, and warmth.
• Pacinian corpuscles detect transient pressure and high-frequency vibration.
• Krause end bulbs detect cold.

• There are three classes of mechanoreceptors:- Tactile, proprioceptors, and baroreceptors.
• Mechanoreceptors sense stimuli due to physical deformation of their plasma membranes.
• They contain mechanically-gated ion channels whose gates open or close in response to pressure, touch, stretching, and sound.
• There are four primary tactile mechanoreceptors in human skin:- Merkel’s disks, Meissner’s corpuscles, Ruffini endings, and Pacinian corpuscle; two are located toward the surface of the skin and two are located deeper.
• A fifth type of mechanoreceptor, Krause end bulbs, are found only in specialized regions.

• Merkel’s disks are found in the upper layers of skin near the base of the epidermis, both in skin that has hair and on glabrous skin; that is, the hairless skin found on the palms and fingers, the soles of the feet, and the lips of humans and other primates.
• Merkel’s disks are densely distributed in the fingertips and lips.
• They are slow-adapting, unencapsulated nerve endings, which respond to light touch.
• Light touch, also known as discriminative touch, is a light pressure that allows the location of a stimulus to be pinpointed.
• The receptive fields of Merkel’s disks are small, with well-defined borders.
• That makes them very sensitive to edges; they come into use in tasks such as typing on a keyboard.

• Meissner’s corpuscles, also known as tactile corpuscles, are found in the upper dermis, but they project into the epidermis.
• They are found primarily in the glabrous skin on the fingertips and eyelids.
• They respond to fine touch and pressure, but they also respond to low-frequency vibration or flutter.
• They are rapidly- adapting, fluid-filled, encapsulated neurons with small, well-defined borders which are responsive to fine details.
• Merkel’s disks and Meissner’s corpuscles are not as plentiful in the palms as they are in the fingertips.

• Deeper in the dermis, near the base, are Ruffini endings, which are also known as bulbous corpuscles.
• They are found in both glabrous and hairy skin.
• These are slow-adapting, encapsulated mechanoreceptors that detect skin stretch and deformations within joints; they provide valuable feedback for gripping objects and controlling finger position and movement.
• Thus, they also contribute to proprioception and kinesthesia.
• Ruffini endings also detect warmth.
• Note that these warmth detectors are situated deeper in the skin than are the cold detectors.
• It is not surprising, then, that humans detect cold stimuli before they detect warm stimuli.

• Pacinian corpuscles, located deep in the dermis of both glabrous and hairy skin, are structurally similar to Meissner’s corpuscles.
• They are found in the bone periosteum, joint capsules, pancreas and other viscera, breast, and genitals.
• They are rapidly-adapting mechanoreceptors that sense deep, transient pressure, and high-frequency vibration.
• Pacinian receptors detect pressure and vibration by being compressed which stimulates their internal dendrites.
• There are fewer Pacinian corpuscles and Ruffini endings in skin than there are Merkel’s disks and Meissner’s corpuscles.

Function of Mechanoreceptors:-

• A person understands that they have had a sensation once the organ responsible for detecting that particular sense sends a message to the brain, which is the primary organ that processes and organizes all of the information.
• Messages are sent from all corners of the body to the brain by wires called neurons.
• There are thousands of tiny neurons that branch out to all parts of the body, and on the ends of many of these neurons are mechanoreceptors.
• To demonstrate what happens when you touch an object, we will use an example.
• Imagine a mosquito lands on your arm.
• The pressure of the insect, ever so light, stimulates mechanoreceptors in that particular area of your arm.
• Those mechanoreceptors send a message along the neuron they are connected to.

• The neuron connects all the way to the brain, which receives the message that something is touching the body at the precise location of the specific mechanoreceptor that sent the message.
• The brain will act on this information.
• Perhaps it will tell the eyes to look at the part of the arm that detected the touch.
• And once the eyes tell the brain that there's a mosquito on the arm, the brain might tell the hand to quickly flick it away!

Types of Cutaneous Mechanoreceptors:-

Cutaneous:- 

• Mechanoreceptors are located in the skin.
• There are many different types of tactile sensations, including touch, pressure, vibration, and temperature.
• There are different types of mechanoreceptors that are better suited for each of these, and the brain knows what the body is feeling based on which category of mechanoreceptor sends the message.

Pacinian Corpuscles:-

• Otherwise known as lamellar corpuscles, detect sudden changes in vibration or pressure.
• If the surface you are walking on changes from carpet to hardwood, or an object moving nearby causes a vibration, it is the Pacinian corpuscle that sends the message to the brain.

Meissner's Corpuscles:- 

• Are similar to Pacinian corpuscles in that they also detect changes in touch, but Meissner's corpuscles are located in a more shallow position beneath the skin and are therefore specialized to detect very light touches.
• They are ideal for detecting texture and patterns on objects that touch the skin.

Ruffini Nerve Endings:- 

• Let you know when your skin is being stretched, and information about deeper, lasting pressure.
• They are especially dense on the fingertips where they are thought to contribute to our ability to grip objects.
• If you are holding a coffee mug and it begins to slowly slip, Ruffini nerve endings will detect the pressure and stretch in your skin and communicate the situation to your brain.

Merkel Nerve Endings:- 

• Also respond to touch, but are fine-tuned spatially to let you know where precisely an object is in contact with your body.

2. Photoreceptors:-

Structure And Function Of Photoreceptors:-

• Photoreceptors are the cells in the retina that respond to light.
• Their distinguishing feature is the presence of large amounts of tightly packed membrane that contains the photopigment rhodopsin or a related molecule.
• The tight packing is needed to achieve a high photopigment density, which allows a large proportion of the light photons that reach the photoreceptor to be absorbed.
• Photon absorption contributes to the photoreceptor’s output signal.

• In the retina of vertebrates the rods and cones have photopigment-bearing regions (outer segments) composed of a large number of pancakelike disks.
• In rods the disks are closed, but in cones the disks are partially open to the surrounding fluid.
• In a typical rod there are about a thousand disks, and each disk holds about 150,000 rhodopsin molecules, giving a total of 150 million molecules per rod.
• In most invertebrate photoreceptors the structure is different, with the photopigment borne on regularly arranged microvilli, fingerlike projections with a diameter of about 0.1 μm.
• This photoreceptor structure is known as a rhabdom.

• The photopigment packing is less dense in rhabdoms than in vertebrate disks.
• In both vertebrate photoreceptors and rhabdoms, each photoreceptor cell contains a nucleus, an energy-producing region with mitochondria and an axon that conveys electrical signals to the next neurons in the processing chain.
• In reptiles and birds the receptors may also contain coloured oil droplets that modify the spectrum of the light absorbed by the photopigment, thereby enhancing colour vision.
• In insects and other invertebrates the receptors may also contain granules of dark pigment that move toward the rhabdom in response to light.
• They act as a type of pupil, protecting the rhabdom in bright conditions by absorbing light.

3. Phonoreceptors:-

• The ear is the site of reception of two senses namely hearing and equilibrium.
• Anatomically, the ear is divided into three regions: the external ear, the middle ear and internal ear.

• The external ear consists of pinna, external auditory meatus and ear drum.
• The pinna is flap of elastic cartilage covered by skin.
• It collects the sound waves.
• The external auditory meatus is a curved tube that extends up to the tympanic membrane.
• The tympanic membrane is composed of connective tissues covered with skin outside and with mucus membrane inside.
• There are very fine hairs and wax producing sebaceous glands called ceruminous glands in the external auditory meatus.
• The combination of hair and the ear wax helps in preventing dust and foreign particles from entering the ear.
• The middle ear is a small air-filled cavity in the temporal bone.
• It is separated from the external ear by the eardrum and from the internal ear by a thin bony partition; the bony partition contains two small membrane covered openings called the oval window and the round window.
• The middle ear contains three ossicles: malleus [hammer bone], incus [anvil bone] and stapes [stirrup bone] which are attached to one another.

• The malleus is attached to the tympanic membrane and its head articulates with the incus which is the intermediate bone lying between the malleus and stapes.
• The stapes is attached to the oval window in the inner ear.
• The ear ossicles transmit sound waves to the inner ear.
• A tube called Eustachian tube connects the middle ear cavity with the pharynx.
• This tube helps in equalizing the pressure of air on either sides of the ear drum.
• Inner ear is the fluid filled cavity consisting of two parts, the bony labyrinth and the membranous labyrinths.
• The bony labyrinth consists of three areas: cochlea, vestibule and semicircular canals. 
• The cochlea is a coiled portion consisting of 3 chambers namely: Scala vestibuli and Scala tympani- these two are filled with perilymph; and the Scala media is filled with endolymph. 
• At the base of the cochlea, the Scala vestibule ends at the ‘oval window’ whereas the Scala tympani ends at the ‘round window’ of the middle ear.
• The chambers Scala vestibuli and Scala media are separated by a membrane called Reisner’s membrane whereas the Scala media and Scala tympani are separated by a membrane called Basilar membrane.

Organ of Corti:-

• The organ of Corti is a sensory ridge located on the top of the Basilar membrane and it contains numerous hair cells that are arranged in four rows along the length of the basilar membrane.
• Protruding from the apical part of each hair cell is hair like structures known as stereocilia. 
• During the conduction of sound wave, stereocilia makes a contact with the stiff gel membrane called tectorial membrane, a roof like structure overhanging the organ of corti throughout its length.

Mechanism of hearing:-

• Sound waves entering the external auditory meatus fall on the tympanic membrane.
• This causes the ear drum to vibrate, and these vibrations are transmitted to the oval window through the three auditory ossicles.
• Since the tympanic membrane is 17-20 times larger than the oval window, the pressure exerted on the oval window is about 20 times more than that on the tympanic membrane.  
• This increased pressure generates pressure waves in the fluid of perilymph.
• This pressure causes the round window to alternately bulge outward and inward meanwhile the basilar membrane along with the organ of Corti move up and down.
• These movements of the hair alternately open and close the mechanically gated ion channels in the base of hair cells and the action potential is propagated to the brain as sound sensation through cochlear nerve.

Defects of Ear:-

• Deafness may be temporary or permanent.
• It can be further classified into conductive deafness and sensory-neural deafness.
• Possible causes for conductive deafness may be due to.

i.  The blockage of ear canal with earwax,

ii.  Rupture of eardrum

iii. Middle ear infection with fluid accumulation

iv. Restriction of ossicular movement.

In sensory -neural deafness, the defect may be in the organ of Corti or the auditory nerve or in the ascending auditory pathways or auditory cortex.

Organ of Equilibrium:-

• Balance is part of a sense called proprioception, which is the ability to sense  the position, orientation and movement of the body.
• The organ of balance is known as the vestibular system which is located in the inner ear next to the cochlea.
• The vestibular system is composed of a series of fluid filled sacs and tubules.
• These sacs and tubules contain endolymph and are kept in the surrounding perilymph.
• These two fluids, perilymph and endolymph, respond to the mechanical forces, during changes occurring in body position and acceleration.

• The utricle and saccule are two membranous sacs, found nearest the cochlea and contain equilibrium receptor regions called maculae that are involved in detecting the linear movement of the head.
• The maculae contain the hair cells that act as mechanoreceptors.
• These hair cells are embedded in a gelatinous otolithic membrane that contains small calcareous particles called otoliths.
• This membrane adds weight to the top of the hair cells and increase the inertia.

• The canals that lie posterior and lateral to the vestibule are semicircular canals; they are anterior, posterior and lateral canals oriented at right angles to each other. 
• At one end of each semicircular canal, at its lower end has a swollen area called ampulla.
• Each ampulla has a sensory area known as crista ampullar is which is formed of sensory hair cells and supporting cells.
• The function of these canals is to detect rotational movement of the head.