Unit 4: the Skeletal System

Part II: The Skeleton- Axial Skeleton

Book Chapter(s): 7 (pgs. 199-226)

Unit Objectives:

• Distinguish the bones and structures of the skull
• Distinguish the structures of the vertebral column and thoracic cage
• Distinguish the structures of the pelvic girdle and upper limb
• Distinguish the structures of the pelvic girdle and lower limb
• Describe bone development

Unit 4: Skeletal System

Table of Contents

Axial Skeleton

The Cranium

The Skull

Facial Bones

Other cranium features

The Vertebral Column

The Thoracic Cage

The Skeleton

• 20% of body mass
• Axial Skeleton (80 bones)
• Skull = 29 bones
• Cranium = 8 bones (4 single bones and 2 paired bones)
• Face = 14 bones (2 single bones and 6 paired bones)
• Ear ossicles = 3 pairs
• Hyoid = single bone in base of tongue
• Vertebral column = 26 bones
• Thorax = 25 bones

Axial Skeleton

The Skull

Cranium: enclose and protect the fragile brain and furnish attachment sites for head and neck muscles

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Facial bones:

1. Form the framework of the face
2. Contain cavities for the special sense organs or sight, taste, and smell
3. Provide openings for air and food passage
4. Secure the teeth
5. Anchor the facial muscles of expression

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The Cranium

The Skull

• Most skull bones are flat
• Firmly connected by interlocking sutures (except mandible)
• Coronal
• Sagittal
• Squamous
• Lambdoid (lam-doyd)

The Cranium

Overview of Skull Geography

Cranium can be divided into a vault and a base:

1. Cranium vault (calvaria [kal’va’re-ah]): forms the superior, lateral and posterior aspects of the skull, as well as the forehead
2. Cranial base: inferior aspect
1. Base is divided into three distinct steps or fossae:
1. Anterior
2. Middle
3. Posterior cranial fossae
3. Small cavities (about 85 named openings--provide passageways for the spinal cord, the major blood vessels serving the brain, and the 12 cranial nerves)
• Middle and inner ear
• Orbits
• Sinuses

The Cranium

Cranium

8 bones -- paired parietal and temporal; unpaired frontal, occipital, sphenoid, and ethmoid

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Frontal Bone

• Shell-shaped and forms the anterior cranium
• Connects to parietal via coronal suture
• Squamous part of frontal bone is the forehead and ends inferiorly at the supraorbital margins
• Anterior cranial fossa forms superior wall of the orbits which supports frontal lobes of the brain
• Supraorbital foramen (notch) allows supraorbital artery and nerve to pass to the forehead
• Glabella: smooth portion of frontal bone between the orbits
• Frontal bone meets nasal bone at frontonasal suture; lateral to the glabella are frontal sinuses

The Skull

Cranium

8 bones -- paired parietal and temporal; unpaired frontal, occipital, sphenoid, and ethmoid

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Frontal Bone - Important Markings and Features

• Anterior cranial fossa forms superior wall of the orbits which supports frontal lobes of the brain
• Supraorbital foramen (notch) allows supraorbital artery and nerve to pass to the forehead
• Glabella: smooth portion of frontal bone between the orbits
• Frontal bone meets nasal bone at frontonasal suture; lateral to the glabella are frontal sinuses

The Skull

The Skull

The Skull

The Skull

The Skull

Cranium

Parietal Bones and Major Sutures

• Curved rectangular bones that form most of the superior and lateral aspects of the skull
• Four largest sutures occur where the parietal bones articulate with other cranial bones
• Coronal suture (kŏ-ro’nul): parietal bones meet the frontal bone anteriorly
• Sagittal suture: parietal bones meet superiorly at the cranial midline
• Lambdoid suture (lam’doid): parietal meets the occipital bone posteriorly
• Squamous suture (one on each side): parietal and temporal bone meet on the lateral aspect of the skull.

The Skull

The Skull

The Skull

The Skull

The Skull

Cranium

Occipital Bone

• Forms most of the skull posterior wall and base
• Articulates anteriorly with the paired parietal and temporal bones via the lambdoid and occipitomastoid sutures.
• Forms the walls of the posterior cranial fossa which supports the cerebellum of the brain
• Interior part of the brain connects with the spinal cord through foramen magnum; flanked laterally by two occipital condyles that articulate with the vertebra of the spinal column (nodding motion)
• Hypoglossal canal is located medially and superiorly to each occipital condyle through which cranial nerve XII passes
• External occipital protuberance: median protrusion just superior to the foramen magnum

The Skull

Cranium

Occipital Bone

• Other ridges
• External occipital crest secures the ligamentum nuchae (lig”ah-men-tum noo’ke)--a sheetlike elastic ligament that connects the vertebrae of the neck to the skull
• Superior and inferior nuchal (nu’kal) lines: anchor many neck and back muscles

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The Skull

The Skull

The Skull

The Skull

The Skull

The Skull

Cranium

Temporal Bone

• Three shapes:
• Squamous part: flaring; abuts the squamous suture;
• has barlike zygomatic process that meets the zygomatic bone of the face anteriorly; these two make up the zygomatic arch;
• mandibular fossa on the interior surface of the zygomatic process receives the condylar process of the mandible forming the freely moveable temporomandibular joint
• Tympanic: the temporal bone surrounds the external acoustic meatus, or external ear canal
• Petrous: thick part that houses the middle and inner ear cavities; sphenoid bone and petrous portions of temporal bones construct the middle cranial fossa.

The Skull

Cranium

Temporal Bone

• Jugular Foramen allow passage of the internal jugular vein and three cranial nerves
• Carotid canal transmits the internal carotid artery into the cranial cavity
• Foramen lacerum is a jagged opening between the petrous temporal bone and the sphenoid bone; closed by cartilage in a living person
• Internal acoustic meatus transmits cranial nerves VII and VIII
• Mastoid process acts as an anchoring site for some neck muscles
• Styloid process is an attachment point for several tongue and neck muscles and for a ligament that secures the hyoid bone of the neck and skull
• Stylomastoid foramen allows cranial nerve VII to leave the skull

The Skull

The Skull

The Skull

Cranium

Sphenoid Bone

• Keystone of the cranium; contributes to the middle cranial fossa and orbits
• Main parts are the body, greater wings, lesser wings, and pterygoid processes

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The Skull

Cranium

Sphenoid Bone - Important Markings

• Sella turcica: hypophyseal fossa portion is the seat of the pituitary gland
• Optic canals: allow passage of optic nerves (cranial nerves II) and the ophthalmic arteries
• Superior orbital fissures: allows passage of the maxillary division of cranial nerves III, IV, part of V (ophthalmic division) and ophthalmic vein
• Foramen rotundum (2): allow passage of the maxillary division of cranial nerve V
• Foramen ovale (2): allows passage of the mandibular division of cranial nerve V
• Foramen spinosum (2): allows passage of the middle meningeal artery

The Skull

The Skull

Cranium

Ethmoid Bone

• Helps to form the anterior cranial fossa
• Forms part of the nasal septum and the lateral walls and roof of the nasal cavity
• Contributes to the medial wall of the orbit

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The Skull

Cranium

Ethmoid Bone - Important Markings

• Crista galli: attachment point for the falx cerebri, a dural membrane fold
• Cribriform plates: allow passage of filaments of the olfactory nerves (Cranial Nerve I)
• Superior and medial nasal conchae: form part of lateral walls of nasal cavity; increase turbulence of air flow.

The Skull

The Skull

Cranium

Auditory Ossicles (malleus, incus, and stapes)

• Found in the middle ear cavity
• Involved in sound transmission

The Skull

Facial Bones

14 bones -- mandible and vomer are the only unpaired; maxillae, zygomatics, nasals, lacrimals, palatines, and inferior nasal conchae are paired bones

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Mandible

• Lower jaw; U-shaped
• Largest, strongest bone in the face

Facial Bones

Facial Bones

Mandible - Important Markings

• Coronoid processes: insertion points for the temporalis muscles
• Condylar processes: articulate with the temporal bones in the temporomandibular joints of the jaw
• Mandibular symphysis: medial fusion point of the mandibular bones
• Dental alveoli: sockets for the teeth
• Mandibular foramina: permit the inferior alveolar nerves to pass
• Mental foramina: allow blood vessels and nerves to pass to the chin and lower lip

Facial Bones

Facial Bones

Maxilla

• Keystone bones of the face
• Form the upper jaw and parts of the hard palate, orbits, and nasal cavity walls

Facial Bones

Facial Bones

Maxilla - Important Markings

• Dental alveoli: sockets for teeth
• Zygomatic processes: help form the zygomatic arches
• Palatine processes: form the anterior hard palate; meet medially in intermaxillary suture
• Frontal process: forms part of the lateral aspect of bridge of nose
• Incisive fossa: permits blood vessels and nerves to pass through anterior hard palate (fused palatine processes)
• Inferior orbital fissure: permits maxillary branch of cranial nerve V, the zygomatic nerve, and blood vessels to pass
• Infraorbital foramen: allows passage of infraorbital nerve to skin of face

Facial Bones

Facial Bones

Facial Bones

Zygomatic

• Form the cheeks and part of the orbits

Nasal

• Form the bridge of the nose

Lacrimal

• Form part of the medial orbit walls
• Lacrimal fossa: houses the lacrimal sac, which helps to drain tears into the nasal cavity

Facial Bones

Facial Bones

Facial Bones

Palatine

• Form posterior part of the hard palate and a small part of nasal cavity and orbit walls
• Median palatine suture: medial fusion point of the horization plates of the palatine bones, which form the posterior part of the hard palate

Vomer

• Inferior part of the nasal septum

Inferior nasal concha

• Form part of the lateral walls of the nasal cavity

Facial Bones

Facial Bones

Facial Bones

Special Characteristics of the Orbits:

Orbits: cone-shaped bony cavities in which the eyes are firmly encased and cushioned by fatty tissue

• The walls of each orbit are formed by 7 bones -- the frontal, sphenoid, zygomatic, maxilla, palatine, lacrimal, and ethmoid bones
• Also contains the superior and inferior orbital fissures and the optic canals

Other cranium features

Special Characteristics of the Nasal Cavity:

Nasal cavity: constructed of bone and hyaline cartilage

• Roof is formed by cribriform plates of the ethmoid
• Lateral walls are largely shaped by the superior and middle conchae of the ethmoid bone, the perpendicular plates of the palatine bones, and the inferior nasal conchae
• The depressions under cover of the conchae on the lateral walls are called meatuses so there are superior, middle, and inferior meatuses
• The floor of the nasal cavity is formed by the palatine processes of the maxillae and the palatine bones

Other cranium features

Special Characteristics of the Nasal Cavity:

Nasal cavity: constructed of bone and hyaline cartilage

• The nasal cavity is divided into right and left parts by the nasal septum; the bony portion of the septum is formed by the vomer inferiorly and the perpendicular plates of the ethmoid bone superiorly
• A sheet of cartilage called the septal cartilage completes the septum anteriorly

Other cranium features

Paranasal Sinuses

• Five skull bones--the front, sphenoid, ethmoid, and paired maxillary bones--that are clustered around the nasal cavity and have a moth-eaten appearance
• Lighten the skull and enhance the resonance of the voice

Other cranium features

Hyoid Bone

• Lies inferior to the mandible in the anterior neck; horseshoe shaped with a body and 2 pairs of horns or cornua
• Does not articulate directly with any other bone; anchored by the narrow stylohyoid ligaments of the styloid processes of the temporal bones
• Acts as movable base for tongue
• Body and greater horns are attachment points for neck muscles that raise and lower the larynx during swallowing and speech

Other cranium features

The Vertebral Column

1. Spine or spinal column that consists of 26 irregular bones connected in a way that a flexible, curved structure results
2. In fetus/infant = 33 vertebrae; 9 inferior vertebrae eventually fuse to form two composite bones = sacrum and coccyx

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Functions:

• Transmits weight to trunk
• Surrounds and protects the delicate spinal cord
• Provides attachment points for the ribs and muscles of the back and neck

The Vertebral Column

The Vertebral Column- Regions and Curvatures

• About 70 cm long with 5 major regions
• Cervical vertebrae: 7 vertebrae of neck
• Thoracic vertebrae: 12 of thoracic region
• Lumbar vertebrae: 5 of lower back
• Scarum: connects to hip and pelvic
• Coccyx: terminus of vertebral column
• Four curvatures that give the column an “S” shape
• Cervical and lumbar curvatures: concave posteriorly
• Thoracic and sacral curvatures: convex posteriorly

The Vertebral Column

Homeostatic Imbalances of the Vertebral Column

1. Scoliosis: abnormal lateral curvature that occurs most often in the thoracic region
2. Kyphosis: a dorsally exaggerated thoracic curvature
3. Lordosis: accentuated lumbar curvature

The Vertebral Column

Ligaments

The major supporting ligaments are the anterior and posterior longitudinal ligaments

• Anterior ligament helps prevent hyperextension of the spine
• Posterior ligament resist hyperflexion of the spine
• Ligamentum flavum: connects adjacent vertebrae and contain elastic CT and is especially strong; stretches as we bend forward and recoils when we resume erect posture

The Vertebral Column

Intervertebral Discs

A cushion like pad composed of two parts:

1. Nucleus pulposus: Inner gelatinous part that gives discs its elasticity and compressibility
2. Anulus fibrosus: outer collar composed of collagen and fibrocartilage
1. Acts a shock absorber; allow spine to flex and extend; bend more laterally
2. Accounts for about 25% height of the vertebral column

The Vertebral Column

Homeostatic Imbalances with Intervertebral Discs

• Herniated (prolapsed) disc: rupture of the anulus fibrosus followed by protrusion of the spongy nucleus pulposus through the anulus

The Vertebral Column

General Structure of Vertebrae

1. Body or centrum: weight bearing region located anteriorly
2. Vertebral arch: posteriorly
3. Vertebral foramen: opening in which the spinal cord passes; together form the vertebral canal
1. Pedicles: short bony pillars projecting posteriorly from the vertebral body
2. Laminae: flattened plates that fuse in the median plane

The Vertebral Column

General Structure of Vertebrae

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Intervertebral foramina: lateral openings between adjacent vertebrae

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Several important processes pass through the vertebral arch:

1. Spinous process
2. Transverse process
3. Superior and inferior articular processes

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Smooth surfaces of processes are called facets and are covered in hyaline cartilage

The Vertebral Column

Regional Vertebral Characteristics:

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Movements that can generally occur between vertebrae are:

1. Flexion and extension
2. Lateral flexion
3. Rotation

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The Vertebral Column

Regional Vertebral Characteristics:

Cervical Vertebrae (C1-C7); most (C3-C7) have:

• An oval body
• Spinous process is short (except C7) projects directly back and is bifid (or split at its tip)
• Vertebral foramen is large and generally triangular
• Each transverse process contains a transverse foramen through which vertebral arteries pass to service the brain

The Vertebral Column

Regional Vertebral Characteristics:

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C7 has a larger spinous process and is not bifid; can be felt through skin; called the vertebra prominens

The Vertebral Column

Regional Vertebral Characteristics:

First two cervical vertebrae:

• No intervertebral disc and highly modified

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Atlas (C1)

• has no body or spinous process
• has a ring of bone consisting of anterior and posterior arches and a lateral mass on each side
• Lateral mass have superior and inferior articular facets
• Superior articular facets receive the occipital condyles of the skull; “Yes” movement
• Inferior articular facets forms joints with the axis below

The Vertebral Column

Regional Vertebral Characteristics:

Axis (C2)

• Has a body and other typical features
• Only unusual feature is the knoblike dens projecting superiorly from its body; missing body of “atlas”
• Acts as a pivot for the rotation of the atlas
• “No” motion

The Vertebral Column

Regional Vertebral Characteristics:

Thoracic Vertebrae (T1-T12); most have:

• Body is roughly heart shaped; two small facets called demifacets on each side (superior costal facet and inferior costal facet); demifacets receive head of ribs (T10-12 only have 1 facet)
• Vertebral foramen is circular
• Spinous process is long and points sharply downward
• Transverse processes have facets that articulate with the tubercles of the ribs (except T11 and T12)
• Superior and inferior articular facets lie mainly in the frontal plane (rotation)

The Vertebral Column

Regional Vertebral Characteristics:

Lumbar vertebrae (L1-L5) have a sturdier structure

• Kidney-shaped bodies
• Pedicles and laminae are shorter and thicker
• Spinous processes are short, flat, and hatchet shaped; robust and project directly backward
• Vertebral foramen in triangular
• Orientation of the facets of the articular processes differ; lock lumbar vertebrae together and provide stability by preventing rotation.

The Vertebral Column

Regional Vertebral Characteristics:

Sacrum:

• 5 fused vertebrae (S1-S5)
• Forms posterior wall of pelvis
• Articulates with L5 superiorly and with articular surfaces of hip bones, forming sacroiliac joints

The Vertebral Column

Regional Vertebral Characteristics:

Coccyx:

• Tailbone
• 3-5 fused vertebrae
• Articulates superiorly with sacrum

The Vertebral Column

The Thoracic Cage

Include the thoracic vertebrae posteriorly, the ribs laterally, the sternum, and costal cartilages anteriorly

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Functions:

• Protective cage around the vital organs of the thoracic cavity
• Supports the shoulder girdles and upper limbs
• Provides attachment points for many muscles of the neck, back, chest, and shoulders
• Intercostal spaces between the ribs are occupied by the intercostal muscles, with life and then depressed the thorax during breathing

The Thoracic Cage

The Thoracic Cage

The Thoracic Cage

Sternum: anterior midline of thorax and has three regions:

1. Manubrium: articulates with clavicle via the clavicular notch; articulates with first two pairs of ribs
2. Body: articulates with costal cartilages of the second-seventh ribs
3. Xiphoid process: articulates only with sternal body

The Thoracic Cage

The Thoracic Cage

Sternum: has three important anatomical landmarks:

1. Jugular notch: point where left common carotid artery issues from the aorta
2. Sternal ridge: horizontal ridge that acts as a hinge, allowing sternal body to swing anteriorly when we inhale
3. Xiphisternal joint (zif”i-ster’nul): point where sternal body and xiphoid process fuse

The Thoracic Cage

The Thoracic Cage

Ribs: 12 pairs

1. True ribs: superior seven rib pairs attach directly to sternum by costal cartilages
2. False ribs: attach indirectly or not at all; ribs 8-10 indirectly (vertebrochondral ribs); ribs 7-10 (costal margin); ribs 11-12 (vertebral ribs or floating ribs)

Ribs

• Increase in length from 1-7; then decrease in length from 8-12
• Bulk of the rib is called a shaft with a smooth superior border and sharp and thin inferior border with a costal groove
• Articulates with the body of vertebrae by the head with two facets
• Tubercle articulates with the costal facet of the transverse process

The Thoracic Cage

The Thoracic Cage

The Thoracic Cage