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Bone and Overview of the Skeletal System

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The Skeleton

  • Includes bones and cartilages.
  • Supports internal organs.
  • Protects internal organs.
  • Provides levers for movement by skeletal muscles.
  • Stores lipids and minerals.
  • Initial site of blood cell formation (hematopoiesis).

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The Axial Skeleton

  • Made up of bones that lie along the midline of the body.
  • Includes the skull, hyoid, vertebrae, ribs, sternum, and sacrum.

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The Appendicular Skeleton

  • Made up of bones that support the limbs and attach them to the axial skeleton.
  • Includes the pectoral girdles, upper limb, pelvic girdles, and lower limb.

Pectoral girdle

Upper limb

Pelvic girdle

Lower limb

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The Appendicular Skeleton

  • The pectoral girdles include the clavicles and the scapulae.
  • The pelvic girdles consist of the innominate bones (os coxae).

Pectoral girdle

Upper limb

Pelvic girdle

Lower limb

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Classification of Bones

  • Most bones contain two different forms of osseous tissue, compact bone and spongy bone. More on this in a moment.
  • Bones may be classified by shape.
    • Long
    • Short
    • Flat
    • Irregular
    • Sesamoid
    • Wormian (sutural)

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Long Bones

  • Long bones make up most of the bones of the appendicular skeleton.
  • Ex:
    • Femur
    • Phalanges
    • Metacarpals/metatarsals
    • Humerus

Humerus

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Short Bones

  • Roughly cubical.
  • Primarily composed of spongy bone.
  • Ex:
    • Carpals
    • Tarsals

Trapezoid (a carpal)

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Flat Bones

  • Two thin layers of compact bone sandwiching a layer of spongy bone.
  • Ex:
    • Skull
    • Ribs
    • Scapula
    • Sternum

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Irregular Bones

  • Irregular in shape and do not fit into any other category.
  • Often made up of several components fused during development.
  • Ex:
    • Vertebrae
    • Innominate bones

A vertebra

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Sesamoid Bones

  • Form within tendons.
  • Patella is the only sesamoid bone that is consistently present in all people.
  • Sometimes considered to be a subcategory of short bones.

Patella

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Wormian (Sutural) Bones

  • Form in the sutures of the skull.
  • Not present in all people or in the same locations.
  • Sometimes considered to be a subcategory of flat bones.

Wormian bones

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Bone Markings

  • Bone markings fall into four categories.
    • Projections for attachment of muscles or ligaments.
    • Projections that form joints.
    • Cavities.
    • Depressions or openings for the passage of blood vessels and nerves.
  • If you familiarize yourself with the types of bone markings now, you will have an easier time remembering the specific bone markings listed on your structure list.

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The Structure of Bones

  • Individual bones are organs: a discrete structure composed of two or more tissue types that performs a specific function.
  • Bones are composed of osseous connective tissue, dense irregular connective tissue, adipose connective tissue, and one or more types of cartilage.
  • Most bones contain both compact and spongy bone:
  • Compact bone
    • provides strength along a single axis but is weak perpendicular to that axis.
  • Spongy bone
    • provides limited strength in many directions and helps to distribute forces.

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Compact Bone

  • Compact bone is built from units called osteons.
  • An osteon (or Haversian system) is composed of a series of concentric rings called lamellae.
  • In between the lamellae, osteocytes sit within spaces called lacunae.

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Compact Bone

  • Osteocytes send cytoplasmic projections into the surrounding lamellae via canaliculi.
  • These projections allow osteocytes to take up and deposit material from the lamellae.
  • They also allow the osteocytes to receive nutrients from the blood supply of the bone.

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Compact Bone

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Compact Bone

  • Blood vessels and nerves travel through the central (or Haversian) canal of the osteon.
  • Blood vessels and nerves also travel perpendicular to the osteons via perforating (or Volkman’s) canals.

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Compact Bone

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Spongy Bone

  • Spongy bone is composed of bars of osseous tissue called trabeculae.
  • Trabeculae are not arranged like osteons.
  • Osteocytes sit within lacunae inside the trabeculae and communicate with each other and the surface of the bar via canaliculi.

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Structure of Long Bones

  • Two epiphyses separated by a diaphysis.
    • composed of compact bone surrounding a central medullary cavity.
  • Epiphyses are composed of spongy bone surrounded by a thin layer of compact bone.

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Structure of Long Bones

  • Entire bone is surrounded by a fibrous periosteum.
  • The ends of the epiphyses are typically covered with hyaline cartilage as protection for the joint.

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Structure of Long Bones

  • The medullary cavity is filled with yellow marrow (adipose tissue).
  • The spaces within the spongy bone of the epiphyses are filled with red marrow, which is the site of hematopoiesis.

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Structure of Long Bones

  • The epiphyseal line is the boundary between epiphysis and diaphysis (metaphysis).
  • It marks the zone of growth in length of long bones.

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Cartilage – This should be review

  • All cartilage contains chondrocytes within lacunae.
  • The matrix is composed of ground substance and different types of protein fibers, depending on the particular variety of cartilage.

  • Cartilage is classified into three types:
    • Hyaline cartilage has a uniform matrix without visible protein fibers.
    • Elastic cartilage has thick branched elastin fibers.
    • Fibrous cartilage has many parallel layers of collagen fibers.

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Hyaline Cartilage

  • Hyaline cartilage provides uniform structure but not much strength.
  • It forms the embryonic skeleton, the articular surfaces of long bones, the costal cartilages, the nasal cartilage and the laryngeal and tracheal cartilages.

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Elastic Cartilage

  • Elastic cartilage provides flexibility.
  • It forms the cartilage of the external ear and the structure of the epiglottis.

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Fibrous Cartilage

  • Fibrous cartilage provides great strength along a single axis.
  • It allows for cushioning under heavy compression forces.
  • It forms the intervertebral discs, the pubic symphysis, and the menisci of the knees.