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Skeletal System

Bones: An Overview

1. Bones support and protect body organs; serve as levers for the muscles to pull on to cause movement at joints; store calcium, fats, and other substances for the body; and contain red marrow, the site of blood cell production.

2. Bones are classified into four groups: long, short, flat, and irregular on the basis of their shape and the amount of compact or spongy bone they contain. Bone markings are important anatomical landmarks that reveal where muscles attach and where blood vessels and nerves pass.

3. A long bone is composed of a shaft (diaphysis) with two ends (epiphyses). The shaft is compact bone; its cavity contains yellow marrow. The epiphyses are covered with hyaline cartilage; they contain spongy bone (where red marrow is found (Figure 5.1).

4. The organic parts of the matrix make bone flexible; calcium salts deposited in the matrix make bone hard.

5. Bones form on hyaline cartilage "models" or fibrous membranes. Eventually these initial supporting structures are replaced by bone tissue. Epiphyseal plates persist to provide for longitudinal growth of long bones during childhood and become inactive when adolescence ends (Figure 5.2).

6. Bones change in shape throughout life. This remodeling occurs in response to hormones (i.e., PTH, which regulates blood calcium levels) and mechanical stresses acting on the skeleton.

7. A fracture is a break in a bone. Common types of fractures include simple, compound, compression, comminuted, and greenstick. Bone fractures must be reduced to heal properly.

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Axial Skeleton

1. The skull is formed by cranial and facial bones. Eight cranial bones protect the brain: frontal, occipital, ethmoid, and sphenoid bones, and the pairs of parietal and temporal bones. The 14 facial bones are all paired (maxillae, zygomatics, palatines, nasals, lacrimals, and inferior conchae), except for the vomer and mandible. The hyoid bone, not really a skull bone, is supported in the neck by ligaments (Figure 5.3).

2. Skulls of newborns contain fontanels (membranous areas), which allow brain growth. The infant's facial bones are very small compared to the size of the cranium.

3. The vertebral column is formed from 24 vertebrae, the sacrum, and the coccyx. There are 7 cervical vertebrae, 12 thoracic vertebrae, and 5 lumbar vertebrae, which have common as well as unique features (Figure 5.4). The vertebrae are separated by fibrocartilage discs that allow the vertebral column to be flexible. The vertebral column is S-shaped to allow for upright posture. Spinal curvatures present at birth are the thoracic and sacral curvatures; secondary curvatures (cervical and lumbar) develop after birth.

4. The bony thorax is formed from the sternum and 12 pairs of ribs. All ribs attach posteriorly to thoracic vertebrae. Anteriorly, the first 7 pairs attach directly to the sternum (true ribs); the last 5 pairs attach indirectly or not at all (false ribs) (Figure 5.5). The bony thorax encloses the lungs, heart, and other organs of the thoracic cavity.

5. The most common Joint problem is arthritis, or inflammation of the joints. Osteoarthritis, or degenerative arthritis, is a result of the "wear and tear" on joints over many years and is a common affliction of the aged. Rheumatoid arthritis occurs in both young and older adults; it is believed to be an autoimmune disease. Gouty arthritis, caused by the deposit of uric acid crystals in joints, typically affects a single joint.

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Appendicular Skeleton

1. The pectoral girdle, composed of two bones, the scapula and the clavicle attaches the upper limb to the axial skeleton (Figure 5.6). It is a light, poorly reinforced girdle that allows the upper limb a great deal of freedom.

2. The bones of the upper limb include the humerus of the arm, the radius and ulna of the forearm, and the carpals, metacarpals, and phalanges of the hand.

3. The pelvic girdle is formed by the two coxal bones, or hip bone's. Each hip bone is the result of fusion of the ilium, ischium, and pubis bones. The pelvic girdle is securely attached to the sacrum of the axial skeleton, and the socket for the thigh bone is deep and heavily reinforced (Figure 5.7). This girdle receives the weight of the upper body and transfers it to the lower limbs. The female pelvis is lighter and broader than the male's; its inlet and outlet are larger, reflecting the childbearing function of the female.

4. The bones of the lower limb include the femur of the thigh, the tibia and fibula of the leg, and the tarsals, metatarsals, and phalanges of the foot.

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Joints

1. Joints hold bones together and allow movement of the skeleton.

2. Joints fall into three functional categories: synarthroses (immovable), amphiarthroses (slightly movable), and diarthroses (freely movable).

3. Joints also can be classified structurally as fibrous, cartilaginous, or synovial joints depending on the substance separating the articulating bones.

4. Most fibrous Joints are synarthrotic, and most cartilaginous joints are amphiarthrotic. Fibrous and cartilaginous joints occur mainly in the axial skeleton.

5. Most joints of the body are synovial joints, which predominate in the limbs. In synovial joints, the articulating bone surfaces are covered with articular cartilage and enclosed within the joint cavity by a fibrous capsule lined with a synovial membrane (Figure 5.8). All synovial joints are diarthroses.

6. The most common Joint problem is arthritis, or inflammation of the joints. Osteoarthritis, or degenerative arthritis, is a result of the "wear and tear" on joints over many years and is a common affliction of the aged. Rheumatoid arthritis occurs in both young and older adults; it is believed to be an autoimmune disease. Gouty arthritis, caused by the deposit of uric acid crystals in joints, typically affects a single joint.