The skeletal system consists of the bones (206 in adults) and joints, along with the cartilage and ligaments that occur at the joints. The skeleton is divided into the axial skeleton (skull, hyoid bone, vertebral column, and thoracic cage) and the appendicular skeleton (girdles and limbs).
The skeleton has the following functions:
1) The skeleton-notably, the large, heavy bones of the legs-supports the body and its organs against the pull of gravity.
2) The skeleton protects soft body parts. For example, the skull forms a protective encasement for the brain.
3) Certain bones, such as those of the skull, ribs, and sternum (breastbone), produce blood cells in both adults and children.
4) Bones are storage areas for mineral salts, notably calcium salts.
5) Bones, especially those of the legs and arms, provide sites for muscle attachment and permit flexible body movement.
Anatomy of a Long Bone
Although the bones of the skeletal system vary considerably in shape as well as size, a long bone, such as one in the arm or leg, can be used to illustrate certain principles of bone anatomy. The bone is enclosed in a tough, fibrous, connective tissue covering called the periosteum (per"e-os'te-um), which is continuous with the ligaments and tendons that anchor bones. The periosteum contains blood vessels that enter the bone and service its cells. At both ends of a long bone is an expanded portion called an epiphysis (e-pif'I-sis); the portion between the epiphyses is called the diaphysis (di-af'I-sis).
When bone is split open, the section shows that the diaphysis is not solid but has a medullary (med'u-lar"e) cavity containing yellow marrow. Yellow marrow contains large amounts of fat. The medullary cavity is bounded at the sides by compact bone. The epiphyses contain spongy bone. Beyond the spongy bone, there is a thin shell of compact bone and, finally, a layer of cartilage called the articular cartilage. Articular cartilage is so named because here bone articulates (meets) another bone. Articulation is the joining together of bones at ajoint. The medullary cavity and the spaces of spongy bone are lined with endosteum, a thin layer of squamous epithelium.
Compact bone, or dense bone, contains osteocytes (os'te-o-sits) (bone cells) in tiny chambers called lacunae. The osteocytes are arranged in a cylinder of concentric layers called lamellae, and the osteocytes and lamellae surrounding a single central canal comprise a Haversian system. Blood vessels and nerves from the periosteum enter the central canal. Osteocytes are connected to the central canal and to each other by passageways called canaliculi. The lacunae are separated by a matrix that contains collagenous protein fibers and mineral deposits, primarily of calcium and phosphorus salts.
Spongy bone, or cancellous bone, contains numerous bony bars and plates, called trabeculae. Although lighter than compact bone, spongy bone is still designed for strength. Like braces used for support in buildings, the trabeculae of spongy bone follow lines of stress.
In infants, red bone marrow, a specialized tissue that produces blood cells, is found in the cavities of most bones. In adults, red blood cell formation, called hematopoiesis (hem"ah-to-poi-e'sis), occurs in the spongy bone of the skull, ribs, sternum (breastbone), and vertebrae, and in the ends of the long bones.
A long bone has a shaft (diaphysis) and tvvo ends (epiphyses). The diaphysis contains a medullary cavity with yellow marrow. and the epiphyses contain spongy bone with red marrow.
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