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 skeletal system organ , function, diagram labled .

The skeletal system may be divided into two functional parts .

Skeletal system.

The  axial skeleton  is composed of the bones of the head (cranium or skull), neck (hyoid bone and cervical spine), and trunk (ribs, sternum, vertebrae, and sacrum). The  appendage bones   consist of the bones of the limbs, including the bones that form the chest girdle (shoulder) and the pelvic girdle.

Cartilage and Bones .

Bones are composed of cartilage and bones. Cartilage is an elastic, semi-rigid connective tissue that forms part of the bone and requires more flexibility. For example, costal cartilage connects the ribs to the sternum.

 In addition, the   articular surface   (support surface) of the bones involved in the synovial joint is covered with articular cartilage, which provides a smooth, low-friction sliding surface for free movement .

Blood vessels do not enter the cartilage (that is, they are avascular); therefore, their cells diffuse to obtain oxygen and nutrients. The ratio of bone to cartilage in bones changes as the body grows; the younger a person is, the more cartilage they have. The bones of newborn babies are soft and elastic because they are mainly composed of cartilage. Bone is a kind of living tissue, a form of tough, highly specialized connective tissue that constitutes most of the bones. Bone supply for adult bones.

Supports the body and its important cavities; it is the main support tissue of the body to protect the mechanical basis of movement of important structures (such as the heart) (leverage), continuous supply of salt (such as calcium) (produced by bone marrow)) in the medullary cavities of many bones middle).

Except where the articular cartilage is located, a covering of fibrous connective tissue surrounds each skeletal element like a sleeve; the periosteum of the surrounding bone is the periosteum , and the periosteum of the cartilage is the perichondrium. Periosteum and perichondrium nourishes the outer surface of bone tissue. They can deposit more cartilage or bone (especially during fracture healing) and provide an interface for the connection of tendons and ligaments.

The two types of bone are compact bone and cancellous bone (trabecular bone). The difference between them lies in the relative amount of solid matter and the amount and size of the space they contain . All bones have a thin layer of dense bone on the surface, surrounding the central mass of cancellous bone, except for the rest is replaced by the medullary cavity (marrow). Between the medullary cavity of adult bones and the spicules (trabeculae) of cancellous bone, there is yellow (fat) or red (forming blood cells and platelets) bone marrow, or a combination of both.

Transverse sections of the humerus. The shaft of a living bone is a tube of compact bone that surrounds a medullary cavity.

The structure and ratio of dense bone and cancellous bone vary by function. The tight bones provide the strength to bear the weight. Among the long bones designed for the stiffness and attachment of muscles and ligaments, the dense bone near the middle of the axis has the largest amount, where the bone can bend. In addition, long bones have bulges (for example, ridges, ridges, and nodules) that serve as support (support) where large muscles meet. Living bone has certain elasticity (flexibility) and great rigidity (hardness).