The Skeletal System and Bones

The human adult skeleton has 206 bones joined to ligaments and tendons to form a supportive and protective framework for underlying soft tissues and muscles. The skeletal system serves several important functions in the body:

• Bones serve as levers that transmit muscular forces.
• Our skeletal system protects our organs.
• Our skeletal system serves as a framework for other tissues and organs.
• Bones serve as banks for storage and release of minerals like calcium and phosphorous.

The skeleton consists of the axial and appendicular skeleton. There are 8o bones in the axial skeleton, consisting of the skull, spine, ribs and and sternum. There are 126 bones in th appendicular skeleton: 60 in the upper extremities, 60 in the lower extremities, 2 in the pelvic girdle, and 4 in the shoulder girdle.

Each of these 206 bones consists of three layers: the bone marrow, compact bone and the periosteum. Within the long bone is a central marrow cavity known as bone marrow. The red marrow produces red blood cell, which carry oxygen, white blood cells, which fight infection, and platelets, that help stop bleeding. Yellow marrow consists mainly of fat cells. Surrounding the marrow is a dense rigid bone called the compact bone. Cylindrical in shape, the dense layers of the compact bone are honeycombed with thousands of tiny holes and passages. Nerves and blood vessels run through the passages that supply oxygen and nutrients to the bone. This dense layer of compact bone supports the weight of the body and is mainly compromised of calcium and minerals. Each bone is covered by the periosteum, which is a layer of specialized connective tissue and acts as the skin of the bone. The inner layer of the periosteum contain cells that produce bone. These three bone layers work together to handle the skeletal system.

The 206 bones that make up the skeleton are divided into two categories, the axial skeleton (trunk and head) and the appendicular skeleton (arms and legs). These bones also vary in shape and size. There are five main categories of the bones: flat bones, short bones, long bones, sesamoid bones and irregular bones.

• Flat bones provide protection and include the ilium, ribs, sternum, clavicle and scapula. They are usually characterized by a curved surface where is either thick at the tendon attachment or very thin.
• Short bones provide some shock absorption and include carpals and tarsals. They are usually small, cubical shaped, shaped bones.
• Long bones provide structural support and include the tibia, fibula, femur (which might be partially replaced when the knee suffers from deceases like arthritis), radius, ulna and humerus. Their long cylindrical shaft with relatively wide protruding ends usually characterize them.
• Sesamoid bones provide protection as well as improve mechanical advantage of musculotendinous units and include units in the patella and the flexor tendons of the toe and thumb. They are usually characterized as small bones embedded within the tendon of a musculotendinous unit.
• Irregular bones serve a variety of purposes in the body and include bones throughout the spine as well as the ischium, pubis and maxilla.

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