The Skeletal and Muscular Systems

Organization and Gross Anatomy of the Skeletal System

The adult human skeleton provides essential mechanical support for the body, protects delicate internal organs, stores vital minerals, and participates in hematopoiesis. Anatomists divide the adult human skeleton into two major functional sections: the axial skeleton and the appendicular skeleton. The axial skeleton forms the central axis of the body and includes the skull, auditory ossicles, hyoid bone, vertebral column, ribs, and sternum. The appendicular skeleton attaches to this central core and includes the pectoral girdle, pelvic girdle, and both the upper and lower limbs.

Within these bones are specialized soft tissues known as bone marrow. Red bone marrow serves as the active site of hematopoiesis, which is the biological production of blood cells. In contrast, yellow bone marrow consists primarily of adipose tissue and serves as an important energy reserve for the body.

Bones are categorized into five distinct groups based on their structural shapes. Long bones are longer than they are wide; familiar examples include the femur, humerus, tibia, and phalanges. Short bones are roughly cube-shaped and include the carpals of the wrist and tarsals of the ankle. Flat bones are thin and curved to provide protective shielding, such as the sternum, ribs, and cranial bones. Irregular bones possess complex shapes that do not fit standard categories, exemplified by the vertebrae and pelvic bones. Finally, sesamoid bones are unique because they develop embedded within tendons, with the patella being the primary example.

A typical long bone exhibits specific anatomical regions suited to support and movement. The elongated tubular shaft of a long bone is called the diaphysis, and each expanded, rounded end is called an epiphysis. In growing bones, a specialized layer of hyaline cartilage known as the epiphyseal plate allows for longitudinal bone growth. The central hollow space running through the diaphysis is the medullary cavity, which houses bone marrow. Covering the entire outer surface of the bone, with the exception of articular surfaces, is a protective, dense fibrous membrane called the periosteum.