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The skeletal system includes the bones of the skeleton and the cartilages, ligaments, and other connective tissues that stabilize or interconnect them. Bones are the organs of the skeletal system, and they do more than serve as racks that muscles hang from; they support your weight and work together with muscles, producing controlled, precise movements. Without a framework of bones to connect to, contracting muscles would just get shorter and fatter. Our muscles must pull against the skeleton to make you sit, stand, walk, or run.
The bones of the skeleton are complex, dynamic organs that contain osseous tissue, other connective tissues, smooth muscle tissue, and neural tissue.
The skeleton has many vital functions:
Support: The skeletal system provides structural support for the entire body. Individual bones or groups of bones provide a framework for the attachment of soft tissues and organs and muscles use to cause movement.
Mineral storage: The calcium salts of bone are a valuable mineral reserve that maintains normal concentrations of calcium and phosphate ions in body fluids. Calcium is the most abundant mineral in the human body. A typical human body contains 1–2 kg (2.2–4.4 lb) of calcium, with more than 99 percent of it in the bones of the skeleton.
Blood cell production: Within certain bones, a connective tissue called red bone marrow produces red blood cells, white blood cells, and platelets, a process called hemopoiesis. Red bone marrow consists of developing blood cells, adipocytes, fibroblasts, and macrophages within a network of reticular fibers. It is present in developing bones of the fetus and in some adult bones, such as the hip (pelvic) bones, ribs, sternum (breastbone), vertebrae (backbones), skull, and ends of the bones of the humerus (arm bone) and femur (thigh bone). In a newborn, all bone marrow is red and is involved in hemopoiesis. With increasing age, much of the bone marrow changes from red to yellow.
Triglyceride storage. Yellow bone marrow consists mainly of adipose cells, which store triglycerides. The stored triglycerides are a potential chemical energy reserve.
Protection: Skeletal system protects and supports body organs and delicate tissues. The ribs protect the heart and lungs, the skull encloses the brain, the vertebrae shield the spinal cord, and the pelvis cradles delicate digestive and reproductive organs.
Leverage: Many bones of the skeleton function as levers. They change the magnitude and direction of the forces generated by skeletal muscles. The movements produced range from the delicate motion of a fingertip to powerful changes in the position of the entire body.
The skeleton is divided into two regions:
The Axial skeleton and
Appendicular skeleton.
The axial skeleton, which forms the central supporting axis of the body, includes:
the skull,
auditory ossicles,
hyoid bone,
vertebral column, and
thoracic cage (ribs and sternum).
The appendicular skeleton includes:
the bones of the upper limb and
pectoral girdle and the bones of the lower limb and pelvic girdle.
It is often stated that there are 206 bones in the skeleton, but this is only a typical adult count, not an invariable number. At birth there are about 270, and even more bones form during childhood. With age, however, the number decreases as separate bones gradually fuse. For example, each side of a child’s pelvic girdle has three bones—the ilium, ischium, and pubis—but in adults, these are fused into a single hip bone on each side. The fusion of several bones, completed by late adolescence to the mid-20s, brings about the average adult number of 206.
This number varies even among adults. One reason is the development of sesamoid bones—bones that form within some tendons in response to strain. The patella (kneecap) is the largest of these; most of the others are small, rounded bones in such locations as the hands and feet. Another reason for adult variation is that some people have extra bones in the skull called sutural bones.
Figure 1. Human skeleton
Note: Green colored bones are Appendicular skeleton. The rest are Axial skeleton.
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