SKELETAL SYSTEM, HUMAN

the internal skeleton that serves as a framework for the body. This framework consists of many individual bones and cartilages. There also are bands of fibrous connective tissue, the ligaments and tendons, in intimate relationship with the parts of the skeleton. This article is concerned primarily with the gross structure and the function of the skeleton of the normal human adult. The human skeleton, like that of other vertebrates, consists of three subdivisions, each with origins distinct from the others and each presenting certain individual features. These are (1) the axial, comprising the vertebral columnthe spineand much of the skull; (2) the visceral, comprising the lower jaw, some elements of the upper jaw, and the branchial arches, including the hyoid bone (the branchial arches are barlike ridges and the bones and cartilages that derive from them on either side of the throat and the head of the embryo, corresponding to the bars that support the gills of fish; for the hyoid, see below The hyoid: example of the anchoring function); and (3) the appendicular, to which the hip and shoulder girdles and the bones and cartilages of the limbs belong. When the relation of these three divisions of the skeleton to the soft parts, such as the nervous system, the alimentary respiratory, and circulatory systems, and the voluntary muscles, are considered, it is clear that the functions of the skeleton are of three different typessupport, protection, and motion. Of these functions, support is the most primitive and the oldest, just as the axial part of the skeleton was the first to evolve. The vertebral column, corresponding to the notochord in lower organisms, is the main support of the trunk. The central nervous system lies largely within the axial skeleton, the brain being well protected by the cranium (see below The cranium: example of the protective function) and the spinal cord by the vertebral column, by means of the bony neural arches (the arches of bone that encircle the spinal cord) and the intervening ligaments. A distinctive characteristic of man as compared with other mammals is his erect posture. The human body is, to some extent, like a walking tower that moves on pillars, represented by the legs. Tremendous advantages have been gained from this erect posture, the chief among which has been the freeing of the arms for a great variety of uses. Nevertheless, erect posture has created a number of mechanical problems; in particular, weight bearing. These problems have had to be met by adaptations of the skeletal system. Protection of the heart, lungs, and other organs and structures in the chest creates a problem somewhat different from that of the central nervous system. These organs, the function of which involves motion, expansion, and contraction, must have a flexible and elastic protective covering. Such a covering is provided by the bony thoracic basket, or rib cage, which forms the skeleton of the wall of the chest, or thorax. The connection of the ribs to the breastbonethe sternumis in all cases a secondary one, brought about by the relatively pliable rib (costal) cartilages. The small joints between the ribs and the vertebrae permit a gliding motion of the ribs on the vertebrae during breathing and other activity. The motion is limited by the ligamentous attachments between ribs and vertebrae. The third general function of the skeleton is that of motion. The great majority of the skeletal muscles are firmly anchored to the skeleton, usually to at least two bones and, in some cases, to many bones. Thus, the motions of the body and its parts, all the way from the lunge of the football fullback to the delicate manipulations of a handicraft artist or of the use of complicated instruments by a scientist, are made possible by separate and individual engineering arrangements between muscle and bone. In this article, the cranium, the hyoid bone, the rib cage, and other parts of the skeleton are considered in terms of their sharing in these functions. Additional reading Detailed description of the anatomy of the human skeleton is given in Cunningham's Textbook of Anatomy, 12th ed. by G.J. Romanes (1981); and in Peter L. Williams and Roger Warwick (eds.), Gray's Anatomy, 36th ed. (1980). Also useful are W.J. Hamilton (ed.), Textbook of Human Anatomy (1956), a concise and readable account of the anatomy of the skeleton of man; J.Z. Young, An Introduction to the Study of Man (1971); Kathy E. Goldberg, The Skeleton (1982), a popularly written anatomy; and J.E. Frazer, Anatomy of the Human Skeleton, 6th ed. (1965), a classic in the field of osteology.