Extremeties
The upper and lower limbs are constructed after a common type, but the different functions for which they have become adapted in man have lead to structural differences of a very definite kind. Each limb consists of a girdle, which connects it to the trunk, and three segments. The terminal segment in the upper limb forms the hand and is specially adapted for prehension. In the lower limb the terminal segment forms the foot and is primarily adapted to constitute an efficient supporting base for the body in the erect attitude, but it is, at the same time, constructed in such a manner as to facilitate locomotion. In order that the full benefit of the prehensile character of the hand may be obtained, the living upper limb is characterized by the wide range of movement which it enjoys --- in some situations, e.g. the shoulder--joint, actually at the expense of stability. In the living lower limb, on the other hand, the demand for stability is the prime factor, and stability is assured, even although some degree of mobility may be sacrificed for the purpose.
The bones by which the upper and lower limbs are attached to the trunk constitute, respectively, the shoulder and pelvic girdles. The shoulder girdle, which is formed by the scapular and the clavicles, is deficient both in front and behind. In front, however, it is completed by the upper end of the sternum, with which the medial ends of the clavicles articulate. Behind, the scapula are separated from each other by a wide gap and are connected to the trunk by muscles only. The pelvic girdle is formed by the hip bones, which articulate with each other in front at the pubic symphysis so that the girdle is complete anteriorly. Posteriorly the girdle is incomplete, but the gap is filled by the upper part of the sacrum, with which the hip bones articulate. The pelvic girdle, with the sacrum, forms a complete ring, massive and comparatively rigid, in marked contrast with the lightness and mobility of the shoulder girdle.
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Vertebral Column