The hand and foot are constructed on somewhat similar principles, each consisting of a proximal part, the carpals or tarsrus, an intermediate portion, the metacarpals or metatarsals, and a distal portion; the phalanges. The proximal part consists of a series of more or less cubical bones; which allow a slight amount of gliding on one another and are chiefly concerned in distributing forces transmitted to or from the bone of the arrn or leg. The intermediate part is made up of slightly movable long bones. which assist the carpals or tarsals in distributing forces, and also give greater breadth for the reception of such forces. The separation of the individual bones from one another allows of the attachments of the interosseous muscles and protects the dorsipalmar and dorsiplantar vascular anastomoses. The distal portion is the most movable, and its separate elements enjoy a varied range of movements; the chief of which are flexion and extension.
Figure 501
Figure 502
The functions of the hand and foot are, however, very different, and the general similarity between them is greatly modified to meet these requirements. Thus the foot forms a firm basis of support for the body in the erect posture, and is therefore more solidly built, and its component parts are less movable on each other than those of the hand. The architecture of the bones of the hand is designed to provide an efficient instrument of prehension, and its component parts are not only capable of a greater range of movement but possess a greater degree of independent mobility. In the case of the proximal phalanges the difference is readily noticeable; those of the foot are smaller and their movements more limited than those of the hand. The difference between the metacarpal bone of the thumb and the metatarsal bone of the great toe is very much more marked. The metacarpal bone of the thumb is constructed to permit of great mobility; as compared with the other metacarpal bones, it is carried forwards and rotated round its long axis through an angle of approximately 90�, and it is capable of a considerable range of movement at its articulation with the carpals. The metatarsal bone of the great toe assists in supporting the weight of the body, is constructed with great solidity, lies parallel with the other metatarsals, and has a very limited degree of movement. The carpals is small in proportion to the rest of the hand, is placed in line with the forearm, and forms a transverse arch, the concavity of which constitutes a bed for the flexor tendons. The tarsals forms a considerable part of the foot, and is placed at right angles to the leg, a position which is almost peculiar to man, and has relation to his erect posture. In order to allow of their supporting the weight of the body efficiently while making provision for the requisite spring and elasticity of the gait, the tarsals and metatarsals are constructed in a series of arches (figs. 501, 502), the disposition of which will be considered after the articulations of the foot have been described.
Applied Anatomy.—Considering the injuries to which the foot is subjected, it is surprising how seldom the tarsal bones are fractured. This is no doubt due to the fact that the tarsals is composed of a number of bones, articulated by a considerable extent of surface, and joined together by very strong ligaments, which serve to break the force of violence applied to this part of the body. When fracture does occur, these bones being composed for the most part of a soft cancellous structure, covered only by a thin shell of compact tissue, are often extensively comminuted, especially as most of the fractures are produced by direct violence; and, as there is only a very scanty amount of soft parts over the bones, the fractures are very often compound, and amputation is often necessary.
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