FOOT, BONES OF THE (in human ana tomy).—The foot (pes ; Gr. 7a- ovc; Fr. le pied ; Germ. der Fuss) forms the inferior segment of the lower extremity, being united to the leg at the ankle-joint nearly at a right angle, so that in the erect position on a plane surfitce the foot is horizontal. The outline of the foot circum scribes an ovoidal figure, the long, axis of which is directed from before backwards ; and in the same direction the foot is divided into three segments, the anterior one surpassing that behind it in mobility, but falling short of it in solidity. These divisions are the tarsus, meta tarsus, and the toes.
The size of the foot, taken as a whole, varies in different individuals: it always exceeds that of the hand, chiefly, however, in length and thickness, its breadth being less than that of the hand. In the hand we find divisions pre cisely analogous to those of the foot above mentioned and similarly constructed, with this difference, that the solid part of the foot is more solid and more developed in every way than the corresponding part of the hand, but the moveable parts possess less mobility than the analogous segments of the hand. The parts of the foot and hand, as Mr. Lawrence observes, are disposed inversely in respect to their importance. The posterior portion of the former and the anterior of the latter are of the most consequence and possess the most remark able characters. In short, the foot is nothing more than the hand so modified as to afford a firm basis of support to the inferior extremity in the erect posture. One of the most remark able of these modifications is that manifest in the metatarsal bone of the great toe, which corresponds to the metacarpal bone of the thumb. The latter bone is connected with the carpus so that it forms an acute angle with the second metacarpal bone. It enjoys at its arti culation with the carpus a considerable degree of mobility, in virtue of which exists the opposable faculty of the thumb. On the other hand, the metatarsal bone of the great toe enjoys but a very limited degree of mobility at its articulation with the tarsus: it lies parallel to the adjacent bone and possesses considerable strength. These remarkable differences, says
Mr. Lawrence, are easily understood when we consider that the great toe, as one of the points on which the body is supported, requires solidity ; while the thumb, being concerned in all the numerous and varied motions of the hand, must be organised for mobility. Those animals in which the inferior segments of both anterior and posterior extremities are eminently required for prehension have the inferior seg ments of all four extremities organised as hands, and are thence denominated Quadrumanous.
The most elevated part of the foot is at its posterior part, where it contributes to form the ankle-joint; thence it inclines forwards, gra dually expanding transversely, and presenting a more or less convex surface from behind forwards. This is the dorsutn pedis, the instep.
The inferior surfitce likewise expands as it proceeds forwards. It is slightly concave in the transverse direction, and more manifestly so in the antero-posterior one ; this latter, how ever, varies in a degree proportionate to the convexity of the dorsum. This is the plants pedis, the sole.
The internal edge of the foot corresponds to the great toe, the external edge to the little toe, the anterior to the ends of the toes, and the posterior extremity of the foot is formed by the os calcis.
I. Tarsus (Germ. die Fussuturzel).—Nearly the posterior half of the foot is occupied by the tarsus, which is arranged in the form of an arch, convex superiorly, on the highest point of which rests the weight of the leg. Seven bones enter into the formation of the tarsus ; they are arranged in two sets or rows. The posterior row is formed by the astragalus and vs caleis, the anterior row by the osnaviculare, the os cuboideum, and the three cuneiform bones. Through the medium of the first two bones of the anterior row that row is articulated with the posterior.