The typical number of the tarsal bones corresponds with that noticed in the carpus. The seven bones described in the human tarsus represent ten primitively distinct ossicles. The single piece, called os naviculare, or scaphoides, is composed of two bones, which answer to the compound os scaphoides of the carpus ; the astragalus is the semilunare ; the os calcis is the cunéiforme and the pisiforme combined. The internal cuneiform supporting the metatarsal bone of the great toe repeats the trapezium ; the middle cuneiform, the os trapezoides ; the external cuneiform, the os magnum. The os cuboides corresponds with the os unciforme : formed by the coalescence of two distinct ossicles, it articulates behind with that portion of the os calcis which is homologous with the os cunéiforme, and supports in front the two last metatarsal bones, as the os unci-forme supports the two last metacarpals.
In most vertebrata the foot is longer and narrower than the hand, which is constructed more for purposes of prehension than of progression. We are not therefore surprised to find that the first row of tarsal bones, unlike those of the carpus, lose the horizontal and transverse arrangement, and are subject to more frequent coalescence. One bone only, the astragalus, receives from the bones of the leg, the weight of the body. The primitively compound os scaphoides receives its anterior rounded extremity, and, removed to the inner border of the foot is interposed between this convex head and the three cunéiforme bones. The two homologues of the cuneiform and pisiform bones of the carpus coalesce to constitute the os calcis ; into which is inserted the tendo Achillis. There can be no need of pointing out the reason why these two bones are not distinct, as in the carpus. A small movable os pisiforme is a sufficiently firm point of insertion for the flexor muscle of the wrist, but would be wholly inadequate as a powerful lever of sufficient strength, when elevated, to sustain the whole weight of the body. The reception of the head of the astragalus into a cup formed by the os calcis, os scaphoides, and the elastic calcaneo-scaphoid ligament constitutes one of the most important modifications in the articulation of the tarsal bones. The sudden shock, which would be communicated to the body, when the feet strike the ground in jumping, running, etc, is by this means avoided, or at least rendered milder; and to its elastic properties may be referred that springiness, and light step, which so distinguishes the progressive movements of the young from those of the old or infirm.
The three cuneiform, and the cuboid bones bear a striking resemblance in their arrangement and connections with the second row of the carpal bones, and much more stress would have been laid in earlier works of Anatomy upon this point, had the error been avoided of comparing the patella with the olecranon. But the necessity which in that case followed, of making the tibial side of the leg correspond with the ulnar side of the forearm, exacted a second necessity of comparing the great, toe with the little finger instead of with the thumb. The consequence of thus confounding the outer and the inner side of a limb was to reduce the matter to a state of inextricable confusion. The firmness of both the hand and the foot requires that the two small external digits, less movable than the rest, should be supported upon a solid single piece, which in the carpus is opposed to the os cuneiforme; and, in the tarsus, to the homotype of the os cuneiforme, viz. the anterior portion of the os calcis.