Fig. 47.—Posterior Limb of the Horse placed in the Position which it should occupy if the Animal were a Plantigrade: Left Limb, External Surface.
1, Tibia; 2, astragalus; 3, calcaneum; 4, metatarsus; 5, first phalanx; 6, second phalanx; 7, third phalanx.
When we studied the anterior limbs, we saw in passing from the plantigrades to the digitigrades, and finally the ungulates, or unguligrades, as the hand became hyperextended, the carpus was raised and more and more removed from the ground. We shall establish the existence of the same condition in the posterior limbs; in the plantigrades the tarsus rests on the ground; in the digitigrades it is removed from it; while in the unguligrades the distance which separates it from the point of support is still more considerable; and it is, indeed, necessary to imagine that if these latter were plantigrades, would occupy the position on the ground which is indicated by [Fig. 47].
In veterinary anatomy the tarsus is called the ham; a name we adopt in conformity with usage, but which we cannot but regret, as in human anatomy the ham is the region of the posterior surface of the knee.
The general arrangement of the region of the digits of the posterior limbs in birds, presents some points of interest.
We shall merely say with regard to the metatarsus, that it is formed by a single bone, which in the cock is furnished towards its inferior third with a pointed process, the spur. At the inferior part, there is, however, found another, which is but very slightly developed, and with which the first phalanx of the innermost toe articulates.
The toes are, in the majority of species, four in number:[15] an internal, which is directed backwards, and corresponds to the great-toe; the others are directed forwards. This arrangement is constant in grallatores (wading birds), gallinaceæ[16] (domestic fowls), and raptores (birds of prey).
[15] In spite of the fact that the custom is to designate the terminal portions of the foot of birds by the name of digits, we prefer to employ here the terms foot and toes. In adopting this decision we believe we are acting according to a more didactic method. Homology of names should, in our opinion, always accompany homology of regions.