In the ass, the ulna is a little longer than in the horse--that is to say, it descends lower; and the radius is a little more convex anteriorly.
The carpal bones are seven in number--four in the superior row, and three in the inferior. The trapezium is wanting in the latter. Sometimes, however, in certain varieties of horses the trapezium is developed, but then it is no more than a very small osseous nodule. The pisiform bone, situated at the external part of the first row of bone, is prominent posteriorly. It is of rounder form and flattened from without inwards. It articulates with the trapezium and the radius. It presents, on its external surface, a groove for the passage of the tendon of the posterior ulnar muscle, which is named by veterinary anatomists the _external flexor of the metacarpus_.
The carpus, as a whole, is of an irregularly cuboid shape; its anterior surface, slightly convex from side to side, forms the skeleton of the region of the _knee_ (wrist). The metacarpus is formed of three bones: the principal metacarpal and the two rudimentary ones.
The principal metacarpal, which forms the region of the _canon_, is directed vertically; its anterior surface is slightly convex transversely. This surface is covered by a number of tendons, which slightly alter its appearance; so that it is the principal base of this part of the fore-limb. Its posterior surface is flattened. The superior extremity of this metacarpal presents plane surfaces, variously inclined, with which the bones of the inferior row of the carpus articulate. On the anterior surface, and a little to the inner side, is found a tuberosity, which is destined for the insertion of _the anterior extensor of the metacarpus_, the homologue of the radial muscles. The inferior extremity is formed by two condyles, an internal and an external; between which is found a median crest.
This extremity, the superior extremity of the first phalanx, which articulates with it, together with two sesamoid bones--the great sesamoids--which are situated on its posterior surface, collectively form the region which from its rounded outlines is called the _ball_.
With regard to the rudimentary metacarpals, external and internal, to which some authors give the name of _fibulæ_, they are applied to the sides of the posterior surface of the principal metacarpal. They are elongated bones, of which the superior extremity, which is a little thickened, is called the _head_; the lateral bones of the second row of the carpus partly rest on the heads of these. They become more slender as they descend, and terminate opposite the inferior fourth of the principal metacarpal. Each ends in a slight swelling, to which the name _button_ has been given. The internal one is the better developed.