Fig. 54.—Tarsus of the Horse: Left Posterior Limb, Anterior Surface.
1, Tibia; 2, internal tuberosity of the inferior extremity of the tibia (homologue of the internal malleolus of man); 3, external tuberosity of the inferior extremity of the tibia (homologue of the external malleolus); 4, median crest lodged in the groove of the pulley of the astragalus; 5, pulley of the astragalus; 6, internal tuberosity of the astragalus; 7, calcaneum; 8, cuboid; 9, scaphoid; 10, great cuneiform, the small cuneiform is placed behind this latter; 11, principal metatarsal; 12, external rudimentary metatarsal. The internal rudimentary metatarsal, being more slender than the external, does not appear in the figure.
The bones of the tarsus are six in number: the calcaneum and astragalus form the upper row; the cuboid, scaphoid, and two cuneiforms form the lower ([Fig. 54]).
The astragalus has not, as in ruminants, an inferior trochlea for articulation with the scaphoid; this portion of the bone presents a surface which is slightly convex. It articulates with the tibia by a trochlea that occupies not only the superior surface, but also the anterior. This trochlea, which is directed slightly obliquely downwards and outwards, has a very pronounced form; its lips, which are extremely prominent, determine by their anterior part one of the features which we recognise on the anterior aspect of the ham—a feature which is still more accentuated when the metatarsus (canon) is extended on the leg. On the internal surface of the astragalus is found a tubercle, which forms a projection in the corresponding region of the ham.
The calcaneum, which is not quite so long as that of the ox, forms by its summit a prominence which is called the point of the ham.
The cuboid is small; the scaphoid is large, and flattened from above downwards. Of the two cuneiforms, the more external is the larger; it closely resembles the scaphoid; it is flattened from above downwards as is the latter; but it is a little smaller in size. The small cuneiform, which occupies the inner side of the tarsus, is the smallest bone in this region; it is sometimes divided into two parts; this raises the number of the cuneiforms to three, and that of the bones of the tarsus to seven.
The bones of the metatarsus and the phalanges are equal in number to the corresponding bones in the anterior limbs; they are formed on a type analogous to that of these latter. Accordingly, we shall merely indicate the differences which characterize them.
The principal metatarsal is longer than the metacarpal of the same class; its shaft is more cylindrical; its inferior extremity is somewhat thicker. The external rudimentary metatarsal is better developed than the internal; in the metacarpus the reverse is the case.
The phalanges so far resemble those of the anterior limb that, as differential characters, we need point out only the following: the first phalanx of the hind-foot is a little shorter than that of the fore-foot; its inferior extremity is a little narrower, and its superior extremity a little thicker. The second phalanx is a little less expanded laterally.