We can easily recognise the trochleæ which we have been discussing, in the little bones which children use ‘to play at bones’; these bones are no other than the astragali of sheep.
We have already mentioned that the scaphoid and the cuboid are ankylosed; they form by their union an irregular bone, on which the astragalus and calcaneum are supported.
The cuneiforms articulate with the internal half of the superior extremity of the principal metatarsal; the external half of this metatarsal articulates with the portion of bone which represents the cuboid.
The metatarsus is represented by a principal metatarsal, formed by the coalescence of two metatarsals; we also find in this region a very small rudimentary metatarsal.
The metatarsus is a little longer than the metacarpus; its transverse measurement is a little less; on the other hand, it is a little thicker in antero-posterior direction; from these two differences it results that the body of the metatarsus is quadrilateral, whereas the metacarpus presents only an anterior and a posterior surface.
The rudimentary metatarsal is a very small roundish bone, situated at the back of the superior extremity of the principal metatarsal.
The phalanges closely resemble those of the anterior limbs; nevertheless, the first and second phalanges differ from the latter in the fact that they are a little longer and narrower.
At the back of the metatarso-phalangeal articulations, as in the corresponding region of the anterior limbs, are found the sesamoid bones. Such also exist at the articulations of the second and third phalanges.
Unguligrades: Horse ([Fig. 40], p. 64).—The pelvis of the horse presents a general form which sharply differentiates it from that of the carnivora; in fact, the ilium is twisted in such a way that the external iliac fossa does not look outwards, but upwards. It results from this twist that the anterior iliac spine, which we have seen to be directed downwards in the carnivora, has become external; and this prominence is much farther removed from the vertebral column than in the dog or cat. On the other hand, the posterior iliac spine, which is directed upwards in the carnivora, has become internal; it is also placed nearer to the vertebral column, with the result that the distance which separates this spine from that of the bone of the opposite side is proportionately less.
The internal iliac spine, which is conical in shape, and curved upwards, forms a prominence known as the angle of the crupper; the external iliac spine, thick and provided with tuberosities, forms a clearly-defined prominence; this is the angle of the haunch.