We have still to describe, in connection with the horse, some epidermic tissues, which are known as _chestnuts_. The chestnut is a small, horny plate which is found on the internal surface of each of the limbs, at a level differing on the anterior from that of the posterior ones. On the anterior limbs the chestnut is situated on the internal surface of the forearm, towards the middle part, or the inferior third of this region. On the posterior limbs it is developed on the back of the superior extremity of the internal surface of the canon, towards the inferior part of the ham--that is, the tarsus. [Illustration: FIG. 105.--FOOT OF THE OX: LEFT SIDE, ANTERO-EXTERNAL VIEW. 1, Internal hoof; 2, external hoof; 3, internal surface of this latter; 4, internal spur.] Inasmuch as some authors consider the chestnuts as being vestiges of the thumb and the great-toe, we propose giving a mnemonic which will enable us to remember their situation, or, rather, their difference of level. If we consider that the thumb, in the human species, is longer than the first toe, we may easily remember that the chestnut is placed higher in the anterior limbs than in the posterior ones. Indeed, if we suppose a digit taking its origin at these points, it will be longer in front (the thumb) than behind (the first toe). =Hoofs of the Ox and the Pig.=--The ox has four hoofs on each foot--two which contain the third phalanges, and two others, rudimentary, situated at the posterior aspect of the limb, at the level of the inferior part of the canon; these latter bear the name of _spurs_. We will occupy ourselves especially with the former (Fig. 105). Each of the hoofs presents three faces which, if we consider them in relation to the median axis of the limb to which they belong, are: external, internal, and inferior. The external surface resembles the wall of the hoof of the horse. The internal surface is slightly concave from before backwards, so that the external and internal hoofs of the same foot are not in contact with each other, except by the extremities of this surface, and that an interval separates them between these two points. The inferior surface, slightly depressed, ends behind in a swelling produced by the plantar cushion, which covers a thin lamina of horn. At the anterior part of the hoof these three surfaces unite in forming a well-marked angle which, on account of the concavity of the internal surface, is slightly curved towards the axis of the foot. The pig has also four hoofs--two for the great digits and two for the lateral digits. They recall those of the ox. CHAPTER IV PROPORTIONS Inasmuch as we have taken for granted, in connection with the present volume, that before entering on the study of the anatomy of quadrupeds the reader was prepared for it by a sufficient knowledge of human anatomy, it is quite natural that we should extend the same supposition to the study of proportions.