Of all the peculiar markings which have been referred to, the most remarkable and least explicable are the horny growths or callosities on the inner sides of the legs and the backs of the fetlock-joints of horses and their allies. It has been remarked by an American naturalist " that whoever discovers the meaning of the horse's callosities will become famous among naturalists all the world over". Why so much thought and speculation has been devoted to these bodies is not at all easy to understand. They are so placed inside the forearm and at the lower and back part of the hind-leg, just below the hock and behind the fetlocks, that they are quite out of the way. They are never affected by or connected with any disease, and when they grow long enough to be unsightly, as they sometimes do, the shoeing-smith pares them down with his knife, just as he pares the sole of the foot. In size the horny structures vary from that of a hazel-nut to that of an oval mass nearly 3 inches long and 1½ inch broad in the centre in coarse-bred horses. Their shape is most commonly an elongated oval, those in the fore-legs being larger and more distinctly pear-shaped than those in the hind-legs. Some of the earlier veterinary writers - Snape (1687), Bracken (1739), Gibson (1751), Blaine, and also James White (1802) - do not mention the chestnuts, although Gibson figures them in his plates of the limbs of the horse in the fore-arm, but not in the hind-legs.
In a later edition (1832) Blaine ascribes to the chestnuts a fanciful value as adjuncts to the generative organs of the stallion, apparently disregarding the fact that they are quite as well developed in the mare. Chauveau, in his Comparative Anatomy (1873), refers to the chestnuts as little horny oval or round plates found in the horse in the inner face of the forearm, and at the upper extremity of the inner surface of the metatarsal bone. They are composed of a mass of epithelial cells, arranged in tubes like the horn of the hoof. "In solipeds," it is said, "the chestnut is the representative of the thumb.
"In fine-bred horses this horny production is much less developed than in coarser breeds. It is always smaller in the hind-limbs.
"In the hind-legs and the fore-legs we also find a similar but smaller horny mass growing from the skin in the tuft of hair behind the fetlock, and named the ergot. Like the chestnut, it bears the same relative development in fine-bred and in coarse-bred horses." This is all that Chauveau has to say on the subject.
Sir William Flower combats the view which has been very generally accepted, that the callosities are the remains of the first digits, and his argument is well worth attention. After a concise description of the skin, with its inner layer of interlacing fibres, blood-vessels and nerves, and glands and follicles, constituting the true skin, and the layer of flattened cells which form a protecting layer of insensitive structure - the epidermis or cuticle, - the author refers to the hairy coat of the horse, with its varieties of fine and coarse hairs; the "chestnuts" are described as "mallenders" and "sallenders", with the remark that they are treated as a disease by the older veterinary writers. It is true that these words are used by ancient and modern veterinary writers to indicate an eruptive affection in the bend of the knee-joints and hock-joints respectively, but the terms have never been applied by them to the horny excrescences called chestnuts or callosities.
Sir W. Flower's chief objection to the view that the chestnuts are rudimentary digits is based on the fact that in the case of the excrescences which are most constant - those on the fore-limbs - the position which they occupy on the forearm, at some distance above the knee, is quite inconsistent with the theory that they represent the thumbs.
Sir W. Flower concludes " that the callosities belong to a numerous class of special modifications of particular parts of the skin surface which occur in many animals, the use of which is in most cases remarkably obscure, Bare spots, thickened patches or callosities, and tufts of elongated or modified hair, often associated with groups of peculiar glands, are very common in various parts of the body, but especially in the limbs of many ungulates, and to this category the chestnuts of the horse undoubtedly belong."
A somewhat similar horny excrescence has already been mentioned as existing at the back of the fetlock of the horse, hidden by the tuft of long hairs which give the name feetlock or fetlock to the joint. To this excrescence, owing to its growth occasionally in the form of a spur, the term ergot is applied, and with regard to its significance Sir W. Flower suggests that it corresponds to the foot-pads of animals which walk more or less on the palm and the sole. As no one has previously offered any explanation of the uses of the horny growths at the back of the fetlocks, it will be interesting to give Sir W. Flower's description verbatim. "If we look at the palms of our own hands (which, as shown before, correspond with the hinder surface of the fore-limb of the horse below the so-called knee) we see slight prominences just behind the root of each finger and opposite the knuckles at the back of the hand, which mark the position of the joint between the metacarpal bones and the first phalanges of the digits. Over these, especially when the palm is subject to pressure and friction from hard manual labour, the epidermis is thickened. The sole of the foot presents exactly the same arrangement.
" In such an animal as a dog or a cat, in which this part of the foot comes to the ground in walking, there is a large, trilobed, prominent, bare pad, composed of a thick, fatty cushion covered with hardened epidermis, generally of a black colour. There are also smaller pads in front of this on the under surface of each of the toes, but the large one corresponds with the coalesced three middle prominences of the human palm or sole just noticed.