This structure may be correctly described as a form of hair cemented together into a dense mass, and employed to protect those parts of the body - the feet, for example - which are subject to constant attrition. In the horse there are first and most important the hoofs, which protect the internal foot, the oval-shaped bodies on the' inside of the fore-legs immediately above the knee-joint, and the inside of the hind-legs, just below the hock, described as chestnuts, and the bodies which have already been mentioned behind the fetlock-joints, the ergots. A careful examination of all these horny productions will prove that in their elementary structure they are composed of the same elements as the hair which covers the animal body. They rest upon, and are secreted by, a papillated membrane, and consist entirely of cells varying in form and character much the same as they do in the several parts of the hair, from the centre to the surface. The horn fibres or hairs passing from the coronet to the ground surface, forming the crust, and the horn fibres passing from the vascular membrane at the bottom of the foot to the ground, forming the sole, may all be resolved into epithelial cells, like those of the cuticle. Altogether, the structure of horn may be said to consist of bundles of hair closely applied to each other to form a compact covering to the parts beneath. In certain diseases, "canker" of the feet, for example, the horn on the diseased surface always appears in the form of tufts of hair, or of horn fibres, which, in consequence of the disease, have failed to adhere and form a compact structure. The study of the anatomy of the hair of different animals, as compared with the horny production, is an exceedingly interesting one, and it is perfectly easy to find the analogues in the hairs of some of the thick-skinned animals, such as the rhinoceros and the elephant, transverse sections of which, when examined by the microscope, can hardly be distinguished from transverse sections of the hoof of the horse.
Fig. 261. - Section of Hair Follicle.
Fig. 262. - Lamella; of Horn.
Fig. 263. - Transverse Section of Horn.
Fig. 264. - Longitudinal Section of Horn.