ANATOMY AND PHYSIOLOGY OF THE HORSE^S FOOT. 319 
In its chemical characteristics, horn appears to be identical 
with epidermis, hair, wool, feathers, and whalebone, in 
yielding “keratin,” a substance intermediate between albumen 
and gelatine, and containing from 60 to 80 per cent, of 
sulphur. Keratin is insoluble in water, alcohol, and ether, 
but dissolves readily in caustic alkalies, sulphuric acid, and 
concentrated acetic acid. 
The minute structure of the hoof has been a fertile source 
of conjecture and discussion for the last hundred years ; and 
it is but recently that microscopic research has been able to 
settle many of the problems connected not only with its 
histological anatomy, but its origin and growth. 
By some it has been looked upon as a mass of hairs 
agglutinated into a dense envelope ; others have compared it 
to a flexible bone ; while others, again, have seen nothing in 
it but an exaggerated and somewhat modified continuation of 
the epidermis. The settlement of this question is of much 
moment in a pathological point of view, as many diseases of 
the foot, now obscure in their nature, and therefore liable to 
be treated empirically, cannot be properly understood until 
the structure and organisation of the hoof are ascertained. 
In the first place, it will be admitted that the hoof, being 
hard, insensible, destitute — it may be said— of blood-vessels, 
and intended for the most part only as a defence or protection 
to the highly organised structures lodged within it, cannot be 
included among the vital textures, and, from its situation, 
chemical composition, and constitution, must be classed with 
the epidermic tissues. 
We have shown that the different parts of the hoof are all 
composed of an aggregation of fibres disposed parallel to each 
other, and running in the same direction as the axis of the 
foot. This fibrous texture is shown in various ways in every 
day experience ; certain diseases, for instance, make it par* 
ticularly manifest, and irregular wear of the lower margin of 
the wall disunites these fibres to a certain extent, and they 
become tufty and hair-like. 
But we have also noticed that one aspect of the hoof — that 
next the living surface — is perforated by innumerable circular 
apertures in the wall, periople, sole, and frog; that each of 
these apertures receives a villus or papilla, multitudes of 
which stud the tegumentary surfaces corresponding to this 
aspect ; that every one of these canals, with its filamentous 
tuft of blood-vessels, is the commencement of a fibre ; and 
that therefore the hoof is composed of tubular fibres. In 
this respect, then, it differs from the epidermis or cuticle^ 
which is simply, as we have seen, an aggregation or stratifica- 
