ANATOMY AND PHYSIOLOGY OF THE HORSE’S FOOT. 663 
The epiderm or cuticle is the outermost, insensible, and 
non- vital portion of the skin. It differs from that we have 
just noticed, besides the absence of vessels, nerves, &c., in 
being composed entirely of cells, arranged horizontally in 
irregular beds of varying thickness and density, according to 
the situation and necessities of the integument. The more 
superficial cells are very thin, small, flat, and light- coloured, 
and often so densely packed together as to form a horny 
covering; but as we pass downwards, we find them becom- 
ing more oblong, vertical, round and soft, until at length 
we reach those which have been last secreted from the 
derm, and which constitute what is sometimes known as 
the rete mucosum. The cells of this lowermost stratum of 
the epiderm are, in the horse, nearly always of a dark colour, 
owing to the presence of the pigment granules to which 
the tint of the skin is due; they also contain well-defined 
nuclei. No other structure can be perceived in this cuticle, 
save the shafts of hairs and the canals of the perspiratory and 
sebaceous glands passing through to reach the upper surface. 
In its chemical composition it is allied to hair and horn, 
consisting of “ keratin,” a substance intermediate between 
albumen and gelatine, and containing sulphur. It is the 
protector of the extremely delicate apparatus contained in the 
derm, and serves at the same time various important offices 
in connection with the sense of touch, the temperature of the 
body, and several of its secretions and excretions, and also 
acts as the chief agent in supporting contact with the ground 
and sustaining weight in a large number of animals. The 
outer or upper strata are being continually detached in scales 
or layers by wear or exposure to the weather, and replaced by 
those which are in process of formation by the derm. The 
latter is separated from the epiderm by an exceedingly fine 
fibrous expansion, which is apparently destitute of any definite 
structure (the membrana propria ) . This is of such extreme 
tenuity that it can only be clearly distinguished in early life. 
It follows the elevation and depression of the derm, forms in 
fact a covering to it, and acts as a medium to the epiderm 
for the transmission of nutrition to its cells from the layers 
of blood-vessels continuously extending beneath it. 
From the fetlock, proceeding downwards to the hoof, the 
skin possesses in every respect these essential characteristics, 
but on a magnified scale. The derm becomes much thickened, 
and its texture is denser and more resisting ; while its blood- 
vessels are largely increased, and on its surface the ultimate 
branches of the arteries form complicated networks. The 
papillse also become greatly augmented in size, the hair folli- 
