450 THE HORSE. 
on each side: there are the bars, which in the natural foot appear as 
sharpened prominences, extending from the heels into the centre of the 
foot, between the sole and the frog, and which are useful as buttresses, 
supporting the crust from being crushed inwards by the superincumbent 
weight. The sole is the plate at the bottom of the foot, which should be 
slightly concave downwards, and is fixed to the inner edge of the crust, and 
the outer sides of the bars, and not to their lower surfaces. Its usual 
thickness is about one-sixth of an inch, but it will vary greatly in different 
horses, and it is thicker where it runs back between the bars and the 
crust. It is secreted in plates, which can readily be separated with a 
knife in that direction. The frog is the prominent, triangular, and elastic 
substance, which fills up the space between the heels posteriorly, the bars on 
each side, and the sole in front. In the middle is a longitudinal fissure, 
called the cleft, the sides of which should form an angle of about forty-five 
degrees. In front of this cleft is a solid wedge of the elastic horny sub- 
stance, constituting the frog, which lies immediately beneath the navicular 
bone, and has received the name of the cushion. Posteriorly it is spread 
out into a thin band on each side which covers the bulbs of the heels, and 
passes round the upper part of the wall constituting the coronary frog- 
band of Bracy Clark, which is continuous with the coronary substance. 
The: structure of the horn which forms these three divisions, varies a 
good deal. In the crust it is fibrous, somewhat resembling whalebone 
in this respect, but not quite so hard; these bristly fibres are united 
by a gelatinous substance, but they are arranged so as to lie in straight 
lines descending from the coronary circle to the ground. The wall may, 
therefore, be considered as composed of hairs agglutinated together, 
and each secreted by one of the vill, which are so thickly spread over 
the surface of the coronary circle. The sole is also fibrous, but not 
nearly so much so as the wall; and the fibres are not arranged in so 
parallel a manner, taking rather an oblique direction from behind 
forwards, and being more easily separated into scales. The frog differs 
from both, in possessing finer fibres and in smaller quantity, in com- 
parison with the gelatine, which formation renders it more soft and elastic 
and also more prone to decomposition. ‘The horny matter is sometimes 
coloured a greyish brown, sometimes white, and sometimes marbled by a 
mixture of the two colours. (These parts are shown more clearly in the 
article treating of Shoeing in the 32d Chapter.) 
THE nor is developed by secretion, which has its seat in the coronary 
substance and lamin. It consists in a pouring out on their surface of a 
plasma, in which rounded cells develop themselves, in correspondence 
with the villi from which the secretion is poured out. These cells are 
arranged in layers, corresponding with the secretory surface. In the crust 
this growth takes place from the superior border to the inferior, but in the 
sole and froe, from the internal surface to the external. This growth is 
constant through the life of the animal, and it would give the hoof an exces- 
sive development if it were not either for the wear of the soil in the unshod 
horse, or of the action of the smith’s knife in the shod one; but the increase 
of the wall being solely from above downwards, it does not require any re- 
duction on its external surface. The coronary substance, sometimes called 
the coronary ligament, is a fibro-cartilaginous band intervening between 
the skin of the leg and the hoof, covered with cuticle externally, and 
with villi, which form a secretory surface on the edge towards the hoof. 
It is most liberally supplied with blood, as we shall presently see, and is 
attached to the upper part of the coffin bone and extensor tendon by 
