REVIEWS. 
45 G 
are called sandcracks. The wall of the hoof is divided into two layers, an 
external and an internal. Taking up a black hoof which has been freed from 
all its adherent parts, it will be found to be white within ; and this white 
part will likewise be found to be much less hard than, and about a quarter 
the thickness of, the coloured portion. The hard black external layer of 
bora is secreted by the coronet ; the internal white and soft layer is secreted 
by the sensitive laminae, and it is thrown into projections which exactly 
fitted into the folds of the secreting surface. The wall is also divided into 
toe, heel, and quarters. The quarters are the lateral parts of the walls ; the 
more forward and thickest portion of horn is called the toe ; the more back- 
ward and thinnest, the heels. In the usual formation of feet, the inner side 
of the hoof is observed to be weaker than the outer ; and as the inner side 
receives most weight, this proves the care taken to secure elasticity in the 
horse’s movements. The horn yields to pressure, and the thinnest, of course, 
yields most. The density of the hoof materially decreases as it approaches 
the heels, evidently to favour the descent of the elastic frog, which likewise 
favours elasticity of motion. The superior marginal part of the hoof is 
softer and thinner than the rest ; and if our eyes be directed inward, we 
shall perceive an indentation called the conorary ring, within which the 
conorary ligament once reposed ; the thin and soft part is horn just secreted, 
and in a single layer ; whereas where the horn is thicker, the conorary liga- 
ment is more bulky, for the horn is secreted in several layers. 
“ To finish our description of the hoof, we must turn our attention poste- 
riorly; where we shall find the laminae inflicting themselves abruptly inwards 
and downwards, forming what are termed the bars ; which are nothing more 
than continuations of the hom of the heels, stimulated and thickened by 
pressure. So also the frog is little else than an adaptation of the coronet 
turned downward, and forced to bear a certain amount of weight. 
“ The horny sole is not so brittle as the crust ; it is thicker at the circum- 
ference, where it unites with the outer wall of the hoof, than it is towards 
its centre. Its concavity enables the foot to retain its situation upon the 
ground ; while it forms an arch which yields by its elasticity, thus affording 
an admirable spring. The horny frog is the triangular portion that fills up 
the natural excavation of the sole. This horny frog swells out, but not so 
much in general cases as to extend beyond the level of the outer surface of 
the crust ; it however is sufficiently prominent in every healthy foot as to 
receive secondary pressure. An inferior view of the foot will show 
that the frog presents a distinct mark of separation between it and the other 
part, which is called its commissure ; wherefore connexion is principally kept 
up by an internal prolongation of the secretive substance of the coronet. 
The horny frog is intimately connected to the sensitive frog, of. which last it 
is indeed the secretion. The frog is divided, or indented with a deep fissure, 
which is called its cleft, within which are numerous sebaceous glands. It is 
these glands, when diseased, which pour forth a stinking fluid which rots 
the horn, and is known by the name of thrush. The sensitive part of the 
secretive frog is covered by the elastic frog. 
“ Horn is entirely unorganized, and when once divided can never again 
unite like flesh. It does not bleed when cut. It has no feeling when a 
knife is drawn across it. It lias none of the properties of living substance, 
and consequently cannot be expected to possess the reparative function that 
appertains to living matter. Horn, although a secretion, and as such pretty 
much the same, is not all of the like hardness. The horny frog is the least 
hard, and the most pliable ; the horny sole ranks the next in these qualities. 
Then come the horny heels ; afterwards the quarters and bars ; and lastly, 
as the most hard, the toe. The inner crust is throughout softer than the 
