ANATOMY AND PHYSIOLOGY OF THE HORSE’S FOOT. 703 
pushed downwards by that incessantly thrown out from the 
coronary zone, and that the Malpighian layer of cells is also 
carried in the same direction, until, reaching the sole, it 
assists in forming the white line described as uniting that 
portion of the hoof to the wall. 
Such, then, is the simple and definite part played by the 
podophyllse in health, as revealed by observation and experi- 
mentation. In extraordinary circumstances, however, whether 
arising from disease or accident, their role is certainly not of 
this passive character. 
We have just now alluded to the fact, that when these leaves 
are denuded of their protection — the wall — they are quickly 
covered by a quantity of epidermic matter thrown out from 
their surface, which effectually shields them from harm, though 
it is far from possessing the ordinary characteristics of the horn 
which has been removed, and may be said to be of a provi- 
sional character. In such a case the podophyllae have brought 
into play the faculty which they, in ordinary physiological 
conditions, had held in reserve, and suddenly, under the 
irritation induced by the denudation, commence to secrete as 
much horn as may serve to repair the breach, and preserve 
them at least for the time being. This reserve faculty, of 
course, is once more held in abeyance when no further injury 
is done to the podophyllge, and they have returned to their 
usual normal condition. In certain diseases of the foot, when 
this latent keratogenesis of the vascular laminae is brought 
into activity, the most serious consequences ensue. Conges- 
tion or subacute inflammation of these leaves, if not properly 
controlled or suppressed, brings into operation their secretory 
power, just as it was manifested when they were experiment- 
ally or accidentally laid bare. But now we have the rigid 
and resisting wall covering them, and the new horn thrown 
out from their surface being continually added to the kera- 
phyllse, these become enlarged, and lead to displacement of 
the pedal hone and deformity of the hoof, particularly towards 
its anterior portion. This supplementary and, as we might 
term it, accidental horn, instead of being useful as a protec- 
tion to the sensitive parts, as in the instance wherein the 
wall has been removed, becomes a source of real injury, and 
constitutes one of the very gravest complications with which 
the veterinary surgeon has to contend. Only too frequently, 
under certain conditions, the secretory action thus set up is 
continued more or less energetically during the lifetime of the 
animal, and, like the coronary cushion, the vascular leaves 
become a permanent source of concressible horn, which is 
added to the hoof leaves as soon as it is exuded ; thus dimi- 
