154 ANATOMY AND PHYSIOLOGY OF THE HORSE 5 S FOOT. 
degree of strength and durability, and these qualities are least 
perceptible at its most depressed point or centre. 
Nowhere is it so hard, however, as the wall, a circum- 
stance well known to the farrier, who invariably attacks it 
with his knife, to fashion it into an abnormal shape or 
attenuated dimensions, in accordance with some barbarous 
and unreasonable whim. 
The colour of the sole depends upon that of that wall ; if 
the latter is wholly or partially black or white, it is the same 
to a corresponding degree. It differs from the wall, never- 
theless, in having its outer surface of a lighter shade than its 
inner. 
The sole is composed of fibres like the wall, and these are 
disposed in straight lines from above obliquely downwards 
and forwards, parallel with those of the wall. 
The frog . — The horny frog ( fourchette of the French, 
hornstrahl of the Germans) and its appendage or continua- 
tion — the “ periople” — will be considered separately, though 
in reality they are but in one structure. 
The frog is a kind of duplicature in horn of the lower 
aspect of that part of the foot we have named the “ plantar 
cushion , 55 being exactly moulded on it, and thus accurately 
reproduces it in form. In shape it is a triangular, or rather 
a pyramidal body, somewhat wedge-like as it fills up the in- 
dentation in the middle of the sole and the central space 
between the bars, which it in this way unites. In this situa- 
tion it might not inaptly be compared to the keystone of an 
arch, if we looked at it as the front of the wall rests on the 
ground or on a table, though this comparison might give a 
false idea of its function. 
It is at once apparent, however, that it completes the floor 
or bottom of the hoof. 
It may be described as possessing two faces, two sides, a 
base, and a point or summit. 
The upper face is a true imprint of the lower surface of the 
plantar cushion, repeating its projections and depressions by 
corresponding cavities and prominences. For instance, it 
shows a triangular cavity, shallow and narrow in front, hut 
increasing in width and depth as it passes backwards. This 
space, at its middle, is divided by a vertical prominence into 
two channels, each of which appears to run into the cavity 
situated within the inflexure of the wall. 
The vertical eminence widens at its base, so as to occupy 
the greater part of the posterior extremity of the frog, and, 
as it rises, it becomes thinner and flatter, until at the heels it 
presents a sharp convex border, standing much higher than 
