436 DISEASES OF THE HORSE’S FOOT 
most marked. This at first sight appears to flatly con- 
tradict what we have said with regard to frog-pressure in 
other portions of this work. With this, however, must be 
reckoned other predisposing causes. In this case it is not 
to frog-pressure alone we must look, but to the condition 
of the frog itself, and that of the neighbouring parts. 
It is when we have a frog which, though well developed 
and apparently satisfying all demands as to size and build, 
is at the same time composed of a hard, dry, and non- 
yielding horn that we must look for trouble. 
‘The foot predisposed to navicular disease is the strong, 
round, short-toed or clubby foot, open at the heels, with a 
sound frog jutting prominently out between them. Here is 
a frog exposed to all the pressure that might be desired for 
it, bounded at its sides by heels thick and strong, and in- 
disposed to-yield, and itself liable, from its very exposure, 
to become, in the warm stable, hard and dry, and incom- 
pressible’ (Percival). . 
Here, instead of acting, as normally it should, as a re- 
silient body, and an aid to the absorption of concussion, it 
seems rather to play the part of a foreign body, and to bring 
concussion about. Seeing, then, that the navicular bursa 
is in very near contact with it, it is conceivable that this 
joint-like apparatus should suffer, and the pedal articula- 
tion be left unaffected, the more so when we take into con- 
sideration the compression theory just described. 
4. A Weak Navicular Bone-—When the disease com- 
mences first in the bone—and there is no denying the fact 
that sometimes, although not invariably, it does—it may be 
explained by attributing to the structure of the bone an 
abnormal weakness in build. 
The navicular bone consists normally of compact and 
cancellated tissue arranged in certain proportions, the com- 
pact tissue without, and the cancellated within. These 
proportions can only be judged of by the examinations of 
sections of the bone, and when it is found in any case that 
the cancellated tissue bulks more largely in the formation 
of the bone than normally it should, we have what we may 
