34 AN ESSAY ON CHRONIC PODOTROCIIOLITIS. 
two, a triangle with unequal sides would be described. It may be 
admitted as a general rule, that this kind of contraction of the wall 
almost invariably occasions partial contraction of the sole and frog; 
for, accordingly as the transverse diameter of the plantar surface of 
the hoof diminishes, the arch of the sole and the elastic property 
of the frog ought to increase. This form of sole causes the crest 
of the frog not only to predominate over its superior surface, but 
also to describe a perfect curve, which shrinks away, in consequence 
of the transverse diameter of the sole. According to the laws of 
mechanics, the tension of the curve increases towards its summit, 
while at the base it entirely loses the power of dilatation; and when 
the lower parts of the quarters incline towards each other, they act 
as opposing powers, and considerably diminish, if not altogether 
annihilate, the dilatative power of the hoof. 
The consequences of these alterations are — lateral compression of 
the ligamentous apparatus and bones of the foot, contraction of the 
laminae of the quarters, and a consequent diminution of their 
elasticity ; for of two cords equally stretched, the longer one will 
always communicate a greater amount of impulsive power than 
the shorter one. Although the coffin-bone takes on all the various 
forms of the hoof, the ligamentous apparatus is not less compressed 
from below, and forced against the perforans by the arching of the 
sole ; the fatty matter deposited in the cells of this apparatus dis- 
appears, and thus is the elasticity deprived of another of its 
elements. If to these circumstances be added the thickness and 
hardness of the sole, the loss of elasticity in the frog consequent 
upon its ascent, and the lateral compression which it undergoes ; 
if we come to reflect that the faculty of yielding is lost to both 
these parts when they are compressed from above, it becomes evi- 
dent that, the weight of the body falling upon the navicular bone 
and the tendon, these organs must undergo a double amount of 
pressure, on the one part acting from above downwards, on the 
other from below upwards. This latter force must naturally con- 
tuse the perforans and the navicular, especially where accessory 
circumstances concur to increase its intensity *. 
All unnatural pressure upon the navicular bone, whether coming 
from above or below, tends to increase the friction of the tendon 
upon this bone, and is liable to create podotrocholitis. To oc- 
casional causes belong the rapid and prolonged paces of saddle 
horses, leaping of ditches, gates, hedges, &c., sudden springs dur- 
ing a hard trot; galloping upon rough uneven ground, or upon 
* I have never yet seen chronic podotrocholitis in fiat feet furnished with 
an abundant frog. Nor am I the only person who has made this observa- 
tion, which, in itself, tends to prove that partial contraction of the sole and 
frog plays a very important partin the affections now occupying our attention. 
