304 DISEASES OF THE HORSE’S FOOT 
peculiarly softened. It afforded a yielding sensation to the 
finger, not unlike that which is imparted by indiarubber, 
and on cutting the altered horn it was almost as easily) 
sliced as cheese-rind. The outer surface being in this way 
slightly pared off, the deeper substance of the horn was dis- _ 
coloured by a pinkish stain. The horn of the frog was in 
-many instances found detaching from the vascular surface, 
which was very disposed to take on a diseased action, some- 
what allied to canker, and became extremely difficult to 
treat. 
Conditions such as these, although not constituting the 
disease itself, certainly lay the frog open to infection, 
especially if afterwards the animal is called upon to work 
in the mud of the streets of a large town, or to stand in a 
badly drained and damp stable. 
A further cause of thrush is to be found in the condition 
of the frog, brought about by contraction of the heels (see 
p- 118). We have already seen that one of the most promi- 
nent factors in the causation of contraction is the removal 
of the frog from the ground by shoeing, with its consequent 
diminution in size and deterioration in quality of horn. 
This leads to fissures in the horny covering, and favours 
infection of the sensitive structures beneath. Thrush is, in 
fact, nearly always present in the later stages of contracted 
foot. 
By some thrush is believed to be but the commencement 
of canker. With this, however, we do not hold. We 
believe both to be due to specific causes as yet undis- 
covered, but that the cause of thrush is not the one operat- 
ing in canker. In arriving at this conclusion we are guided 
by clinical evidence. The two conditions are quite dis- 
similar, even in appearance, and, while one is readily 
amenable to treatment, the other is just as obstinately 
resistant. 
Symptoms.—The symptoms of thrush are always very 
evident. Probably the first thing that draws one’s atten- 
tion to it is the stench of the puriform discharge. The foot 
is then picked up and the characteristic putrescent matter 
