320 DISEASES OF THE HORSE’S FOOT 
this purpose it is best used as a dry powder. Under this 
dressing any remaining spot of canker is readily detected 
by the wet condition of the calomel when the shoe is 
removed the next day. In dealing with such a spot, a very 
good plan, after all apparently diseased tissue has been 
excised, is to touch the cankered part with solid nitrate of 
silver, or a feather dipped in one of the strong mineral 
acids, and then reapply calomel over the surface. The 
result of this treatment is frequently very gratifying. 
‘In successful treatment the shoe must be removed each 
time—an adjustable plate will not do, as no man can 
thoroughly pare and examine a foot with the shoe on, and 
imperfect dressings are worse than useless. Indeed, it is 
better not to pare or thin the horn at all, than to imper- 
fectly pare, since canker, if undestroyed, develops far more 
rapidly under thin horn than under thick. 
‘In conclusion, I would again urge the necessity, at the 
very first operation, when the horse is down, of removing 
every single particle of the diseased tissue, either by excision 
or effectual cauterization, but at the same time taking very 
great care to guard against the latter being too destructive. 
The cautery should be laid aside as soon as the tissue 
cauterized ceases to burn white. The moment at which the 
canker has thus been eradicated without destroying sound 
tissue is indicated by the appearance of healthy horn, by 
the intimate union of that with the secreting surface, and 
by the healthy aspect of the exuded blood when paring has 
been carried to the quick. 
‘Should subjacent healthy structures be destroyed during 
the process, that is shown by the production of a raw sore, 
or of a sore to which a “‘sit-fast,” coextensive to the injury, 
is firmly attached. This seriously retards recovery. The 
secreting surface having been destroyed, no new horn can 
be produced directly from the part, and a new secreting 
surface and new horn have now to grow inwards from the 
surrounding undestroyed tissue, and that is a slow process. 
At the same time, on the principle of choosing the least of 
two evils, practical experience teaches that. it is better to 
