306 DISEASES OF THE HORSE’S FOOT 
With restoration, so far as is possible, of the frog functions, 
and with careful dressing, a cure is nearly always obtained. 
6. CANKER. 
Definition.—Under this unscientific, yet expressive term, 
is indicated a chronic diseased condition of the keratogenous 
membrane, commencing always at the frog, and slowly 
extending to the sole and wall, characterized by a loss of 
normal function of the horn secreting cells, and the discharge 
of a serous exudate in the place of normal horn. 
Causes.--The exact cause of canker has still to be dis- 
covered. Therefore, before expressing an opinion as to 
what the probable cause may be, we may state here that 
such opinion can only be based upon clinical observation. 
Such being the case, we are almost duty bound to give the 
views of older authors before those of more modern writers. 
From the mass of material ready to hand we may select 
the following as serving our purpose. 
The earliest opinion appears to have been that canker, as 
the name indicates, was of a cancerous or cancroid nature. 
This was also believed by Hurtrel D’Arboval, who looked 
upon canker as carcinoma of the recticular structure of the 
foot. The same theory we find enunciated in the Veterinary 
Journal so late as 1890. Although the word ‘ cancer’ or 
‘carcinoma’ is not there used, the author employs the 
terms ‘Papilloma’ and ‘Epithelioma’ with the evident 
intention of expressing his belief in the malignant nature of 
the disease. 
Another early opinion was that the disease was a spreading 
ulcer, gradually extending and changing the tissues which 
it invaded. . 
A further early theory, and one which if not still believed 
in, has died a hard death, is the constitutional theory. This 
was believed in by nearly all the older writers, and is 
mentioned so late as 1872 by the late Professor Williams. 
In his ‘ Principles and Practice of Veterinary Surgery,’ he 
says: ‘Canker is a constitutional disease due to a cachexia or 
habit of body, grossness of constitution, and lymphatic 
