f a
serous exudate in the place of normal horn.

_Causes_.--The exact cause of canker has still to be discovered. 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 temperament.' This, we
believe, is credited to-day by some, and yet, quite 100 years before the
date of the 1872 edition of Williams's work--in 1756, to be exact--we find
a veterinary writer when talking of grease (a disease, by-the-by, very
closely allied to canker) exclaiming against this habit of referring
everything which we do not rightly understand to some ill-humour of the
body. The wisdom his words contain justifies us in giving them mention
here. 'It is a very foolish and absurd Notion,' he says, 'to imagine a
Horse full of Humours when he happens to be troubled with the Grease. But
such Shallow Reasoning will always abound while Peoples' Judgments are
always superficial. Therefore, to convince such unthinking Folks, let them
take a thick Stick and beat a Horse soundly upon his Legs so that they
bruise them in several Places, after which they will swell, I dare say,
and y