RESEARCHES INTO THE CAUSES OF BLINDNESS* 345 
have just been describing. The external symptoms are so trifling, 
that they escape on a superficial examination, and are rarely ob- 
served by those who have the care of the animal. The eye is 
very sensitive to light. In the stable it is always partially 
covered, but, as soon as the horse is brought into daylight, the lids 
are lowered over the globe of the eye. The same occurs when 
any attempt is made to approach the hand towards the orbital 
region. There is no perceptible inflammation of the conjunctiva : 
if it exists at all, it is merely of an increased irritation of the 
lachrymal apparatus. Hence we perceive that the internal oph- 
thalmia, which usually terminates in blindness, commences in 
two different ways : — in the first, with an alteration or perversion, 
if we may so call it, of the function of the secretive tissue of the 
aqueous humour, followed by inflammatory symptoms in the 
accessory organs of the eye : in the second case there is little or 
no exterior irritation — all is concentrated in the lower parts of the 
globe of the eye. We shall refer to each of these two varieties 
separately ; for, although the same in essence, they present great 
differences in their symptoms and progress. 
First variety . — The inflammation of the conjunctiva and the 
flow of tears continue for eight or ten days with an unvaried in- 
tensity, yet never becoming so bad as to cause the owner of 
the animal to deem it necessary to discontinue working him. 
About this period the symptoms assume a new character ; the 
secretion of tears becomes less abundant, but there is still some 
discharge from the eye, and it is exceedingly sensitive of light as 
well as of the approach of external objects. The opacity of the 
cornea is less decided ; the aqueous humour is simply strabismus 
instead of being clouded — the iris no longer reflects those various 
colours as in the healthy animal — the pupillar opening, although 
still capable of motion, no longer possesses its natural contractile 
power — the animal continues to see, but his vision is obscure and 
indistinct, and he is easily frightened by any motion near the 
eyes. 
Such is the progress of this disease during the thirty-five or 
forty days that follow its commencement. Sometimes, however, 
each symptomatic appearance is so changed, that it is easy to 
see it does not pursue one invariable course. In horses harnessed 
to heavy carriages a violent inflammation of the conjunctiva is 
observed, accompanied by the swelling of that mucous membrane 
termed chemosis — excoriation of the edge of the eyelids, loss 
of the eye-lashes, and ulcers on the cornea. These acute symp- 
toms are not the consequences of the disease itself, but in the 
generality of cases, if not always, they proceed from the animal 
having been badly treated or from other incidental causes. 
