TUMOUR IN THE LATERAL VENTRICLE. 
21 
that accompanies it, whether or not the phenomena are pre¬ 
sented on one or both sides of the body. Numerous 
instances have occurred in which the pupil of one eye was 
dilated, and impaired volition, or paralysis, and insensibility 
existed on the opposite side of the body to the lesion in the 
brain, particularly at the outset of the disorder; and 
frequently, later in the case, both eyes and both sides have 
become equally affected. The pupils are usually dilated in 
cases of congestion, extravasation of blood, effusions of 
serum, and from the pressure of tumours ; and, when this is 
the condition of one pupil only, it is very significant of the 
opposite side of the brain being the seat of the lesion. But 
it is far from being a constant circumstance. Cases will 
occur when the same lesions exist, and in the same parts of 
the encephalon in which the pupils are contracted, or one 
contracted and the other dilated ; and both pupils have been 
observed to be contracted in cases where dissection has pre¬ 
sented considerable lesion and extravasation. The pupils 
have been observed unaltered,—neither dilated nor con¬ 
tracted,—though the pressure of an abscess in one hemis¬ 
phere of the cerebrum was sufficient to produce loss of 
vision, insensibility, and paralysis of the extremities. 
Uneven pressure upon the brain is probably the cause of 
dilatation of one pupil and contraction of the other. The 
contraction of the pupil is produced by a branch of the 
third pair of nerves acting on the circular muscular fibres of 
the iris, and though the radiant fibres dilate the pupil, 
without doubt, from whence they derive their action is by 
no means clear ; therefore it is a little difficult to say what 
the altered conditions of the pupil are owing to, under the 
influence of encephalic lesions. We know that the pupil 
dilates and contracts with the decrease and increase of light, 
in the healthy state of the eye, with an adjustment, exact¬ 
ness, and precision, quite remarkable ; but the altered condi¬ 
tion of the pupil, under morbid influence of the brain, is an 
action of the iris, independent of light, having no relation 
with the faculty of vision. Vision may be seriously im¬ 
paired, the animal unable to see, without any lesion of the 
visual organ whatever ; without any paralysis of the optic 
nerve, and entirely independent of it; purely from a fixed 
position of the iris. The diagnosis from altered volition, 
convulsions, and palsy, I consider less to be depended upon 
than the size of the pupil, in the horse. The muscles of all 
the extremities are affected full as frequent when the disorder 
is on one side of the brain, as one or both extremities of one 
side; nevertheless, when the muscles of one side are exclu- 
