64 
MR. YOUATT’s VETERINARY LECTURES. 
spond with the superior surface of the spinal chord; and they 
occupy a central situation upon that surface. The optic nerves 
are also a palpable prolongation oj the medullary substance of 
the brain . The course is somewhat longer than that of the 
olfactory nerve, but through the whole of that course they have 
not a single anastomosis . They are also nerves of peculiar sensa¬ 
tion, and their minute ramifications are affected only by the 
particles of light. 
Peculiarities of the Optic Nerves ,— Their Decussation .—I have 
shewn you these medullary chords, winding their way over the 
crura, and meeting at the centre, and not only meeting, but 
apparently decussating—crossing. There is a certain identifica¬ 
tion of medullary matter of the two nerves, to secure the more 
perfect discharge of their function—to assist experience in cor¬ 
recting the errors which would arise from the somewhat dis¬ 
similar impressions made, or the pictures (if we so'dare to call 
them) painted on the two retinae, and to ensure perfect and dis¬ 
tinct vision. The impression made by the odoriferous particles 
of bodies on the two nostrils, or the two olfactory nerves, is, 
in a state of health, precisely the same; the vibrations of the 
air, as they reach the tympanum of the ear on either side are the 
same; but from the construction of the organ of sight, the 
picture on each retina must differ, and that very materially with 
regard to a near object. Look at any thing first with one eye 
and then with the other, and you will understand what I mean. 
This partial mingling of medullary matter in the two nerves 
was probably designed to prevent, or to remedy, the confusion of 
vision which would otherwise result. We have,however, abundant 
proof that there is no actual decussation. This brain was taken 
from a dog that had been blind in the right eye for several years. 
Observe how the nerve on the right side is shrunk, and w T hat a 
curious semi-transparent hue it has assumed. Our principal 
patient, the horse, is sadly exposed to inflammation of the eye, 
which too often destroys the sight: but there is this peculiarity 
about the disease, that when blindness is produced in one eye, 
the other is spared for a long time, and often for life. If we 
examine the brain of one of these horses, we uniformly find the 
nerve on the blind side shrunk : it obeys the law which governs 
every other part of the frame, and when it is no longer useful, it 
gradually withers away. It has been long settled, I believe, 
among human anatomists, that there is no decussation of the 
nerves. The examination of some Russian criminals determined 
the matter. Theft was in some part of that vast empire punished 
with the loss of an eye for the first offence, and death for the 
second. Some pirates who had suffered the first penalty, were 
