70 MR. youatt’s veterinary lectures. 
Difficulty about the Fourth Pair of Nerves. —Sir Charles Bell, 
to whom, more than to any other physiologist, we are indebted 
for our knowledge of the nervous system, considers this as a re¬ 
spiratory nerve. He does so partly from its situation, in which I 
confess I am a good deal disposed to acquiesce with him ; but I do 
not understand the function which he assigns to it, of relaxing , 
not contracting , the trachealis muscle, and thus causing the eye 
to roll upwards, under the superior lid, in every forcible effort of 
respiration, as sneezing or coughing, and so tending to the security 
and preservation of the eye. I can form no clear concep¬ 
tion of this relaxation of a muscle by the direct agency of the 
nerve that governs it; nor can I see the great danger to which 
the eye is exposed in more than usually forcible respiratory ef¬ 
forts, nor a purpose sufficiently important to lead to the introduc¬ 
tion of a separate nerve, and from so singular an origin. I must 
therefore, at present, class it with the nerves of voluntary motion ; 
I know it to be a voluntary motor nerve as it regards the usual 
action of the superior oblique muscle. There is, however, some¬ 
thing about this nerve which I do not understand, and w r ith which 
I am not quite satisfied. I shall enter a little more into this, 
and you will be better prepared for the inquiry, when the muscles 
of the eyeball come in order before us. 
The AbducenteSy or Sixth Pair of Nerves. —This nerve arises 
from a transverse sulcus on the medulla oblongata, between the 
pons varolii and the proper motor origin of the seventh pair ; and 
it is clearly referable to the central column of this prolonga¬ 
tion of the spinal chord. Therefore we find it on the inferior 
surface of the spinal chord, and springing from its central column ; 
and when we look more attentively at it, we can trace the Jibri - 
culi springing in a line from which it derives its origin, and uniting 
to form it. We trace it to the cavernous sinus, and through the 
foramen lacerum into the orbit, where it divides : some branches 
of it are distributed over the retractor muscle, a muscle pe¬ 
culiar to quadrupeds, answering the purpose of hands in with¬ 
drawing the eye from danger, if that danger cannot be warded oil; 
but the main bulk of the nerve goes on to the rectus externus or 
the abductor of the eye, by means of which the eye rotates 
outwardly: this branch is also a kind of antagonist to the rectus 
interims and the superior oblique. Then I can clearly under¬ 
stand that this is a nerve of voluntary motion . 
Its Anastomoses with the Great Organic Nerve .— There is a 
peculiarity about it, however, which I do not understand. There 
are some minute anastomoses with the other motor nerves of the 
eye , and with the ophthalmic branch, that I can comprehend ; 
but there is a branch, and a very considerable one, given.to each 
