66 
MR. YOUATTS VETERINARY LECTURES. 
but the optic nerve is an exception to this rule, for the dura 
mater continues to encircle it until it reaches the outer or 
sclerotic coat of the eye. This, according to some, is for the 
purpose of forming the sclerotic coat, which certainly bears con¬ 
siderable resemblance to the dura mater; while the pia mater, 
according to the same anatomists, is expanded within the globe 
of the eye. More of this when we treat of the structure of the 
eye: at present I will only remark, that the difference in situation 
and length of course, between this and the other two nerves of 
peculiar sensation, and the difference in function too, will 
sufficiently account for this prolongation of the dura mater. It 
is necessary that the impression or intelligence conveyed by a 
nerve of peculiar sensation should be distinct and pure. The 
olfactory, the moment it quits the cranium, ramifies upon the 
membrane to which it is destined. We shall presently see that 
the auditory nerve is defended by a bony canal until it arrives at 
the labyrinth of the ear. Nothing can cause confusion in the 
impressions which are conveyed along these nerves ; but the 
optic nerve has to travel through the orbit before it reaches its 
point of destination, and is encircled by muscles of strong and 
rapid action : this dense covering, therefore, is necessary to pro¬ 
tect it from that compression which might occasionally interfere 
with the perfect discharge of its function. 
The Optic Nerve in Birds .—The structure of this nerve is sin¬ 
gular in birds. The fibrous conformation is evident; but these 
fibres are curiously collected into laminae in birds of prey, and I 
think I have seen something like it in a few of those which we 
have partially domesticated, and particularly in the pheasant. The 
nerve cannot be dissected, as in the quadruped, into a collection 
of fibres more or less numerous and complicated, but into a con¬ 
geries of leaves; it seems like the folding of one continuous 
membrane, and the retina is the expanding of the plaits. We 
can connect this, as we shall have to do many other peculiarities 
in the eye of the bird, with the almost inconceivable acuteness 
of sight in them and in birds of prey particularly, of which natural 
history affords us a thousand proofs. 
The Auditory Nerve. —This nerve is the eighth in order, reckon¬ 
ing anteriorly posteriorly. English anatomists have termed it 
the portio mollis of the seventh pair : I will venture, following 
the example of most continental anatomists, and for reasons that 
shall be stated hereafter, to call it simply the auditory or eighth 
pair of nerves. 
Its Character as a Nerve of peculiar Sensation .—I find it on the 
floor of the fourth ventricle, which is not only a prolongation 
but may almost be said to be the superior surface of the spinal 
