corpus striatum
and the base of the
corpus callosum
. They are prolongations of the medullary substance of the central portion of the brain. They are the largest of the cerebral nerves. Their course is exceedingly short; and they have not a single anastomosis, in order that the impression made on them may be conveyed undisturbed and perfect to the brain.
The olfactory nerve is a prolongation of the substance of the brain, and it abuts upon the cribriform bone, of which mention has been made. I will not speak of the singular cavities which it contains, nor of their function; this belongs to the sensorial system: but its pulpy matter has already been traced to the base of the ethmoid bone, and the under part of the septum, and the superior turbinated bone. Although we soon lose it in the mucous membrane of the nose, there is little doubt that in a more filmy form it is spread over the whole of the cavity, and probably over all the sinuses of the face and head. It is, however, so mingled with the mucous membrane, that no power of the lens has enabled us to follow it so far. It is like the
portio mollis
of the seventh pair, eluding the eye, but existing in sufficient substances for the performance of its important functions.
We have frequent cases of
ozæna
in old dogs, and sometimes in those that are younger. The discharge from the nostril is abundant and constant, and sometimes fetid. The Schneiderian membrane, of more than usual sensibility in this animal, is exposed to many causes of irritation, and debilitated and worn out before its time. Pugs are particularly subject to ozæna. I scarcely ever knew a very old pug that had it not to a greater or less degree. The peculiar depression between the nasal and frontal bones in this breed of dogs, while it almost totally obliterates the frontal sinuses, may narrow the air-passage at that spot, and cause greater irritation there from the unusual rush of the air, and especially if the membrane becomes inflamed or any foreign body insinuates itself.