THE OLFACTORY NERVES. 
147 
the ethmoid bone this canal terminates in a blind pouch, so that 
there is no truth in the old opinion that it was a channel through 
which any fluid could be conveyed from the brain into the 
nose. The communication between the ventricles and the cavity 
within the nerve is effected by small circular canals, which Mr. 
Percivall calls the olfactory sinuses. No satisfactory physiological 
explanation has been given of the function of these canals. 
Union. Thus the two branches of the olfactory nerve travel on, 
beneath and on either side of the mammillary processes, and gra- 
dually approaching to each other, until, a little before they reach 
the ethmoid bone, they unite. Their canals are now all oblite- 
rated their substance is blended, and they are pressing forward to 
their destined place of expansion. 
Bulk . — We are struck with their bulk. They are the largest of 
the cerebral nerves, with the exception of the fifth ; and that, the 
most important of the whole, requires a considerable development 
of medullary matter in order to enable it to discharge all its duties. 
We can now, even before its separation from the brain, observe the 
varying bulk of the olfactory nerve in different animals, and bear- 
ing an evident and beautiful proportion to the necessities of the 
animal. The structure differs in each, and also the colour and consist- 
ence of the nervous cord. We shall find, by and by, that they are 
distributed on the olfactory membrane in different ways. We 
shall not, perhaps, be able to describe the design or the effect of 
many of these varieties, but still it is useful, and not uninteresting, to 
attend to them. 
Observe the development of this nerve, in evident relation with 
the necessities of the animal. Compare the olfactory nerve of 
man — in him, connected with pleasure far more than with existence, 
or even with health— compare it with that of the brute. In the brain 
of the horse, not more than half the size of that of the human bein^, 
the olfactory nerve is four times as large as that in man. Compare 
it in our different domesticated animals. In the horse it is large, 
for in a state of nature it affords the only warning against poison- 
ous plants : but, almost over the world, he has become our stabled 
servant, comparatively rarely sent to collect his own nutriment 
amidst the herbage of the field, and having the greater part of his 
food provided for him. 
The ox is oftener driven to shift for himself, or, if worked by 
day, he is usually turned out at night, and needs a somewhat 
acuter sense of smell. His brain is but little more than half the 
size of that of the horse, yet the olfactory nerve is nearly as large. 
In the sheep it is comparatively larger than in the ox. 1 will state 
another reason presently for the great development of this nerve in 
these animals. 
