282 COMPARATIVE ANATOMY AND PATHOLOGY. 
seen. In the carnivorous birds, although the eye is keen and pe- 
netrating, they cannot always avail themselves of its agency in dis- 
covering their food; therefore their olfactory powers are exceed- 
ingly strong, whether we refer to the magnitude of the olfactory 
nerve, or of the auxiliary nerves, of which mention will presently 
be made, or of the ample provision for the distribution of these 
nerves. The convolutions are either partially ossified, or remain 
entirely cartilaginous. They are limited in their extent by the size 
of the olfactory nerves, and they are all covered by a highly vas- 
cular pituitary membrane, over which the nerve ramifies. In some 
birds, even cartilage is not found, but the olfactory apparatus is en- 
tirely membranous. This difference of mechanism does not seem 
to have any connexion with the degree of olfactory power, but that 
depends on the quantity of nervous matter which ramifies over the 
nostril. 
In graminivorous birds the nerve is small, and its distribution is 
likewise confined, or at least it cannot be satisfactorily traced. In 
none of these birds is it very considerable. 
In omnivorous birds, if the term may be used, the passage of 
the nerve and its increased bulk are easily to be traced ; and in 
these respects the dissection of the heads of a duck and a turkey 
would be a very pleasing and useful lesson. The nervous ramifi- 
cations, both olfactory, and from the fifth, or auxiliary pair — just 
now alluded to — spread over the mandibles both within and with- 
out, and their terminations may be traced towards the edges of the 
mandibles. The beaks of graminivorous birds are hard and solid, and 
the surrounding bones are composed of their osseous plates without 
the interposition of any spongy substance. The beaks of the omni- 
vorous or carnivorous birds are still harder and stronger, for the 
purpose of crushing or tearing their prey; but the surrounding 
parts abound with cells, over which the olfactory ner.ve ramifies. 
The air has free access to these cells, and passes through them in 
order to reach the lungs. 
In the carnivorous birds, these contrivances are carried to their 
fullest perfection. The whole of the upper mandible is furnished 
with a net-work of bony fibres, covered with a sensitive mem- 
brane, on which the olfactory nerve abundantly ramifies ; and, in 
point of fact, on the proportional extent of cellular texture, and 
mucous membrane and nervous ramification in the upper mandi- 
ble, each bird depends for his degree and superiority of smell. 
Fishes . — The first observation I have to make respecting them 
is, that although we have irresistible proof of the acuteness of their 
smell, there are none of the cavities in the nostrils which are found 
in the quadruped. The organs of smell in fishes I cannot describe 
better than in the words of Dr. Grant, whose “ Outlines of Com- 
