622 ANIMAL PHYSIOLOGY. 
fifth nerve. It is possible that the auditory nerve may also 
participate, though certainly not so as to produce a pure sen- 
sation of smell. 
Like the other sense-organs, that of smell is readily fa- 
tigued; and perhaps the satisfaction from smelling a bouquet 
of mixed flowers is comparable to viewing the same, one scent 
after another being perceived, and no one remaining predomi- 
nant. 
Our judgment of the position of bodies possessing smell is 
less perfect even than for those emitting sounds; but we always 
project our sensations into the outer world, never referring the 
object to the nose itself. Subjective sensations of smell are 
rare in the normal subject, though common enough among the 
diseased, as is complete or partial loss of smell. It has been 
found that injury to the fifth nerve interferes with smell, which 
is probably due to trophic changes in the olfactory region. 
Comparative.—The investigation of the senses in the lower 
forms of life is extremely difficult, and in the lowest presents 
almost insurmountable obstacles to the physiologist, because 
their psychic life is so far removed from our own in terms of 
which we must interpret, if at all. 
The earliest form of olfactory organ appears to be a depres- 
sion lined with special cells in connection with a nerve, which, 
indeed, suggests the embryonic beginnings of the olfactory 
organ in vertebrates, as an involution (pit) on the epithelium of 
the head region. It would appear that we must believe that in 
some of the lower forms of invertebrates the senses of smell 
and taste are blended, or possibly that a perception results 
which is totally different from anything known to us. The 
close relation of smell and taste,even in man, will be referred to 
presently. There are, perhaps, greater individual differences in 
sensitiveness of the nasal organ among mankind than of any 
other of the sense-organs. Women usually have a much keener 
perception of odors than men. The sense of smell in the dog 
is well known to be of extraordinary acuteness; but there are 
not only great differences among the various breeds of dogs, 
but among individuals of the same breeds; and this sense is 
being constantly improved by a process of “ artificial selection” 
on the part of man, owing to the institution of field trials for 
setters and pointers, the best dogs for hunting (largely deter- 
mined by the sense of smell) being used to breed from, to the 
exclusion of the inferior in great part. Our own power to 
think in terms of smell is very feeble, and in this respect the 
