566 HANDBOOK OF PHYSIOLOGY. 



olfactory nerves, that Magendie was led to the erroneous belief that the 

 fifth nerve might exercise this special sense. 



Varieties of Odorous Sensations. Animals do not all equally perceive 

 the same odors; the odors most plainly perceived by an herbivorous ani- 

 mal and by a carnivorous animal are different. The Carnivora have the 

 power of detecting most accurately by the smell the special peculiarities 

 of animal matters, and of tracking other animals by the scent; but have 

 apparently very little sensibility to the odors of plants and flowers. Her- 

 bivorous animals are peculiarly sensitive to the latter, and have a nar- 

 rower sensibility to animal odors, especially to such as proceed from other 

 individuals than their own species. Man is far inferior to many animals 

 of both classes in respect of the acuteness of smell; but his sphere of sus- 

 ceptibility to various odors is more uniform and extended. The cause 

 of this difference lies probably in the endowments of the cerebral parts 

 of the olfactory apparatus. The delicacy of the seuse of smell is most 

 remarkable; it can discern the presence of bodies in quantities so minute 

 as to be undiscoverable even by spectrum analysis; TumrthnrTnr ^ a grain 

 of musk can be distinctly smelt (Valentin). Opposed to the sensation 

 of an agreeable odor is that of a disagreeable or disgusting odor, which 

 corresponds to the sensations of pain, dazzling and disharmony of colors, 

 and dissonance in the other senses. The cause of this difference in the 

 effect of different odors is unknown; but this much is certain, that odors 

 are pleasant or offensive in a relative sense only, for many animals pass 

 their existence in the midst of odors which to us are highly disagreeable. 

 A great difference in this respect is, indeed, observed amongst men: 

 many odors, generally thought agreeable, are to some persons intolerable, 

 and different persons describe differently the sensations that they sever- 

 ally derive from the same odorous substances. There seems also to be 

 in some persons an insensibility to certain odors, comparable with that 

 of the eye to certain colors; and among different persons, as great a dif- 

 ference in the acuteness of the sense of smell as among others in the 

 acuteness of sight. We have no exact proof that a relation of harmony 

 and disharmony exists between odors as between colors and sounds; 

 though it is probable that such is the case, since it certainly is so with 

 regard to the sense of taste; and since such a relation would account in 

 some measure for the different degrees of perceptive power in different 

 persons; for as some have no ear for music (as it is said), so others have 

 no clear appreciation of the relation of odors, and therefore little plea- 

 sure in them. 



Subjective Sensations. The sensations of the olfactory nerves, inde- 

 pendent of the external application of odorous substances, have hitherto- 

 been little studied. The friction of the electric machine produces a 

 smell like that of phosphorus. Hitter, too, has observed, that when gal- 

 vanism is applied to the organ of smell, besides the impulse to sneeze, 



