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HAND-BOOK OF PHYSIOLOGY. 



have nearly the same action upon the mucous membrane of the eyelids. 

 It was because the common sensibility of the nose to these irritating sub- 

 stances remained after the destruction of the olfactory nerves, that Magen- 

 die 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 herbiv- 

 orous animal and by a carnivorous animal are different. The Oarnivora 

 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 lit- 

 tle sensibility to the odors of plants and flowers. Herbiv- 

 orous animals are peculiarly sensitive to the latter, and 

 have a narrower 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 susceptibility to various odors is more uniform and ex- 

 tended. The cause of this difference lies probably in the 

 endowments of the cerebral parts of the olfactory appa- 

 ratus. The delicacy of the sense of smell is most remark- 

 able; it can discern the presence of bodies in quantities 

 so minute as to be undiscoverable even by spectrum an- 

 alysis; Too"i~o1)~o",o~oo~ f a grain of musk can be distinctly 

 smelt (Valentin). Opposed to the sensation of an agree- 

 able 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 

 (Max 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 per- 

 sons intolerable; and different persons describe differently the sensations 

 that they severally 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 difference 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 har- 

 mony 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 



E 



FIG. 356. Epithe- 

 lial and olfactory 

 cells of man. The 

 letters are placed 

 on the free surface. 

 E, E, epithelial 

 cells; CM/., olfac- 

 tory cells. 

 Schultze.) 



