SENSE ORGANS. jol 



Hearing may be involuntary, but listening is voluntary. 

 So smelling may be involuntary, but sniffing is a volun- 

 tary act of smelling. In sniffing we draw in the air 

 suddenly and then stop it. The air is thus forced 

 more thoroughly into the upper chambers. 



Odoriferous particles being air-borne, it is evident 

 that a substance can not be smelled unless it is volatile. 

 The amount of matter in the air which may be detected 

 by this sense is so infinitesimally small that it can not 

 be estimated. No chemical test can compare with it in 

 delicacy. 



COMPARATIVE PHYSIOLOGY OF SMELL. 



We judge of the delicacy of this sense in other ani- 

 mals in three ways — viz., by the size of the olfactory 

 lobes, by the complexity of the olfactory surfaces, and 

 by the habits of the animal. Judging by either or all of 

 these, there can be no doubt of the enormous superiority 

 of mammals over man. This is especially true of car- 

 nivores, which hunt by smell, and of herbivores, which 

 detect danger by the same sense. Referring to Fig. 41 

 (page 76), we see at once the great size of the olfactory 

 lobes in mammals. Making a transverse section through 

 the nostrils of a dog, a horse, or a cow, we observe at 

 once the great complexity of its chambers. The same 

 superiority is brought out still more strongly by obser- 

 vation of habits. Think of the keenness of the smell of 

 a dog, who follows his master's tracks an hour after he 

 has passed ; or of a hound tracking a deer ; or, again, of 

 a deer sniffing the air and detecting the hunter a mile 

 away. It is probable that among mammals and lower 

 vertebrates generally smell, not sight, is the most impor- 

 tant and most objective sense. In passing through a 

 strange country we take ocular notes, and may return 

 the same way by the use of these. A dog under simi- 

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