THE SENSE OF SMELL. 299 



way is continued along the olfactory tract to the hippocampal 

 region of the brain. As we would expect, this portion of the 

 brain is much developed in these animals having a very acute 

 sense of smell. 



The olfactory epithelium is kept constantly moist with fluid 

 and substances cannot be smelled unless the odorous particles 

 which they give off become dissolved in this fluid. These odor- 

 ous particles diffuse into the upper nares from the air currents 

 which, with each respiration, are passing backwards and for- 

 wards along the lower nasal passages. There is no actual move- 

 ment of air over the olfactory epithelium. 



NATURE OF STIMULUS. It is impossible to state just exactly 

 what it is that emanates from an odorous body to excite the ol- 

 factory sense. All we can say is that it does not require to be 

 present in more thUn the merest traces in the air in order to un- 

 fold its action. Thus even in the case of man, with his undevel- 

 oped sense of smell, 0.000,000,000.04 of a gramme of mercaptan. 

 suspended in a litre of air, can be smelled, and in the case of the 

 dog, the dilution may no doubt be many thousand times greater. 

 The sense of smell is the most important of the projicient sensa- 

 tions in certain aquatic animals, and is very closely associated 

 with the sexual functions of the animal. Just as in the case of 

 taste, certain substances owe their peculiar odors to simultane- 

 ous stimulation of the olfactory epithelium and the receptors of 

 common sensation. Thus the pungency of acids, of ammonia, 

 chlorine, etc., is due to stimulation of the endings of the fifth 

 nerve. Attempts have been made to classify odors, as has been 

 done for tastes, but with no success. 



