174 SMELL, TASTE, ALLIED SENSES 



flavor, for only sour, saline, sweet, and bitter tastes can 

 be sensed and onion produces the same sweetish, taste 

 that apple does. The separateness of smell and taste 

 depends doubtless upon the conditions already described. 

 Smell is excited in general by on© set of substances ; taste 

 by another. Smell calls for only very weak solutions; 

 taste requires relatively strong ones. It may also be that 

 these two senses differ in the nature of the solutions that 

 activate them; taste is attuned to substances that form 

 aqueous solutions, smell to those that dissolve in oil. 

 Cell surfaces are commonly believed to be diphasic in 

 that they are composed of a mixture of two materials one 

 oily and the other aqueous. The gustatory hairs may be 

 so constituted that the aqueous constituent is the avenue 

 of entrance for the stimulating substance and the olfac- 

 tory hairs so that the oily one is the inlet. If such is the 

 case, this feature may also be an important difference 

 between smeU and taste. 



3. Groups of Chemical Receptors. Taste and smell 

 are two of the five senses ordinarily attributed to man. 

 But in the detailed study of the human senses not one 

 has escaped a kind of functional subdivision whereby it 

 has been sho^vn to be more than a single sense. Thus the 

 internal ear originally regarded by physiologists as 

 purely an organ of hearing, was shown by Flourens in 

 1828 to be concerned in a most important way with bodily 

 equilibrium. From this standpoint the ear takes on the 

 character of a double sense organ. This duplicity is 

 especially well marked in certain fishes in which the 

 membranous labyrinth is completely divided in two cor- 

 responding to the functional differentiation already in- 

 dicated ; one of these parts consists of the utriculus with 



