INTERRELATION OF THE CHEMICAL SENSES 175 



its three semicircular canals and has to do with equi- 

 librium, and the other of the sacculus and its appended 

 lagena and is concerned with hearing. Even so unified 

 an organ as the human eye is made up of an intermingling 

 of two receptive fields, for, as originally suggested by 

 Schultze (1866) and as elaborated by von Kries (1904), 

 the retinal rods are concerned with colorless vision in 

 dim light and the cones with color vision in bright light. 

 Thus the eye is differentiated for two kinds of sight, one 

 by night and the other by day. The integumentary sense 

 originally supposed to be unitary, was shown by Bli:x; in 

 1884 to consist of at least three senses, cold, warm, and 

 pressure. To these were added in 1896 by von Frey a 

 fourth, pain. Thus it is clear that the conception of five 

 senses for man is wholly inadequate and though numbers 

 are perhaps not the best way of indicating the sensory 

 equipment of human beings or in fact of any other ani- 

 mal, it is not without interest to record the opinion of 

 Herrick (1918) that the classes of human receptors are 

 now known to be more than twenty. 



The chemoreceptors, represented in the older accounts 

 by the organs of taste and smell, have no more escaped 

 this process of increase than have the other sense organs. 

 The vomero-nasal organ appears to be a kind of accessory 

 receptor for smell and the common chemical sense is ap- 

 parently a primitive form of gustatory organ. But in 

 addition to these subsidiary receptors, the true olfactory 

 surfaces as well as the gustatory areas are not homo- 

 geneous, but are marked by local receptive differentiation. 

 This is especially well illustrated by the so-called sense 

 of taste. This, as has already been shown in the preceding 

 chapter, is in reality not a single sense, but, in accordance 



