ON THE PHYLOGENETIC DIFFERENTIATION OF 

 THE ORGANS OF SMELL AND TASTE. 



BY 



C. JUDSON HERRICK. 



{From the Anatomical Laboratory of the University of Chicago.) 



There are in vertebrates two systems of sense organs adapted 

 to respond directly to peripheral chemical excitation, the organs of 

 smell and taste. In this respect they are in contrast with the other 

 sense organs of the body; but when we come to compare the two 

 chemical senses with one another we find it difficult to discover 

 any objective difference between their stimuli or any explana- 

 tion for the development of two chemical senses in the primitive 

 aquatic vertebrates. And yet the very lowest vertebrates exhibit 

 important morphological differences between the peripheral organs 

 of smell and taste, a complete separateness in the nervous path- 

 ways to the brain and still more important differences in the 

 central reflex connections within the brain. In view of the simi- 

 larity in the nature of the stimuh to which the peripheral organs 

 respond, these fundamental central differences have thus far baffled 

 explanation. 



Let us first consider briefly the criteria by which in the case of 

 human beings the modalities of sense may be distinguished, (i) 

 Doubtless the most important criterion for us is direct introspec- 

 tive experience, the psychological criterion. (2) The adequate 

 stimuli of the various senses exhibit characteristic physical or 

 chemical differences, the physical criterion. (3) The data of 

 anatomy and physiology may differentiate structurally the recep- 

 tive organs and conduction paths of the several types of sensation, 

 the anatomical criterion. (4) The type of response varies in 

 a characteristic way for the different senses, the physiological 

 criterion. 



It is impossible in the present state of our knowledge to frame 

 adequate definitions of all of the senses in terms of any one of 

 these criteria alone. Thus, we are not able introspectively to dis- 



