l62 'Journal of Comparative Neurology and Psychology. 



food, and the undifferentiated primordial chemical sense may 

 have been as important in determining the chemical character of 

 the environing water as of the food eaten. 



Be that as it may, with the appearance of teeth which pierce or 

 crush the food, the organs of chemical sense within the mouth and 

 pharynx assumed an important function as guardians of the en- 

 trance to the cesophagus, an interoceptive function which they per- 

 form in all gnathostome vertebrates — the organs of taste. Parallel 

 with this differentiation within the mouth, the organs of chemical 

 sense lying outside the mouth at the rostral end of the body would 

 assume more and more importance as organs for detection of 

 chemical differences in the surrounding water, differences result- 

 ing usually from the presence of sources of chemical alterations 

 of the water lying outside the body of the fish. These external 

 organs of chemical sensation in the leading segments of the body 

 were finally aggregated as the organ of smell. 



The differences in the character of the stimulus applied to these 

 two organs may have been very slight at the beginning (and indeed 

 may be so still); but in the case of any organism possessing the 

 power of free locomotory movement the physiological significance 

 of the stimulation of the two sense organs may be very different 

 indeed. The object which acts as a stimulus to taste buds is 

 already within the mouth. The appropriate reaction is typic- 

 ally a contraction of the visceral musculature of the mouth and 

 pharynx adapted either to masticate and swallow or to eject 

 the object, as the case may require. The somatic musculature 

 is not necessarily brought into play. The olfactory organ, on 

 the other hand, has become a distance receptor and the appro- 

 priate reaction is a movement, usually locomotor in type, of the 

 somatic muscles, taking the animal toward or away from the 

 source of the stimulus. Even though the stimuli in the two cases 

 were identical, it is evident that the difference in the character of 

 the response would bring into play a very different central reflex 

 apparatus for the distance reaction from that for the mastication 

 or swallowing reflex. 



This difference between the characteristic reaction of the intero- 

 ceptor and the distance receptor is in my opinion the sufficient 

 explanation for the most important structural differences between 

 the olfactory and gustatory systems of vertebrates. This same 

 feature involves, it is true, a certain degree of difference between 



