THE RECEPTORS AND EFFECTORS 75 



less of the nature of the exciting substance or of the subjective 

 quality of the sensation. 



There are, in reality, four different factors which must be 

 taken into account in defining a "sense." (1) Doubtless with us 

 human folk the most important criterion is direct introspective 

 experience, the psychological criterion. Ordinarily this is ade- 

 quate, but, as we have just seen, there are some cases where 

 it alone cannot be depended upon to distinguish between two 

 senses. (2) The adequate stimuli of the various senses exhibit 

 characteristic physical or chemical differences, the physical 

 criterion. This factor, too, must be carefully investigated or we 

 may be led astray. (3) The data of anatomy and experimental 

 physiology may differentiate structurally the receptive organs 

 and conduction paths of the several types of sensation, the ana- 

 tomical criterion. (4) Finally, the type of response varies in a 

 characteristic way for the different senses, the physiological 

 criterion. 



The fourth criterion has been applied to solve the problem of 

 the reason for the development of two very different types of 

 sense organs and cerebral connections for the senses of smell and 

 taste, both of which are chemical senses with similar subjective 

 qualities. It has been pointed out by Sherrington that taste is 

 an interoceptive sense, calling forth visceral responses within the 

 body, while smell is, in part at least, an exteroceptive sense, being 

 excited by objects at a distance from the body and calling forth 

 movements of locomotion carrying the whole body toward or 

 away from the source of the odorous emanations. Thus the 

 form of the response is here the distinctive factor, and incidental 

 to this feature the organs of smell are sensitive to far smaller 

 quantities of the stimulating substance than are the taste-buds. 

 Parker and Stabler have shown that the human organ of smell 

 is sensitive to alcohol at a dilution 24,000 times greater than that 

 necessary to stimulate the organs of taste (see p. 218). 



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

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

 these four criteria alone, although it is a reasonable hope that 

 this may at some future time be attained. Even when all of 

 these criteria are taken into account, it is by no means easy to 

 determine how many separate senses the normal human being 



