CHAPTER XIII 



THE RECEPTOR SYSTEM. INTERNAL AND CUTANEOUS 

 SENSATIONS 



The Receptor System constitutes the Body's means of gain- 

 ing information of its surroundings and of such internal condi- 

 tions as it needs to know about. Since the surroundings may play 

 upon the Body in many different ways and through the operation 

 of many forms of energy, receptors are provided which respond 

 to all sorts of stimuli. Inasmuch as proper adaptation requires 

 that different sorts of stimuli affect the Body differently partic- 

 ular receptors are specialized to respond most readily to partic- 

 ular kinds of stimulation. 



An interesting thing about the responses of the different re- 

 ceptors is that while their adaptation to special forms of stimula- 

 tion does not exclude the possibility of their being aroused by 

 other sorts of stimuli than the normal ones, when so aroused the 

 effect in consciousness is as though the normal stimulus had been 

 applied. Pressure on the eyes gives rise to sensations of light; 

 electrical stimulation of the tongue may cause sensations of taste. 

 This fact has led physiologists to take the view that the quality 

 of any sensation depends on the region of the cerebrum to which 

 it comes, and that it is quite independent of the structure of the 

 receptor or the manner of its stimulation. If this is true it ac- 

 cords well with another conception which most physiologists 

 find very attractive, that the nerve impulse, whatever it may be, 

 is the same sort of process wherever it occurs. It is, of course, 

 evident that this idea, the so-called "doctrine of specific nerve 

 energies," cannot be true if the quality of sensation depends in 

 any manner upon the nature of the receptor or the way in which 

 it is stimulated. It must be confessed that many known facts 

 about the senses, that of sight particularly, cannot at present be 

 explained upon any basis which excludes differences in the re- 

 ceptor as determining factors of the quality of sensation. 



The Differences between Sensations. We distinguish among 



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