CHAPTER X 

 SENSATIONS AND THE SENSE-ORGANS 



We have spoken in a general way of the receptors 

 of the body as sources of nerve-impulses which are con- 

 veyed to the central nervous system and bring about 

 reflexes which, as a rule, are* clearly valuable to the 

 organism. Now we must give some attention to the 

 states of our consciousness which apparently result 

 from the stimulation of receptors. Our subject is that 

 of the sensations and it is one common to physiology 

 and psychology. 



We cannot take up this matter from a metaphysical 

 standpoint. On the physical side a sensation is the 

 accompaniment of a certain process in the brain which 

 we usually assume to have been induced by external 

 forces acting upon the receptors. One of the very first 

 things to be insisted upon is that sensations may be 

 experienced when the external stimuli are absent, pro- 

 vided the brain process takes place. This is often 

 the case, we may suppose, in our dreams. Sensations 

 that do not have any external cause demonstrable to 

 other people are called hallucinations. The more one 

 studies the nervous system the more the wonder grows 

 that we are not often deceived in regard to the origin 

 of our sensory experiences. 



A question that has to be dealt with at the outset is 

 whether or not nerve-impulses are all alike. It is plain 

 that they are most unlike in their effects. Those that 

 go to the skeletal muscles throw them into contraction, 

 others running to the glands set them to secreting, still 

 others led to the heart may restrain it from beating. 

 On the afferent side. the impulses that enter the brain 



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