CHAPTER VIII 

 THE PHYSIOLOGY OF SENSATION 



SECTION I 

 ON THE RELATION OF SENSATION TO STIMULUS 



UP to the, present we have dealt with the central nervous system as a 

 machine for the conversion of afferent into appropriate efferent impulses, 

 and have regarded the sense-organs simply as mechanisms by means of 

 which stimuli of various quality, arising from events in the environment 

 of the animal, could give rise to nerve impulses. We have seen reason 

 to assign these afferent impressions to various classes, according to the 

 physical character of the stimulus involved and according to the quality 

 of the response. The nature of the physiological processes evoked in the 

 organism depends on the physical nature of the stimulus and the locus of its 

 incidence on the surface of the body. Thus a nocuous stimulus applied to 

 the foot causes flexion of the leg, whereas steady pressure on the sole evokes 

 a ' stepping ' reflex with extension of the leg. A beam of light falling on the 

 eye calls forth movements involving contractions of the intrinsic and ex- 

 trinsic ocular muscles. The same beam of light falling on the skin is devoid 

 of effect unless it is so strong as to burn the skin, when a reflex is -excited 

 similar to that evoked by a nocuous stimulus. As our study of the adaptive 

 nerve mechanisms becomes more detailed, and especially when we take 

 into account the activities of the association centres in the cortex, it becomes 

 more and more difficult to follow the chain of processes which lead to any 

 given reaction ; in order to advance further in our knowledge of the activity 

 of the receptor organs, we have frankly to abandon the objective method 

 which has served us in the study of all the other functions of the body, and 

 appeal to our own consciousness for information as to the effects of their 

 excitation. Each one of us is aware that stimulation of an afferent nerve 

 may cause a change in consciousness which we denote as a sensation, and we 

 attribute to other living organisms, presenting similar reactions to ourselves, 

 similar changes in consciousness in consequence of like stimuli. 



Certain sensations differ one from the other to such an extent that com- 

 parison among them becomes impossible. Thus in the skin and underlpng 

 parts we have, as a result of stimulation, sensations of touch and pressure, 

 sensations of heat and of cold, and sensations of pain. The contact of 



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