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 physio- 

 logical 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 

 extrinsic 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 asso- 

 ciation 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 conscious- 

 ness in consequence of like stimuli. 



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



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