PART NINTH. 



OEGANS OF THE SENSES. 



OUR internal as well as our external surfaces are subject 

 to the influence of exterior agents : most of these, under the 

 form of mechanical, physical, or chemical excitants, affect the 

 peripheral origins of the centripetal or sensory nervous 

 system, and give rise to nervous phenomena, the greater 

 number of which we have already studied with that system. 

 Thus we know that there are impressions which may pass 

 ii perceived by the cerebral centre, of which we are uncon- 

 scious, and yet cause reactions by their reflections in the 

 medullary system. These impressions and their results 

 belong to the system described by Marshall Hall under the 

 name of excito-motor system, and by Magendie under that 

 of unconscious sensation or sensibility, and which we have 

 studied under the name of reflex phenomena : such, for in- 

 stance, is the sensation which causes the saliva to be 

 secreted ; and such also are the phenomena which give rise 

 to the pulsations of the heart, for we have seen that this 

 organ contracts under the exciting, or rather excito-reflex, 

 influence of the blood upon its walls. 



In studying the nervous system, we have also pointed out 

 what is understood by sensation, or sensibility properly so 

 called (p. 55). We have seen that the phenomena of sensa- 

 tion may be divided into phenomena of general sensibility, 

 comprising the sensations which warn us, in a vague (senti- 

 ment) or more or less well-defined (sensation) manner, of the 

 changes taking place in our bodies ; and of phenomena of 

 special sensibility, which, occurring in special organs, inform 

 us by the modifications produced in these of certain special 

 qualities of the objects by which we are surrounded. 



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