OF TOUCH. 417 



CHAPTER XXII. 



OF TOUCH, AND THE DETERMINATION OF PRESSURES AND TEMPERA- 

 TURES. 



Functions of the tactile Mechanism: its Structure. Regions of different Sensitiveness. Compar- 

 ative Physiology of Touch. Estimate of physical Qualities. 

 Perception of Temperature. Subjective Sensations of Temperature. 



THE tactile organ is the skin, or some part, modification, or append- 

 age of it. The general functions of the skin have Ibeen al- Functions of 

 ready described. It remains to speak of it in connection with mecifanism 

 the sense of touch. 



An impression has long prevailed among physiologists that this sense 

 should be considered as offering several subdivisions. Thus, for in- 

 stance, we have a consciousness of the general condition of the muscular 

 system muscular sense, as it might be termed and this, in some cases, 

 is exquisitely perfect, as may be gathered from what has been said re- 

 garding the tensor tympani and stapedius muscles in the chapter on 

 hearing. Distinct from this is our appreciation of pain or pleasure, and 

 so also our estimation of temperatures. Adelon has indeed maintained 

 that the cognizance of temperatures is the primary or chief function of 

 this sense. It will be sufficient, however, for our purpose, leaving out 

 these minor subdivisions, to direct our attention to the more important, 

 and to consider the tactile organ as devoted to two uses : 1st, the appre- 

 ciation of pressures ; 2d, of temperatures. Pressures doubtless act upon 

 the skin in a purely mechanical way ; temperatures operate by inducing 

 a variation in the rate of waste and nutrition. .At a certain point, even 

 this distinction ceases, for pressures, when they reach a sufficient intensi- 

 ty, interfere with the supply of arterial blood or the removal of venous, 

 and thereby change the rate of nutrition and waste, acting, as far as 

 this goes, in a manner not unlike that of the variations of temperature. 



In man, the skin possesses tactility to a different degree in different 

 regions. On the tips of the fingers and on the lips the sen- Re ional dif 

 sory perception is most acute, while it is at a minimum on ference in tac- 

 the trunk and thigh. In other mammals, which are covered tx lty ' 

 with hair or wool, the sense of touch is much more restricted. Its 

 proper organ is to be regarded as arising from a concentration of general 

 sensibility of the skin upon a special construction, the papillary body, 

 as it is termed. The organs of vision and hearing consist essentially of 



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