PART V 

 CUTANEOUS SENSATIONS 



Tin: skin, being the outermost layer of the body, represents the tissue or 

 organ by which the organism is brought into relationship with its environ- 

 ment. In the widest sense of the term the skin is protective. This function 

 it discharges by virtue not only of its physical properties but also of its rich 

 endowment with sense organs, by means of which the intracorporeal events 

 can be correlated with those occurring outside and immediately affecting the 

 organism. 



We are accustomed to distinguish several qualities of sensation among 

 those having their origin in the skin, the chief of which are the sense of touch, 

 including that of discrimination, the sense of pain and the sense of tempera- 

 ture. The very different qualities of sensation included under these three 

 classes suggest that there may be a special mechanism, or class of mechanism, 

 for each sense, and a careful investigation of the sensory qualities of the 

 skin surface bears out this idea. Isolated stimulation of minute areas on the 

 skin does not excite all the sensations together, but only a sense of touch or 

 of pain, or a Sense of cold or warmth. We are therefore justified in dealing 

 with each of these sensations separately. 



THE TEMPERATURE SENSE 



By means of the skin we can appreciate that a body coming 

 in contact with the skin is either cold or warm. If the body is at 

 the same temperature as the skin, as a rule no sensation of tempera- 

 ture is excited. It was formerly thought that the sensations both of heat 

 and cold were determined by the excitation of one and the same end or^an. 

 Warming of this end, organ would produce a sensation of warmth, while a 

 diminution of its temperature would produce the sensation of cold. ( 'areful 

 investigations by Blix and Donaldson of the distribution of the temporal ur<- 

 sense has shown that this opinion cannot be maintained. If a small surface 

 \\ aimed to a few degrees above the temperature of the skin )><> moved over 

 any part of the surface of the body, e.g. the back of the hand, it is found that 

 the warmth of the instrument is not appreciable equally at all parts of the 

 surface of the skin. At some points the sensation of warmth will be very 

 pronounced, but between these points the sensation of warmth may be 

 entirely wanting and the instrument may be judged to be of the same tem- 

 perature as the hand itself. In this way a series of ' warm points ' may be 

 mapped out. On now cooling the instrument a few degrees below the 



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