SECTION II 

 CUTANEOUS SENSATIONS 



THE skin, being the outermost layer of the body, represents the 

 tissue or organ by which the organism is brought into relationship 

 with its environment. 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 temperature. The very different qualities of sensa- 

 tion 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 warm. 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 ccming 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 temperature 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-organ. Warming of this end- 

 organ would produce a sensation of warmth, while a diminution of 

 its temperature would produce the sensation of cold. Careful 

 investigations by Blix and Donaldson of the distribution of the tem- 

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

 small surface warmed to a few degrees above the temperature of the 

 skin be 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 temperature as the hand 



542 



