522 COMPARATIVE PHYSIOLOGY. 



PRESSURE SENSATIONS. 



1. There is a relation betweeD the intensity of the stimulus 

 and the sensation resulting, and this limit is narrow. The 

 greater the stimulus the more pronounced the sensation, though 

 ordinary sensibility soon passes into pain. 2. The law of con- 

 trast may be illustrated by passing the finger up and down in a 

 vessel containing mercury, when the pressure will be felt most 

 distinctly at the point of contact of the fluid. 3. Pressure is 

 much better estimated by some parts than others ; hence the use 

 of the employment of those to sq large an extent. 



THERMAL SENSATIONS. 



1. The law of contrast is well illustrated by this sense; in 

 fact, the temperature of a body exactly the same as that of the 

 part of the skin applied to it can scarcely be estimated at all. 

 The first plunge into a cold bath gives the impression that the 

 water is much colder than it seems in a few seconds after, when 

 the temperature has in reality changed but little ; or, perhaps, 

 the subject may be better illustrated by dipping one hand into 

 warmer and the other into colder water than that to be ad- 

 judged. The sample feels colder than it really is to the hand 

 that has been in the warm water, and warmer than it is to the 

 other. 2. The limit within which we can discriminate is at most 

 small, and the nicest determinations are made within about 27° 

 and 33° C. — i. e., not far from the normal temperature of the 

 body. 3. Variations for the different parts of the skin are 

 easily ascertained, though they do not always correspond to 

 those most sensitive to changes in pressure. The cheeks, lips, 

 and eyelids are very sensitive to pressure. 



Eecent investigations have revealed the fact that there are in 

 the human skin " pressure-spots," and " cold-spots," and " heat- 

 spots" — i. e., the skin may be mapped out into very minute 

 areas which give when touched a sensation of pressure di£Eerent 

 from that produced by the same stimulus in the intermediate 

 regions ; and in like manner are there areas which are sensitive 

 to warm and to cold bodies respectively, but not to both ; and 

 these do not correspond with the pressure-spots, nor to those 

 that give rise when touched to the sensation of pain. 



It has been shown, also, that the extent of the area of skin 

 stimulated determines to a large degree the quality of the re- 



