792 



THE CENTRAL NERVOUS SYSTEM 



also very infrequent in the scalp. They are almost absent from the mucous 

 membranes, which explains why one is able to swallow a liquid that is too 

 hot for the hand. 



The acuteness of the temperature sensation, as with all the other cu- 

 taneous sensations, depends very much on the condition of the skin, 

 being most sensitive when this is at the ordinary temperature, but very 

 imperfect when it is either very hot or very cold. There is also very 

 marked adaptation of the sense. This can be very well shown by the simple 

 experiment of taking three vessels of water, one at a moderate tempera- 

 ture, one very hot and one very cold. If a finger of one hand is placed 

 in the hot water and a finger of the other in the cold, and they are left 

 there for a short time, until the skin has assumed the same temperature 

 as the water, and then transferred to the lukewarm water, the finger 



Fig. 205. — Cold spots (A) and heat spots (B) of an area of skin of the right hand. In each 

 case the most intense sensations were experienced in the hlack areas, less intense in the lined, 

 and least in the dotted. The blank areas represent parts where no special sensation of either 

 kind was experienced. (From Goldseheider.) 



transferred from the cold water will feel hot, and that transferred from 

 the hot water will feel cold. Temperature sensation also produces a 

 marked positive after-effect. Thus, if a cold coin is placed on the fore- 

 head and then removed, the cold sensation will persist for some time in 

 the area of skin on which the coin was laid. 



That the receptors for heat and cold respond only to one kind of 

 stimulus, or if to others, only when these are excessive, can be well il- 

 lustrated by the experiment of touching a cold spot with a very hot ob- 

 ject: the sensation will be that of cold. The hot object has so pronounced 

 a power of stimulation that it has overstepped the threshold for heat 

 of the cold-adapted receptors. The sensation of cold is elicited more 

 promptly than that of warmth. The distinction between a warm and a 



