442 



THE SPECIAL SENSES 



The above results show that while less stimulus is neces- 

 sary to arouse a sense of pressure in the face than in the 

 fingers, the power to discriminate is particularly acute in the 

 finger tips and tongue. This second method is also of service 

 in locating the distribution of nerve ends, since to feel two 

 points as two means that two nerves are being stimulated. 



Another interesting feature of this discrimination ense 

 is that it can be increased with practice, notably in the 

 case of the bhnd. This power is probably due to in- 

 creased branching in the nerve ends thus increasing the 

 nimiber of spots on a given area. 



Temperature sensations. — These are of two kinds, heat 

 and cold, each with separate spots of stimulation and 

 separate fibers. Their location may be determined by 

 passing gently over the skin of the bhndfolded subject, 

 metal points which are colder or 

 warmer than the skin surface. As 

 this is done there will be aroused 

 at definite points distinct sensa- 

 tions of heat or cold, which are at 

 the same time entirely distinct from 

 pressure sensations. If a certain 

 area be explored in this manner 

 and the spots located by ink spots, 

 of different color, say red for cold 

 spots and black for warm, the 

 extent of the nerve ends and the 

 relative number of heat and cold 

 nerves for a given area can readily 

 be determined. Figure 206 shows such a map. The results 

 of such tests applied to all areas of the body may be sum- 

 marized as follows: 



Fig. 208 — A portion of the 

 skin mapped for hot and 

 cold spots; white spots, cold; 

 black spots, hot. 



