860 



CENTRAL NERVOUS SYSTEM 



which they give rise. These are touch, heat, cold and pain. When an 

 area of skin is examined carefully it is found that these sensations 

 are not elicited with equal readiness from all parts, but that definite 

 spots exist which may give rise to one or another of these sensations. 

 Some give rise to touch alone, others to the sensation of warmth, others 

 to that of cold, and still others to a feeling of pain. Each spot evidently 

 marks the location of one or more receptors for the stimulus in ques- 

 tion. The spots giving rise to the four different qualities of sensation 

 frequently do not coincide, nor do their numbers correspond, i.e., cold 

 spots are more numerous than heat spots. In many regions of the 

 body one quality of sensation may be lacking altogether. Thus pain is 

 absent from the inner surface of the cheek opposite the second molar; 

 it .alone occurs on the cornea. These facts furnish proof that the qual- 



A. 



Fig. 213. Cold spots {A) and heat spots (5) of an area of skin of the right hand. In each 

 case the most intense sensations were experienced in the black 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.) 



ity of the cutaneous sensations depends on the stimulation of sense or- 

 gans which are specialized each for a different type of stimulus. 



Touch. Touch spots are stimulated by pressure which deforms the 

 tissue. This may be seen to be so by dipping a finger into mercury 

 when it will be found that the sensation arises only from a band of 

 skin at the surface which is bent by the pressure of the mercury. The 

 skin deeper down is pressed upon uniformly and consequently is not 

 deformed and no sensation is set up. Touch spots are not uniformly 

 distributed on all parts of the body, as we have already indicated. On 

 the hairy regions it is found that they are most numerous at the base 

 of the hairs, especially on the "windward" side. These spots are stim- 

 ulated when the hair is bent, and since the hair acts as a lever, very 



