726 



CUTANEOUS SENSATIONS 



[CH. LII. 



however, follow that the acuteness of the pressure sense varies 

 exactly as the ability of accurately localising sensations ; for instance, 

 the skin of the forearm is as sensitive to pressure changes as that 

 of the palm ; and the tip of the tongue which is the most discrim- 

 inative region of the body for locality is not so for pressure. For 

 pressure stimuli which are near the limen or threshold of sensation, 

 the hair aesthesiometer is much used; this is a hair or bristle 

 mounted in a holder ; the bristle can be shifted backwards or for- 

 wards in the holder, and the amount of pressure it exercises can 

 thus be varied. It is used for the exploration of " touch spots," and 

 these are found most numerously around the hair follicles. The 

 touch spots are more numerous in some parts than in others, but 

 fifteen for each square centimetre of skin is a rough average. To 

 explore "pain spots" a mounted needle is used; in Griesbach's 

 instrument the needle shifts up and down in the holder, and works 

 against a spring which registers the amount of pressure exerted to 

 evoke a painful sensation. In a "pain spot" the sensation is 

 unaccompanied by "cold" or "warmth," even if a cold or warm 



needle is used. For the exploration of 

 " heat spots " a small, hollow, metallic 

 pencil is kept warm by a stream of 

 warm water; this is moved over the 

 surface; there are some points where 

 the sensation is purely tactile, but at 

 the "heat spots" the pencil will feel 

 uncomfortably warm. "Cold spots" 

 can be similarly mapped out by the 

 use of a cold pencil. The accompany- 

 ing figure (fig. 528) indicates a small 

 piece of the skin of the thigh with the 

 " heat spots " horizontally and the " cold 

 spots " vertically shaded. 



All these facts clearly indicate that 

 different varieties of sensation are the result of the stimulation of 

 different end organs, and that the impulses are conveyed to the 

 central nervous system by different groups of nerve-fibres; they 

 moreover form the clearest piece of evidence we have that pain is 

 a distinct kind of sensation. 



The question is more difficult to answer, which particular end 

 organ is concerned with each variety of sensation. There is, how- 

 ever, little doubt that the nerve-fibrils around the hair follicles of 

 the short hairs are the terminations most affected by changes of 

 pressure, and also that Meissner's corpuscles are purely tactual, 

 taking the place of hairs in hairless parts. In the palmar surface 

 of the last phalanx of the index finger, there are 21 Meissner's 



Fro. 528. Heat and cold spots. 

 (Waller, after Goldscheider.) 



