CH. MIL] 



VARIETIES OF CUTANEOUS SENSATIONS 



773 



evoke a painful sensation. The sensation evoked by a " pain spot " is 

 unaccompanied by "cold" or "heat," even if a cold or hot 

 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 ; at the site of the " heat spots " the pencil 

 will feel peculiarly warmer. " Cold spots " can be similarly mapped 

 out by the use of a cold pencil. The accompanying figure (fig. 477) 

 indicates the distribution of cold and heat spots over six squares, 

 each of 1 sq. cm., on the back of the left hand. The black dots 



Fio. 477. Heat and cold spots. (Somewhat enlarged ; after Donaldson.) 



represent cold spots, their size indicating the strength of the 

 reaction. The open circles represent heat spots. 



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 

 corpuscles per square centimetre; in other parts of the palm and 

 sole the number varies from 2 to 8. End-bulbs are believed to 



