794 THE CENTRAL. NERVOUS SYSTEM 



causing pressure to be exerted on the hair follicles, so that the touch 

 corpuscles in the neighborhood of the follicles, or perhaps the fine nerve 

 plexus which surrounds them, becomes excited. The influence of hairs 

 in increasing the touch sensation can be demonstrated by the von Frey 

 method; for example, in one experiment over an area of 9 square mil- 

 limeters of skin with hairs present, 2 milligrams were found to produce the 

 sensation, whereas after the hairs had been removed, it required 36 milli- 

 grams. 



The frequency of touch corpuscles differs very much in different parts 

 of the body. They are most plentiful on the fingers, relatively infrequent 

 over the skin of the back, and very scarce in the skin; directly over bony 

 surfaces. They are entirely absent from the cornea, the conjunctiva 

 of the upper lid, and the glans penis. The adequate stimulus for touch 

 is evidently deformation of the surface. Pressure exerted over all the 

 touch corpuscles of a portion of skin is not felt. This can be demon- 

 strated by dipping the finger into mercury. The pressure of the mercury 

 is felt on the surface but not in the submerged portion of the finger. 

 Touch is the most responsive of all the sensations. Thus, as has already 

 been noted, a tuning fork can be felt vibrating by the finger when to 

 the ear its note is a continuous one, and the stimuli produced by a re- 

 volving serrated wheel can be felt by the fingers as separate even up 

 to a rate of five or six hundred stimuli per second. Adaptation is also 

 a marked feature of the touch sense, as is the experience of every one 

 who has worn flannel underclothing or a plate of false teeth. 



Closely related to the tactile sense is the power of discrimination 'be- 

 tween two points. This is tested by finding at what distance the two 

 points of a pair of calipers stand in order to be distinguished as separate. 

 The result in any given part of the body varies a little according to 

 whether the points rest on touch corpuscles and according to the rela- 

 tionship of the calipers to the hair follicles. On an average, however, 

 we may take the following distances in millimeters as being those at 

 which the two points can be distinguished over different areas of the 

 body: 



mm. 



Tip of tongue 1.1 



Volar surface of finger tip 2.3 



Dorsal of first phalanx 6.8 



Palm of hand 11.3 



Back of hand . 31.6 



Back of neck 64.0 



Middle of back, upper arm and side. 67.1 



It is clear from this list that the power of discrimination tends to 

 diminish in proportion to the lessening mobility of the part. It is greatest 



