290 ILLUSIONS OF TOUCH. 



points, about half an inch apart, be passed from the palm to 

 the tips of the fingers, it will at first seem one line gradually 

 separating into two diverging ones, owing to the keener local- 

 izing power as the finger-tips are approached. 



" The reference of the sensations aroused by the excite- 

 ment of definite nerve-cells to definite parts of the periphery is 

 a power acquired through the physiological experiences of the 

 earliest months of life. Through the sense of sight the seat 

 of irritation is recognized, and through muscular sensation its 

 relation to surrounding parts is experimentally explored, so 

 that cumulative harmonious experiences of tactile, visual, 

 and muscular sensations finally bring into correspondence the 

 various areas with definite varieties of touch sensation." 

 SEWALL. 



The- skin is more sensitive when moist; the barber there- 

 fore moistens the ball of his thumb before feeling the edge of 

 the razor to see if it is sharp enough. 



Illusions of Touch. Aristotle's experiment has already 

 been mentioned. If the " funny bone," or " crazy bone," be 

 hit, i.e., if the ulnar nerve be bruised against the bone, sharp 

 pain may be felt in the wrist and hand, and soreness of these 

 parts may be felt for days, though they are not in the least 

 injured, but only the nerve at the elbow. The currents along 

 this nerve rouse sensation that we have learned to localize at 

 the endings of the nerve fibers. So, too, after amputation of 

 a hand or foot, there may for years be sensations referred to 

 the missing member, probably due to irritation of the nerves 

 of the stump. There is, then, no certainty of getting rid of a 

 corn by amputating a toe. 



The Temperature Sense. Many cases are on record in 

 which, from accident or disease, the pressure sense was lost 

 and the temperature sense retained, or vice versa. Such facts 

 have led to the belief that the temperature sense is distinct 



