104 TACTILE SENSATION. 



Between an excitation of any end-organ and a voluntary 

 motion prompted by it, a time elapses, usually called the 

 " personal time," which is made up of the time required 

 for recognizing the sensation (perception) and the time 

 required for transforming it into muscular action (intention). 

 In the simplest possible case, that in which the person 

 under observation signifies his recognition of an expected 

 excitation by a preconcerted signal, the personal time is 

 about one-sixth of a second for sound, light, and touch. 

 By increasing the intensity of the excitation the time may 

 be somewhat diminished. If the sensation is of such a 

 character as to require interpretation or discrimination 

 before it is acted upon, the time is longer. 



Tactile Sensation. 



Tactile sensation is regarded as the function of the 

 so-called " tactile corpuscles " of the skin, and of analogous 

 end-organs which exist in the exposed mucous membranes. 

 According to the mode in which the end-organs are 

 affected, tactile sensation is divisible into that of Pressure, 

 that of Temperature, and that of Locality. As regards 

 pressure, Weber found that an increment of pressure on 

 the hand must amount to at least one-thirtieth of the 

 previous pressure to be felt. As regards temperature, the 

 degree of excitation is estimated by the difference between 

 the temperature of the object applied, and the actual 

 temperature of the skin. A difference of about one-eighth 

 of a degree can be felt. The sensation of locality may be 

 tested either by " interrogation," or by measuring the dis- 

 tance at which two points of excitation must be apart in 

 order that they may be felt as two. In relation to the 

 latter method, any area on the surface of the skin within 

 which two such points cannot be distinguished, is called a 

 "sensation area." The widths of sensation areas for 



