194 



RECEPTORS 



amount evokes a response from receptors in the muscle, while 

 pain results from great pressure. The total number of tactile 

 corpuscles (excluding those on the head) has been estimated as 

 500,000. These are not evenly distributed over the skin but are 

 more numerous and more sensitive on certain of the more mobile 

 parts of the body, e.g. tongue and fingers. The degree of sensitive- 

 ness of the skin may be determined by some form of aesthesiometer 



STIMULUS 



_ SENSATION 



LATENT PERIOD 

 OR THRESHOLD 



FIG. 41. Diagram to show relationship between stimulus and sensation. 



(say a pair of compasses) by means of which one may measure 

 the smallest distance at which impress of the two points may be 

 perceived as two distinct sensations. The following table gives 

 the activity of the discriminating sense for different parts of the 

 skin : 



TABLE XXIX. 



Tip of the tongue - 

 Third phalanx of finger, volar surface - 

 Red part of the lip 



Second phalanx of finger, volar surface 

 First phalanx of finger, volar surface - 

 Third phalanx of finger, dorsal surface 

 Tip of nose - 



Head of metacarpal bone, volar surface 

 Hall of thumlt 

 Hall of little finger 



( 'tMitre of palm - - 



Dorsum and side of tongue ; white of lips ; meta- 

 carpal part of the thumb 



Third phalanx of the great toe, plantar surface 

 Second phalanx of the fingers, dorsal surface 



Back - 



Eyelid 



Centre of hard palate - 



Millimetres. 

 M 



- 2-2-3 



4-5 



- 4-4-5 



- 5-5-5 



6-8 

 6-8 

 5-6-8 



- 6-5-7 

 5-5-6 



- 8-9 



- 9 



11-3 

 11-3 

 11-3 

 11-3 

 13-5 



