SENSATIONS AND THE SENSE-ORGANS 141 



and only one sensory property presumably has just 

 beneath it a receptor for one kind of stimulation. The 

 capacity for sensation which we can demonstrate for 

 the skin holds with minor differences for the lining of 

 the mouth. In the esophagus we have a persistence 

 of the response to temperature changes but a practical 

 loss of the other feelings. 



Liminal Distance. A general idea of the relative 

 refinement of the pressure sense in various parts of the 

 body may be secured by finding how far apart two blunt 

 points must be placed so that when they are applied 

 to the skin they shall be felt as two rather than one. 

 The interval necessary for any part of the surface of 

 the body is called the liminal distance. It is least on 

 the tip of the tongue where two points need be separated 

 by only J/ 5 inch to be felt distinctly. The dis- 

 tance is about twice as great on the finger tips. This 

 explains why a cavity in a tooth feels much larger when 

 the tongue is presseed into it than when it is examined 

 with the finger. Some portions of the skin are in- 

 credibly deficient in discrimination; on the back it may 

 be necessary to touch points 2 inches apart to have their 

 separate nature apparent to the subject. 



Taste. The receptors for this sense are mostly on 

 the tongue. They are stimulated by many substances 

 which must be in solution to have an effect. There are 

 probably but four distinct kinds of taste: sweet, sour, 

 bitter, and salt. The countless other qualities which 

 we detect in our food are compounded of odors, con- 

 tacts, temperature impressions, and in some cases an ele- 

 ment of irritation verging on pain. There are probably 

 four sets of fibers leading from the tongue to the brain 

 and their endings have a somewhat characteristic 

 distribution. 



Sweet substances are most stimulating to the tip of 

 the tongue. (Observe the happy child boring into the 

 interior of a bonbon with that part of the little member). 

 Salt affects rather strongly the edges of the tongue and 



