242 OUTPOSTS OF THE INTELLIGENCE SERVICE 



III. POSTURAL RECEPTORS 



In this class fall the receptors in muscles, tendons and joints, 

 the proprioceptors of Sherrington's classification. They convey 

 information to the central nervous system about the position of 

 the limbs, etc. (To this group also belong the organs which have 

 to do with the sensations of equilibrium, which will be considered 

 in the next chapter.) Of their physics nothing is known. Recently 

 Adrian has studied the electrical variations in the sensory nerves 

 supplied by them and finds that the electrical discharge is con- 

 tinuous with the stimulation for quite a long time. That is, 

 adaptation is slow. A frog's muscle spindle may continue to 

 discharge impulses for 10 minutes or more when a steady stretching 

 force is applied to the muscle during that time. (See also the 

 vestibular apparatus at the end of the next chapter.) 



IV. SPECIAL RECEPTORS 



Taste and smell are the chemical senses (partly chemical and 

 partly physical) and are closely allied to touch. To stimulate the 

 end-organs of chemical sense, the substance must be in a fine state 

 of division and capable of going into solution in the fluid on the 

 superficies of the sense organ. 



6. Taste. The end-organs for the sense of taste, the so-called 

 taste-buds, are found on the tongue except in the mid-dorsal 

 region, on both the anterior and posterior surfaces of the epiglottis, 

 on the inner surface of the arytenoid process of the larynx, on the 

 soft palate above the uvula, on the anterior pillars of the fauces, 

 and on the posterior wall of the pharynx. They differ in structure 

 and markedly in threshold value with their position. For instance, 

 at the tip of the tongue the structure of the end-organs is such that 

 substances like sugars, amino acids, etc., penetrate easily into the 

 cells surrounding the nerve endings and cause the liberation of 

 something which stimulates the terminations of the nerve. Bitter 

 substances, like quinine, saccharine, etc., affect the receptors near 

 the back of the tongue, the hydrogen ion finds a lower threshold 

 in the receptors at the sides, while chlorides and other salty anions 

 are catered for by end-organs all o^'er the surface of the tongue. 

 Most work has been done on the receptor structures at the tip and 

 on the sides of the tongue. 



Sweetness. This has been investigated almost solely from the 

 chemical standpoint, and the result has been the drawing up of 

 two lists of groupings, e.g. gluciphores and auxoglucs. A substance 

 is sweet, i.e. capable of penetrating the cells and stimulating the 



