Chap. 17 RESPONSIVENESS THE SENSE ORGANS 313 



In order to taste something, mammals must have the substance on their 

 tongues. Nearly all of them are adept at stretching these tongues outside their 

 mouths, cattle licking salt blocks and human beings licking anything. 



Taste Receptors in Man. Special sense organs known as taste buds are 

 imbedded in the mucous membrane of the soft palate and upper surface of 

 the tongue. Their name comes from their bud-like shape, but they are quite 

 as much like bottles with small mouths, the pores that open into furrows 

 that surround them (Fig. 17.3). 



Anything which is tasted must get into the bottle and bring about the 

 chemical reaction with the receptor cells with which the dendrites of the 

 facial or glossopharyngeal nerve are in contact. These reactions start impulses 

 to the brain, ending in the sour taste of pickles, or the sweet taste of sugar. 

 Salty substances are tasted quickly, bitter ones more slowly, due partly to the 

 distribution of the taste receptors. All four kinds of taste receptors are on 



bitter 



Fig. 17.3. A, diagrams of the human tongue and the distribution of the four 

 tastes, sweet, sour, bitter, salt; the central part of the tongue is insensitive to taste. 

 The closeness of the dots represents the number of the sense organs. The rings at 

 the back mark the papillae, each holding a battery of taste buds. B, section of a 

 papilla of the tongue (much enlarged) with taste buds in the groove that surrounds 

 the papilla. C, human taste bud greatly magnified. Saliva mixed with food juice, for 

 example onion, enters the bud through the poie at the top and chemical reactions 

 take place between it and the sensory cells. Impulses pass over the sensory fibers to 

 the brain, where the little understood process of interpretation occurs and maybe 

 the flavor of candy or of onion is revealed. (After Parker: Smell, Taste and the 

 Allied Senses in th^ Vertebrates. Philadelphia, J. B. Lippincott Co., 1922.) 



