PHYSIOLOGY OF GUSTATION 133 



are concerned with taste; the beginning of the gullet, 

 the region of the arytenoid cartilages within the larynx, 

 the epiglottis, the soft palate, and particularly the tongue. 

 In all these regions taste-buds have been identified. On 

 the tongue of adult human beings taste is limited to the 

 tip, the lateral margins and the dorsal surface of the 

 root, the large central area on the upper surface of this 

 organ being devoid of taste. In children, as contrasted 

 with adults, the whole upper surface of the tongue in- 

 cluding the central area is said to be sensitive to taste 

 as is also the inner surfaces of the cheeks. 



2. Gustatory stimuli. The stimulus for taste is an 

 aqueous solution of a great variety of substances. Mate- 

 rials insoluble in water are tasteless,, but not all substances 

 that form aqueous solutions have taste. Thus oxygen, 

 hydrogen, and nitrogen, though freely soluble in water, 

 are without taste. Piutti (1886) long ago showed that 

 laevo-asparagine is tasteless, although its stereoisomer 

 dextro-asparagine is sweet. Other organic compounds, 

 such as the carbohydrates raffinose and alpha-galaoctite 

 are said to be almost, if not quite, tasteless. 



When solids or semi-solids are chewed in the mouth, 

 they not only become mixed with the saliva whereby many 

 of their components become dissolved, but they are 

 spread over the surface of the tongue and are thus 

 brought into intimate contact with its taste-buds. In fact 

 it is not improbable that the movement of the tongue fa- 

 cilitates the entrance of these solutions into the pores of 

 the taste-buds. At least solutions placed upon the 

 tongue, particularly near its root, are tasted with greater 

 certainty, when this organ is moved about than when it 

 is held still. 



