112 ESSENTIALS OF PHYSIOLOGY. 







on both sides of the vallum which surrounds each papilla. A taste 

 bulb is an oval body bounded externally by spindle-shaped cells, arid 

 containing in its interior other cells which are fusiform ; each of the 

 latter terminates at its peripheral extremity by a hair-like process, which 

 projects into the vallum through an aperture called the gustatory 

 pore. The terminations of the fibres of the nerves of taste penetrate 

 the other pole of the taste bulb, and end by arborisation round the 

 fusiform cells, which are the end-organs for the sense of taste. Serous 

 glands lie in the connective tissue subjacent to the vallate papillae, and 

 their ducts open into the lower part of each vallum. 



The substances which act as stimuli for the sense of taste must 

 be in solution. Ordinary foodstuffs possess both taste and flavour, 

 the appreciation of the latter depending on the sense of smell, so that, 

 if the nose be firmly held so as to prevent air currents reaching the 

 olfactory membrane while food is in the mouth, flavours are not appre- 

 ciated. There are only four true taste sensations sweet, bitter, salt, 

 and sour. Any other sensation excited in the mouth, for example, 

 astringency, is due to stimulation of the nerves of common sensibility. 



As in the case of the skin, so in the case of taste, different nerves 

 are concerned in the different sensations. This is shown (1) by the fact 

 that some areas of the tongue are more sensitive than others to the 

 different sensations, and (2) by the effect of drugs. Thus (Ij the tip 

 of the tongue is most sensitive to substances giving rise to the sensation 

 of sweetness, thg_back to those which arouse a bitter sensation, the 

 sides and upper surface to appreciation of sourness, while a salt taste 

 may be excited over the surface generally. There are, moreover, 

 substances which give different taste sensations according to the part 

 of the tongue on which they are placed. Parabrom-benzoic sulphinid, 

 for example, excites a sweet sensation if placed on the tip of the tongue, 

 but only a bitter sensation if placed on the posterior part. Further, 

 the individual papillae have been tested, with the result that some are 

 found to be more sensitive to substances which give rise to a sweet 

 sensation, others to those which excite the other taste qualities, while 

 most are sensitive to more than one quality. (2) Further evidence for 

 the existence of specific nerves for the various taste sensations is derived 

 from the effect of drugs. Cocaine applied to the papillae has no effect 

 on the production of salt sensations, but it abolishes the other three 

 qualities in a definite order, bitter being the first and sour the last to 

 disappear. Gymnenic acid, from the leaves of Gymnena sylvestre, on 

 the other hand, abolishes only the production of sweet and bitter 

 sensations, the former going first. 



The nerves of taste are the chorda tympani to the anterior two- 



