500 PHYSIOLOGY 



as in the eye, where a sensation of white light may result from stimulation of 

 the retina by two complementary colours. Stimulation of one kind of taste- 

 organ heightens the sensibility of the other taste- organs. Thus after the 

 application of salt, distilled water may taste sweet. 



That these primitive taste sensations are served by different nerve- 

 endings is shown by the following facts : 



(a) The tongue is not equally sensitive at all points to all four tastes. 

 Thus the back of the tongue is more sensitive to bitter, while the tip and 

 sides of the tongue react more easily to sweet and sour substances. A differ- 

 ence may be detected between even the circumvallate papillae themselves ; 

 a mixture of quinine and sugar applied to one papilla may excite chiefly 

 a bitter taste, while with an adjacent papilla a sweet taste may predominate. 



(6) By certain drugs we can depress the sensibility of the taste-organs, 

 and we then find that the various tastes are affected to different degrees. 

 Thus on painting the tongue with cocaine the first effect is a diminution 

 of tactile and pain sensibility, so that the application of acid evokes a very 

 sour taste without any of the astringent or stinging sensations normally 

 aroused by the contact with the acid. After this point the taste sensations 

 are also abolished. The bitter sensation disappears first, then the sweet, 

 and then the sour, while the taste of salt appears to remain unaffected. On 

 the other hand, if the leaves of Gymnema sylvestre be chewed, the sensations 

 of bitter and sweet are abolished, leaving intact the acid and salt tastes, 

 and also the general sensibility of the mucous membrane. 



There is no doubt that the stimulating effect of any chemical substance 

 on the taste-nerves has relation to its chemical constitution. Thus a sour 

 taste is determined by the presence of H ions ; the alkaline taste by that 

 of OH ions. The fact that certain acids, e.g. acetic, have a stronger sour 

 taste than would correspond to their dissociation, i.e. to the number of H 

 ions present, is due to the fact that these acids penetrate more easily into 

 the gustatory cells than the mineral acids with a larger dissociation co- 

 efficient. All the oc-amino-acids have a sweet taste. On the other hand, 

 the polypeptides produced by the combination of these amino-acids, as 

 well as the peptones derived from the hydrolysis of proteins, have a bitter 

 taste. Most of the alcohols and sugars have a sweet taste, while the metallic 

 derivatives of these substances are bitter. We do not yet understand the 

 law which determines whether any given substance shall have a taste at 

 all, and what its taste should be. 



The nerves of taste are the glossopharyngeal, which supplies the back 

 part of the tongue, and the lingual branch of the fifth nerve and the chorda 

 tympani, which supply the front part. All these fibres are probably con- 

 nected with a continuous column of grey matter in the brain stem, which 

 represents the splanchnic afferent nucleus of the fifth nerve, the nervus 

 intermedius, and the glossopharyngeal. Some authors have stated that 

 all the taste fibres of the fifth nerve are derived from the glossopharyngeal 

 by the communication through the tympanic plexus and the chorda tympani 

 nerve, while Gowers has recorded a case of complete unilateral loss of taste 



