138 HUMAN PHYSIOLOGY 



of the large circumvallate papillae which form the prominent 

 V-shaped line on the posterior part of the dorsum. 



Each is composed of a covering of elongated cells like the 

 staves of a barrel, enclosing a set of spindle-shaped cells with 

 which the dendrites on the end of the nerve fibres are closely 

 associated. 



These nerve fibres pass to the brain in the fifth nerve. 

 Several cases of complete loss of taste have been recorded 

 by Gowers in which the root of the fifth nerve alone was 

 destroyed, but it is usually thought that the glosso-pharyngeal 

 also carries nerve fibres connected with taste, and the recent 

 observations favour this view. 



Physiology. As to the way in which this mechanism 

 is stimulated our knowledge is very imperfect. In order 

 to act the substance must be in solution. The strength of 

 the sensation depends on the concentration of the solution, 

 upon the extent of the surface of the tongue acted upon, 

 upon the duration of the action, and upon the tempera- 

 ture of the solution. If the temperature is very high or 

 very low the taste sensation is impaired by the feelings 

 of cold or heat. 



It is most difficult to classify the many various taste sensa- 

 tions which may be experienced, but they may roughly be 

 divided into four main groups : 



1. Sweet. 3. Acid. 



2. Bitter. 4. Saline. 



Whether different sets of terminations react specially to 

 each of these is not known, but it has been found that sub- 

 stances giving rise to the sensation which we call bitter act 

 best on the back of the tongue, while substances producing 

 sweet or acid sensations act on the sides and front. Again, 

 chewing the leaves of gymnema sylvestre abolishes sensations 

 of sweet and bitter, but does not interfere with those of acid 

 and saline, and leaves the tactile sense unimpaired. On the 

 other hand, cocaine paralyses the tactile sense before it inter- 

 feres with the sense of taste. 



The sense of taste is very closely connected with the sense 

 of smell, and, when the latter is interfered with, many sub- 



