U2 



AN AMERICAN TEXT-BOOK OF I'll YSIOIJ Hi Y 



The excitement of the taste-nerves appears to depend not so much on the 

 absolute amount of the substance to be detected as on the concentration of the 

 solution containing it. Thus, when, 1 part of common salt to 213 of water 

 was tasted by Valentin, 1 11 cubic centimeters of the fluid was sufficient to give 

 a saltish taste; when diluted so that the ratio of salt to water was 1 to 426, 

 12 cubic centimeters taken in the mouth scarcely gave the salt taste. Sulphate 

 of quinine dissolved in the proportion 1 to 33,000 gave a decided bitter taste, 

 but a solution 1 to 1,000,000 was with difficulty perceived as bitter. 



It has generally been conceded that all gustatory sensations may be built 

 up out of four primary taste-sensations — namely, bitter, sweet, sour, and salt. 

 Some authors even limit the list to tastes of bitter and sweet (V. Vintschgau). 



A uditory. 



Gustatory. 



Tactile. 



PIG. 207-Diagram showing the mode of termination of sensory nerve-fibres in the auditory, gustatory, 

 and tactile structures of vertebrata (from Quain, after Retzius). Each sense organ may be considered as 

 essentially constructed of a nerve-cell with two processes, one finding its way centrally to cluster round 

 other nerve-cells or their processes, and the other to terminate in the periphery. In the organ of smell 

 the peripheral process is very short and is directly irritated by foreign particles, the original nerve-cell 

 being represented by the olfactory cell (Fig. 291). In the organs of touch the nerve-cell is found in the 

 ganglion of the posterior spinal nerve-root; the peripheral process is very long Mini is acted on indirectly 

 through the modified epithelium round which it clusters. The same may be said of the other sense 

 organs. See Quain's Anatomy, 10th ed., vol. iii. pt. 3, p. 152. 



There is strong reason to believe that corresponding to the four primary taste- 

 sensations there are separate centres and nerve-fibres, each of which, when 

 excited, gives rise only to its appropriate taste-sensation. Substances which 

 arouse the sense of taste are not appreciated in uniform degree over the surface 

 of the tongue. Thus, to V. Vintschgau, at the tip of the tongue acids were 

 perceived acutely, sweets somewhat less plainly, and bitter substances hardly 

 at all. It is generally admitted that sweet and sour tastes are recognized 

 chiefly at the front, and bitter, together with alkaline tastes, by the posterior 



1 Lehrbuch der Physiohgw, 1848. 



