174 HAND-BOOK OF PHYSIOLOGY. 



marked manifestations of pain to which its division in animals gives rise, 

 that it is likewise a nerve of common sensibility. The glosso-pharyngeal 

 also seems to contain fibres both of common sensation and of the special 

 sense of taste. 



The functions of the tongue in connection with (3) speech, (4) masti- 

 tication, (5) deglutition, (6) suction, have been referred to in other 

 chapters. 



Taste and Smell; Perceptions. The concurrence of common 

 and special sensibility in the same part makes it sometimes difficult to 

 determine whether the impression produced by a substance is perceived 

 through the ordinary; sensitive fibres, or through those of the sense of 

 taste. In many cases, indeed, it is probable that both sets of nerve-fibres 

 are concerned, as when irritating acrid substances are introduced into the 

 mouth. 



Much of the perfection of the sense of taste is often due to the sapid 

 substances being also odorous, and exciting the simultaneous action of 

 the sense of smell. This is shown by the imperfection of the taste of 

 such substances when their action on the olfactory nerves is prevented by 

 closing the nostrils. Many fine wines lose much of their apparent excel- 

 lence if the nostrils are held close while they are drunk. 



Varieties of Tastes. Among the most clearly defined tastes are 

 the sweet and bitter (which are more or less opposed to each other), the 

 acid, alkaline, and saline tastes. Acid and alkaline taste may be excited 

 by electricity. If a piece of zinc be placed beneath and a piece of copper 

 above the tongue, and their ends brought into contact, an acid taste (due 

 to the feeble galvanic current) is produced. The delicacy of the sense of 

 taste is sufficient to discern 1 part of sulphuric acid in 1000 of water; but 

 it is far surpassed in acuteness by the sense of smell. 



After-tastes. Very distinct sensations of taste are frequently left after 

 the substances which excited them have ceased to act on the nerve; and 

 such sensations often endure for a long time, and modify the taste of 

 other substances applied to the tongue afterward. Thus, the taste of sweet 

 substances spoils the flavor of wine, the taste of cheese improves it. There 

 appears, therefore, to exist the same relation between tastes as between 

 colors, of which those that are opposed or complementary render each 

 other more vivid, though no general principles governing this relation have 

 been discovered in the case of tastes. In the art of cooking, however, at- 

 tention has at all times been paid to the consonance or harmony of flavors 

 in their combination or order of succession, just as in painting and music 

 the fundamental principles of harmony have been employed empirically 

 while the theoretical laws were unknown. 



Frequent and continued repetitions of the same taste render the per- 

 ception of it less and less distinct, in the same way that a color becomes 

 more and more dull and indistinct the longer the eye is fixed upon it. 



