562 HANDBOOK OF PHYSIOLOGY. 



Taste and Smell: Perceptions. The concurrence of common and 

 special sensibility in the same part makes it sometimes difficult to deter- 

 mine 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 

 excellence 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 bv 

 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-taste. 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 afterwards. 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, attention has at all times been paid to the conso- 

 nance 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. 

 Thus, after frequently tasting first one and then the other of two kinds 

 of wine, it becomes impossible to discriminate between them. 



The simple contact of a sapid substance with the surface of the gus- 

 tatory organ seldom gives rise to a distinct sensation of taste; it needs 

 to be diffused over the surface, and brought into intimate contact with 

 the sensitive parts by compression, friction, and motion between the 

 tongue and palate. 



