6fi8 HANDBOOK OF PHYSIOLOGY. 



also of its papillae, is endued (2) especially at its side and tip, with uvery 

 delicate and accurate sense of touch, which renders it sensible of the 

 impressions of heat and cold, pain and mechanical pressure, and conse- 

 quently of the form of surfaces. The tongue may lose its common sen- 

 sibility, and still retain the sense of taste, and vice versa. This fact 

 renders it probable that, although the senses of taste and of touch may 

 be exercised by the same papillae supplied by the same nerves, yet the 

 nervous conductors for these two different sensations are distinct, just 

 as the nerves for smell and common sensibility in the nostrils are dis- 

 tinct; and it is quite conceivable that the same nervous trunk may con- 

 tain fibres differing essentially in their specific properties. Facts already 

 detailed seem to prove that the lingual branch of the fifth nerve is the 

 conductor of sensations of taste in the anterior part of the tongue ; and 

 it is also certain, from the marked manifestations of pain to which its 

 division in animals gives rise, that it is likewise a nerve of common sen- 

 sibility. The giosso-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) mas- 

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

 chapters. 



Taste and Smell: Perceptions. -The concurrence of common and two 

 kinds of special sensibility, i.e., touch and taste 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 

 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, salt, and metallic tastes. Acid and alkaline taste may be ex- 

 cited 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. Ex- 

 periments have shown that it is possible to entirely do away with the 

 power of tasting bitters and sweets while the taste for acids and salts 



