538 OF SENSATION. 



nerve, through which the sense of Taste, as well as that of Touch, is 

 exercised, in the parts of the tongue to which it is specially distributed, 

 which are those that possess both senses in the most acute degree ; 

 and that the Glosso-pharyngeal is subservient to the same functions in 

 the parts supplied by it, being probably the exclusive channel, also, 

 through which the impressions made by disagreeable substances taken 

 into the mouth, are propagated to the Medulla Oblqngata, so as to pro- 

 duce nausea and excite efforts to vomit. The latter nerve is also, as 

 we have seen, the principal channel of the impressions that give rise to 

 the reflex act of swallowing ; with which the fifth pair is concerned in a 

 much inferior degree ( 897). 



945. A considerable part of the impression produced by many sub- 

 stances taken into the mouth, is received through the sense of Smell, 

 rather than through that of Taste. Of this, any one may easily satisfy 

 himself, by closing the nostrils, and breathing through the mouth only, 

 whilst holding in his mouth, or even rubbing between his tongue and his 

 palate, some aromatic substance ; its taste is then scarcely recognised, 

 although it' is immediately perceived when the nasal passages are re- 

 opened, and its effluvia are drawn into them. There are many sub- 

 stances, however, which have no aromatic or volatile character ; and 

 whose taste, though not in the least dependent upon the action of the 

 nose, is. nevertheless of a powerful character. Some of these produce, 

 by irritating the mucous membrane, a sense of pungency, allied to that 

 which the same substances (mustard, for instance) will produce, when 

 applied to the skin for a sufficient length of time, especially if the Epi- 

 dermis have been removed. Such sensations, therefore, are evidently 

 of the same kind with those of Touch, differing from them only in the 

 degree of sensibility of the organ through which they are received. But 

 there are others which produce sensations entirely different from any 

 that can be received through the skin, and which are properly distin- 

 guished, therefore, as gustative ; such are common Salt, which may be 

 considered as a type of the saline taste, Sugar, the type of the saccharine, 

 Quinine of the bitter, and Tannin of the astringent, and Citric acid of 

 the sour. All such substances, therefore, are said to possess sapid pro- 

 perties, exciting distinctive tastes, quite irrespectively of any aromatic 

 or odoriferous properties which they may also possess, as well as of their 

 stimulating action on the skin. 



4. Of the Sense of Smell 



946. Certain bodies possess the property of exciting sensations of a 

 peculiar nature, which cannot be perceived by the organs of taste or 

 touch, but which seem to depend upon the diffusion of the particles of 

 the substance through the surrounding air, in a state of extreme minute- 

 ness. As the solubility of a substance in liquid seems a necessary con- 

 dition of its exciting the sense of Taste, so does its volatility, or tendency 

 to a vaporous state, appear requisite for its having Odorous properties. 

 Most volatile substances are more or less odorous : whilst those which 

 do not readily transform themselves into vapour, usually possess little 

 or no fragrance in the liquid or solid state, but acquire strong odorous 



