486 THE SPECIAL SENSES. 



bility, a direct and simple one. The sapid substances in solution 

 penetrate the lingual papillae by endosmosis, and, coming in actual 

 contact with the terminal nervous filaments, excite their sensibility 

 by uniting with their substance. We have already seen that the 

 rapidity with which endosmosis will take place under certain con- 

 ditions is sufficiently great to account for the almost instantaneous 

 perception of the taste of sapid substances when introduced into the 

 mouth. 



It is on this account that a free secretion of the salivary fluids is 

 so essential to the full performance of the gustatory function. If 

 the mouth be dry and parched, our food seems to have lost its taste : 

 but when the saliva is freely secreted, it is readily mixed with the 

 food in mastication, and assists in the solution of its sapid ingredi- 

 ents ; and the fluids of the mouth, thus impregnated with the savory 

 substances, are absorbed by the mucous membrane, and excite the 

 gustatory nerves. An important part, also, is taken in this process 

 by the movements of the tongue ; for by these movements the food 

 is carried from one part of the mouth to another, pressed against 

 the hard palate, the gums, and the cheeks, its solution assisted, and 

 the penetration of the fluids into the substance of the papillae more 

 rapidly accomplished. If a little powdered sugar, or some vege- 

 table extract be simply placed upon the dorsum of the tongue, but 

 little effect is produced ; but as soon as it is pressed by the tongue 

 against the roof of the mouth, as naturally happens in eating or 

 drinking, its taste is immediately perceived. This effect is easily 

 explained ; since we know how readily movement over a free sur- 

 face, combined with slight friction, will facilitate the imbibition of 

 liquid substances. The nervous papillae of the tongue may there- 

 fore be regarded as the essential organs of the sense of taste, and 

 the lingual muscles as its accessory organs. 



The full effect of sapid substances is not obtained until they are actu- 

 ally swallowed. During the preliminary process of mastication a 

 sufficient degree of impression is produced to enable us to perceive 

 the presence of any disagreeable or injurious ingredient in the food, 

 and to get rid of it, if we desire. But it is only when the food is 

 carried backward into the fauces and pharynx, and is compressed 

 by the constrictor muscles of these parts, that we obtain a complete 

 perception of its sapid qualities. For at that time the food is spread 

 out by the compression of the muscles, and brought at once in 

 contact with the entire extent of the mucous membrane possessing 

 gustative sensibility. Then, it is no longer under the control of the 



