CHAP. XXIII.] THE TONGUE. 165 



ward, downward, backward, forward, and lateral motions, by means 

 of the muscles of mastication acting on the bone in which they are 

 implanted. By these motions of the lower teeth upon the upper, 

 the food is comminuted. A more detailed description of some of 

 the organs of mastication may now be given. 



The cheeks form the outer wall of that part of the mouth which 

 lies outside the teeth. Like the lips, with which they are continuous, 

 they consist of a muscular stratum interposed between the skin and 

 the mucous membrane of the mouth. They admit of distension and 

 compression, and form pouches which receive portions of food escaping 

 outside the teeth during mastication, and from which it is continually 

 returned towards the inner cavity, to be submitted to the grinding 

 action. This use of the cheek is well exemplified in cases of paralysis 

 of the buccinator muscle, in which the food collects in the flaccid 

 bag to which this part is then reduced. 



The tongue is an important agent of mastication, and has been 

 already spoken of as the seat of taste and of an exquisite sense of 

 touch. By virtue of the latter it receives accurate impressions of the 

 tangible qualities of the food, and of its situation in the mouth ; and 

 by its great mobility, it is constituted the main instrument by which 

 the food is moved within the mouth, so as to be effectually brought 

 within the range of the masticating organs. The tongue rests upon 

 the hyoid arch, to which, and to the concavity of the lower jaw, it 

 is fixed by the muscles. It lies within the curve of the teeth, and is 

 covered on its free surface with mucous membrane, which has been 

 before described. The muscular fibres which compose this organ in- 

 tersect one another in an intricate mariner in its interior, but they 

 all appear to arrive ultimately at its dorsal surface, and to be there 

 implanted, in small sets or bundles, into the submucous stratum of 

 dense areolar tissue, a good deal of fat being disseminated through- 

 out, but especially in the intervals between the muscular bundles at 

 their insertion. We refer to works on descriptive anatomy for the 

 anatomy of the muscles. By their action the upper surface of the 

 tongue may be made convex or hollow, or may be pressed forcibly 

 against the roof of the mouth ; the tip of the organ may be protruded . 

 or moved in any direction, and to any recess within the cavity 

 where food might lodge, and the whole organ may be lowered or 

 drawn back. These several actions are so performed as to exem- 

 plify, in the most perfect manner, the concert which has been already 

 mentioned to occur between many muscular and sensitive parts. 



Of the Teeth. These, in the widest acceptation of the term, and 

 in reference to the whole animal scale, are hard organs situated on 



