372 DEGLUTITION. [BOOK n. 



the two coats must contract at different times, otherwise they 

 would neutralise each other's action. Most probably a section of 

 the longitudinal coat contracts in front of the section of the cir- 

 cular coat which is about to contract, thus affording room for the 

 contents which are about to be driven on, or even itself drawing 

 them forward ; but a contraction of the longitudinal coat, even if 

 it followed after that of the circular coat, might still be useful in 

 helping to bring back the tube to its normal width. 



In the small intestine the tube is hung loosely and much 

 twisted so that many loops are formed; the contents moreover aie 

 largely fluid. Hence the steady onward movement, such as is seen 

 when more solid contents pass along the straight and somewhat 

 firmly attached oesophagus, is complicated by movements due to a 

 loop being projected forward by the entrance of fluid from above, 

 or being dragged down by the weight of its new contents, or, on 

 the other hand, due to a loop being retracted by the driving on- 

 ward of its contents and the emptying of itself, and the like. In 

 this way a peculiar writhing movement of the bowel is brought 

 about, and the phrase * peristaltic movement ' is generally used to 

 denote this total effect of the contraction of the muscular coats ; it 

 will however be best to restrict the meaning to the progressive 

 contraction of the circular coat assisted, in most cases, by a similar 

 progressive contraction of the longitudinal coat. We may con- 

 sider the several special movements of the different parts of the 

 canal. 



Mastication. This in man consists chiefly of an up and down 

 movement of the lower jaw, combined, in the grinding action of 

 the molar teeth, with a certain amount of lateral and fore-and-aft 

 movement. The lower jaw is raised by means of the temporal, 

 masseter, and internal pterygoid muscles. The slighter effort of 

 depression brings into action chiefly the digastric muscle, though 

 the mylohyoid and geniohyoid probably share in the matter. 

 Contraction of the external pterygoids pulls forward the condyles, 

 and thrusts the lower teeth in front of the upper. Contraction 

 of the pterygoids on one side will also throw the teeth on to the 

 opposite side. The lower horizontally placed fibres of the tempo- 

 ral serve to retract the jaw. 



During mastication the food is moved to and fro, and rolled 

 about by the movements of the tongue. These are effected by the 

 muscles of that organ governed by the hypoglossal nerve. 



The act of mastication is a voluntary one, guided, as are so 

 many voluntary acts, not only by muscular sense but also by con- 

 tact sensations. The motor fibres of the fifth cranial nerve convey 

 motor impulses from the brain to the above-mentioned muscles ; 

 but paralysis of the sensory fibres of the same nerve renders 

 mastication difficult by depriving the will of the aid of the usual 

 sensations. 



219. Deglutition. The food when sufficiently masticated is, 



