PASSAGE OF FOOD FROM MOUTH TO STOMACH 681 



peristaltic wave forces it through the orifice. When several acts of deglu- 

 tition succeed one another, the cardiac sphincter shares in the inhibition 

 of the cesophageal walls, and offers no resistance to the direct propulsion 

 of food from the mouth to the stomach. 



Cannon has shown that the relaxation of the cardiac orifice which 

 accompanies swallowing extends also to the cardiac end of the stomach, 

 This relaxation lowers the pressure within the stomach, and makes room for 

 the incoming food. 



If the stomach be filled with a fluid such as starch solution, the cardiac 

 sphincter may be seen to relax rhythmically, allowing of the regurgitation of 



FIG. 335. Tracings of respiratory movementsFto show the effect of stimulating the 



central end of the glossopharyngeal nerve. (MABCKWALD.) 



The point of stimulation is marked with a cross. Note that the stoppage 



may occur at any phase of the respiratory movement. 



the stomach contents into the lower part of the oesophagus. Their entry 

 into this tube is at once followed by a peristaltic contraction of this part of the 

 oesophagus (apparently entirely unconscious), which drives the fluid back 

 into the stomach. These movements of regurgitation become more and 

 more infrequent as the gastric contents become acid, and are not observed 

 at all if the stomach be filled with a fluid already acid. This phenomenon 

 has been spoken of as the ' acid control of the cardia.' 



THE NERVOUS MECHANISM OF DEGLUTITION 

 Deglutition is a reflex act. When we swallow voluntarily we supply the 

 necessary initial stimulus either by touching the fauces with the tongue or 

 by forcing a certain amount of saliva into the fauces. The afferent channels 

 of the reflex are contained in the second division of the fifth nerve, the glosso- 

 pharyngeal nerve, and the pharyngeal branches of the superior laryngeal 

 nerve. We can excite a single act or a whole series of acts of deglutition by 

 electrical stimulation of the central end of the last-named nerve. The 

 efferent fibres which determine the contraction of muscles engaged in the 

 act of deglutition travel by the hypoglossal nerve to the muscles of the 



22* 



