718 PHYSIOLOGY OF DIGESTION AND SECRETION. 



cerned. As in the case of other complicated reflex acts, we can only 

 say that the deglutition reflex is controlled by a definite nervous 

 mechanism the final motor cells of which are scattered in the several 

 motor nuclei of the efferent nerves mentioned above. 



So far as the esophagus is concerned, the motor fibers are 

 received from the vagus, and in normal swallowing these fibers are 

 excited reflexly from the pharynx at the beginning of the act. 

 That is to say, the initial sensory stimulus in the pharynx liberates 

 a series of reflex movements which begin with the contraction of the 

 mylohyoid muscle and end with a peristaltic wave that progresses 

 in orderly fashion along the esophagus. It has been shown, 

 however, that the bolus when it reaches the esophagus may start 

 a different order of reflexes by local stimulation of the sensory fibers. 

 These stimuli lead to reflex contraction of the musculature above 

 the bolus, and thus a series of reflexes are liberated which are suffi- 

 cient to move the bolus downward. If the primary reflex initiated 

 at the beginning of the swallow proves inefficient, that is to say, 

 if it succeeds only in forcing the bolus into the top of the esophagus, 

 it may be assumed that this secondary or accessory mechanism 

 comes into play and provides for the transportation of the bolus 

 to the stomach. In this series of secondary reflexes, as in the more 

 complicated primary reflex, the vagus nerve forms a part of the 

 path and the reflex center lies, therefore, in the medulla. 



Cardiac Sphincter. At the cardia or cardiac orifice the cir- 

 cular layer of muscle acts as a sphincter. When the stomach is 

 empty this sphincter is probably relaxed. When the stomach 

 contains food the sphincter is thrown into tone, and thus shuts off 

 the gastric cavity from the esophagus. The sphincter receives 

 both motor and inhibitory fibers from the vagus and inhibitory 

 fibers from the sympathetic system by way of the celiac ganglion. 

 In addition it is supplied from the intrinsic plexus, plexus of 

 Auerbach, which, as elsewhere in the alimentary canal, seems to 

 be capable of regulating the movements of the musculature inde- 

 pendently of the extrinsic nerves. The tonic contraction of the 

 sphincter that occurs when the stomach contains food is main- 

 tained, according to Cannon,* by a reflex through this intrinsic 

 plexus, the stimulus initiating the reflex being due to the acid 

 present in the gastric secretion. The peristaltic wave sweeping 

 down the esophagus is preceded, in the lower part of the esophagus 

 at least, by a wave of inhibition or relaxation. This latter phase 

 affects the sphincter and the adjacent musculature of the stomach 

 and clears the path for the bolus, so that the following wave of 

 contraction may squeeze it through the relaxed orifice without 

 serious resistance. After the passage of the bolus into the stomach 

 * Cannon, "The Mechanical Factors of Digestion," 1911. 



