!8o TEXT-BOOK OF PHYSIOLOGY 



are also contracted and the orifices they surround are more or less tightly 

 closed. 



The Movements of the Sphincter Car dice. The sphincter cardice muscle 

 surrounding the esophago-gastric orifice is always, under normal conditions, 

 tonically contracted and the orifice closed. This contraction is partly due 

 to inherent causes as shown by the fact that it persists for from 24 hours to 

 several days after division of all nerves distributed to it. The contraction 

 may be so pronounced as to offer considerable resistance not only to the pass- 

 age of food but even to the introduction of a sound into the stomach. (Can- 

 non.) That the normal contraction is under the influence of the central 

 nerve system is shown by the effects which follow division and stimulation 

 of the peripheral end of the vagus. If it is stimulated with weak induced 

 currents, the contraction of the sphincter is somewhat inhibited and the 

 orifice enlarged; if it is stimulated with strong currents the contraction is 

 markedly increased and the orifice diminished. Apparently there are in the 

 vagus two sets of efferent nerve-fibers, one of which inhibits while the other 

 augments the contraction, and corresponding to the nerves there must be 

 in the medulla oblongata two centers from which they arise, an augmentor 

 and an inhibitor. 



Observation has also shown that at the beginning of each act of degluti- 

 tion, there is an inhibition of the sphincter muscle, and if the acts follow each 

 other in quick succession, the inhibition and relaxation are increased. 

 (Meltzer.) With the passage of food into the stomach the tonic contrac- 

 tion again supervenes. These effects also follow stimulation of the glosso- 

 pharyngeal nerve. Whether the sphincter inhibition is the result of an 

 inhibition of the center which maintains the tonus, or a stimulation of an 

 inhibitor center, is uncertain. 



Though the tonus of the sphincter cardise is capable of being inhibited 

 and augmented by the central nerve system, the most frequent cause under 

 physiological conditions for an augmentation of the tonus, is the presence of 

 hydrochloric acid on the gastric side of the cardia. In the early stages of 

 digestion when the percentage of the acid is below the normal, the orifice 

 frequently opens and food is regurgitated for a short distance into the esopha- 

 gus. The ascent of the food far into the esophagus is prevented by a con- 

 traction and peristalsis of its muscle walls, by which the food is returned to 

 the stomach. As the free acid accumulates the sphincter tightly closes the 

 orifice. As this takes place after division of the nerves passing to this region 

 of the esophagus the inference is that the contraction of the sphincter is 

 brought about through the intermediation of a local reflex nerve mechanism 

 in the walls of the esophagus. 



It has recently been reported by Cannon that a similar inhibition or 

 relaxation of the musculature of the cardiac end of the stomach is occasioned 

 by each act of deglutition and that it continues and increases if the acts 

 follow each other in quick succession. As the bolus descends the esophagus 

 and before it reaches its termination there is a relaxation of the musculature of 

 the cardiac end, a fall of intragastric pressure, an enlargement of the stomach 

 capacity and hence a readier receptivity of the bolus. That this inhibition 

 is caused by impulses descending the vagus is shown by the effects which 

 follow a moderate stimulation of the vagus nerve and by the fact that it 

 does not take place if the vagus nerves are divided. To this inhibition and 



