i8o TEXT-BOOK OF PHYSIOLOGY. 



nerves distributed to it. The contraction may be so pronounced as to offer 

 considerable resistance not only to the passage of food but even to the intro- 

 duction of a sound into the stomach. (Cannon.) That the normal con- 

 traction is under the influence of the central nerve system is shown by the 

 effects which follow stimulation of the peripheral end of the divided vagus. 

 If it is stimulated with weak induced currents, the contraction is somewhat 

 inhibited and the orifice enlarged; if it is stimulated with strong currents the 

 contraction is markedly increased. Apparently there are in the vagus two 

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

 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 shown that at the beginning of each act of deglutition, 

 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 the food into the stomach the tonic con- 

 traction 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. 



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 

 enlargement of the cardiac end of the stomach the term receptive relaxa- 

 tion has been given. 



The degree of activity of both the pyloric sphincter and the antral muscles 

 is modified also by the central nerve system either in the way of inhibi- 

 tion or augmentation and in response to gastric stimulation. The nerves 

 more especially concerned in the maintenance and regulation of the gastric 

 contractions are the vagi and the splanchnics. The afferent fibers through 

 which nerve impulses pass to the nerve centers are in all probability con- 

 tained in the trunk of the vagus nerve; the efferent fibers through which 

 nerve impulses from the centers reach the stomach, are contained partly in 

 the trunk of the vagus and partly in the trunk of the splanchnic nerve. 



If the vagus nerves are divided in the neck, there is a loss of muscle tonus 

 though the contractions do not wholly disappear. Stimulation of the per- 

 ipheral end of one divided vagus is followed by an augmentation in the 

 vigor of the contraction of the antral muscles, an increase in the tone of the 

 fundus muscles, as well as an increase in the contraction of the sphincter 

 pylori and sphincter cardiae. Though this is the usual result there maybe a 

 primary relaxation or inhibition of short duration of one or all of these 

 structures before the augmentation occurs. May states that this was always 



