!8 4 TEXT-BOOK OF PHYSIOLOGY 



in the stomach because of the closure of the cardia (the esophago-gastric 

 orifice) and of the pylorus (the gastro-duodenal orifice) both orifices being 

 tightly closed by the tonic contraction of sphincter muscles; that both sphinc- 

 ters relax from time to time, the one to permit the entrance of food into the 

 stomach for further digestion, the other to permit the exit of food into 

 the intestine after its more or less complete digestion, after which in both 

 instances the sphincters again contract and close the orifices; that the fundus 

 muscles are tonically contracted and steadily pressing the food into and 

 through the cardiac region to the vestibule; that the pyloric or vestibular 

 muscles are vigorously active throughout the digestive period, triturating 

 the food, mixing it with gastric juice, and finally driving it through the 

 temporarily open pylorus into the intestine. 



These separate but related groups of muscle-fibers, by reason of their 

 physiologic endowments, and possibly by virtue of the presence of local nerve 

 mechanisms, exhibit activities which are independent of the central nerve 

 system. Thus the isolated stomach of the dog and of other animals as well, 

 if kept warm and moist, will exhibit rhythmic movements for a period of 

 time varying from an hour to an hour and a half. Though nerve-cells and 

 nerve-fibers (Auerbach's plexus) are present in the walls of the stomach be- 

 tween the layers of muscle-fibers, it is not believed that they are the immedi- 

 ate sources of the stimulus to the contraction, though they may act as a coordi- 

 nating mechanism. 



Though the activities of the pyloric muscles and of the pyloric sphincter, 

 if not the entire gastric musculature, are due in large measure to conditions 

 referred to in foregoing paragraphs, nevertheless the activities of all portions 

 of the musculature are susceptible to modifications by the central nerve sys- 

 tem either in the way of augmentation or inhibition and in response to intra- 

 gastric stimulation. The nerves more especially concerned in the main- 

 tenance and regulation of the gastric tonus and contractions are the vagi and 

 the splanchnics. The afferent fibers through which nerve impulses pass to 

 the nerve centers are in all probability contained 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. 



The Vagi. 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 peripheral 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 may be 

 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 

 the case in his experiments. A similar inhibition may be brought about 

 reflexly by stimulation of the central end of a divided vagus. This result 

 will not be produced if the opposite vagus has previously been divided. The 

 vagi, therefore, apparently contain both inhibitor and augmentor nerve- 

 fibers for the gastric musculature. 



The Splanchnics. If the splanchnic nerves are divided and the peripheral 

 end stimulated with induced electric currents there follows an inhibition of 

 the peristalsis and a loss of tone. Morat, however, has observed a primary 



