184 TEXT-BOOK OF PHYSIOLOGY 



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 pyloric 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 

 opposite effect. The splanchnic nerves therefore apparently contain both 

 inhibitor and augmentor fibers for the gastric musculature though the inhibi- 

 tor fibers largely predominate. From these facts it would appear that the 

 gastric muscles receive both inhibitor and augmentor fibers from two differ- 

 ent sources. 



Vomiting. Vomiting may be defined as a spasmodic ejection through 

 the esophagus and mouth of the contents of the stomach. This act, 

 which is rather complicated, involves the cooperative activity of the walls 

 of the stomach, the diaphragm and the abdominal muscles. 



The vomiting is preceded by a feeling of nausea, a flow of saliva into 

 the mouth and short spasmodic inspirations. Preceding the act of ejection 

 peristaltic waves run from the pyloric end of the stomach toward the 

 cardiac end, the cardiac sphincter of which is partially relaxed. Suddenly 

 the diaphragm vigorously contracts and forces the stomach downward, 

 while simultaneously the abdominal walls spasmodically contract and 

 press the stomach upward against the resisting diaphragm. By this 

 increase of the abdominal pressure the contents of the stomach are driven 

 through the cardiac orifice, through the esophagus and out of the mouth. 



The Nerve Mechanism of Vomiting. This mechanism includes 

 efferent nerves, a central mechanism and afferent nerves. 



The efferent nerves are those which innervate the diaphragm (the 

 phrenics), the abdominal muscles (the lower thoracics) and the walls 



