168 SPECIAL MUSCULAR MECHANISMS 



of these foods. For it has been shown that dilute hydrochloric 

 acid will remain in the stomach longer than water. The acid 

 chyme, when it reaches the duodenum, brings about a reflex 

 tonic contraction of the pyloric sphincter until the acid has 

 been neutralized by the alkaline bile and pancreatic secretion. 

 During the sojourn of the food in the stomach the fundus 

 remains in a state of tonic contraction, so that as the chyme 

 leaves the antrum it is gradually replaced by the contents of 

 the fundus. The peristalsis of the stomach is independent 

 of the nervous system. There is, however, a rich supply of 

 nerve fibers from two sources: From the vagi and from the 

 solar plexus. Stimulation of the first causes a contraction, 

 and stimulation of the second an inhibition of the stomach. 

 Their function is probably a regulatory one. 



Vomiting. Vomiting is a complex reflex action. It is pre- 

 ceded by a feeling of nausea, a flow of saliva, and retching 

 movements which are brought about by spasmodic contrac- 

 tions of the diaphragm with a closed glottis. This causes the 

 negative pressure in the thorax to increase and open the esopha- 

 gus, while simultaneously pressure is brought to bear upon 

 the stomach. The abdominal muscles, vigorously contracting, 

 force the contents out of the cardiac end of the stomach and 

 up through the mouth. The glottis is closed, and the nasal 

 passages also. The stomach during vomiting may not be in- 

 active, but the main factor in the ejection of its contents is the 

 contraction of the abdominal muscles. This was shown by 

 Magendie, who substituted a bladder of water for the stomach 

 and produced vomiting by injection of an emetic. It has been 

 shown, moreover, that an emetic is without effect in a curarized 

 animal. Vomiting is brought about by local irritation of the 

 mucous membrane of the stomach, by tickling the pharynx, 

 by psychical states, lesions of the brain, by toxic substances 

 in the blood, etc. The afferent path when the reflex is from the 

 stomach is through the vagus. A centre has been described 

 near the calamus scriptorius, but its existence is not certain. 

 The efferent impulses pass over fibers of the vagus, phrenics, 

 and spinal nerves that supply the abdominal muscles. 



Movements of the Small Intestine. The movements of the 

 small intestine are of two or three types. Peristalsis is here 

 developed to a high degree. Such movements may pass over 



