736 PHYSIOLOGY OF DIGESTION AND SECRETION. 



over the pyloric portion in the period preceding the vomiting and 

 that finally a strong contraction at the "transverse band" com- 

 pletely shuts off the pyloric portion from the body of the stomach, 

 which at this time is quite relaxed. The act of vomiting is, in fact, 

 a complex reflex movement into which many muscles enter. The 

 following events are described : The vomiting is usually preceded by 

 a sensation of nausea and a reflex flow of saliva into the mouth. 

 These phenomena are succeeded or accompanied by retching move- 

 ments, which consist essentially in deep, spasmodic inspirations with 

 a closed glottis. The effect of these movements is to compress the 

 stomach by the descent of the diaphragm, and at the same time to 

 increase decidedly the negative pressure in the thorax, and therefore 

 in the thoracic portion of the esophagus. During one of these 

 retching movements the act of vomiting is effected by a convulsive 

 contraction of the abdominal wall that exerts a sudden additional 

 strong pressure upon the stomach. At the same time the cardiac 

 orifice of the stomach is dilated, probably by an inhibition of the 

 sphincter, and according to the above description the fundic end 

 of the stomach is also dilated, while the pyloric end is in strong 

 contraction. The stomach contents are, therefore, forced vio- 

 lently out of the stomach through the esophagus, the negative 

 pressure in the latter probably assisting in the act. The passage 

 through the esophagus is effected mainly by the force of the con- 

 traction of the abdominal muscles; there is no evidence of anti- 

 peristaltic movements on the part of the esophagus itself. Dur- 

 ing the ejection of the contents of the stomach the glottis is kept 

 closed by the adductor muscles, and usually the nasal chamber is 

 likewise shut off from the pharynx by the contraction of the 

 posterior pillars of the fauces on the palate and uvula. In violent 

 vomiting, however, the vomited material may break through 

 this latter barrier and be ejected partially through the nose. 



Nervous Mechanism of Vomiting. That vomiting is a reflex act 

 is abundantly shown by the frequency with which it is produced in 

 consequence of the stimulation of sensory nerves or as the result 

 of injuries to various parts of the central nervous system. After 

 lesions or injuries of the brain vomiting often results. Disagreeable 

 emotions and disturbances of the sense of equilibrium may produce 

 the same result. Irritation of the mucous membrane of various 

 parts of the alimentary canal (as, for example, tickling the back 

 of the pharynx with the finger); disturbances of the urogenital 

 apparatus, the liver, and other visceral organs; artificial stimula- 

 tion of the trunk of the vagus and of other sensory nerves, may all 

 cause vomiting. Under ordinary conditions, however, irritation of 

 the sensory nerves of the gastric mucous membrane is the most 

 common cause of vomiting. This effect may result from the prod- 



