MOVEMENTS OF THE ALIMENTARY CANAL. 731 



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- 

 ucts of fermentation in the stomach in cases of indigestion, or may 

 be produced intentionally by local emetics, such as mustard, taken 

 into the stomach. The afferent path in this case is through the 

 sensory fibers of the vagus. The efferent paths of the reflex are 

 found in the motor nerves innervating the muscles concerned in the 

 vomiting, — namely, the vagus, the phrenics, and the spinal nerves 

 supplying the abdominal muscles. Whether or not there is a defi- 

 nite vomiting center in which the afferent impulses are received 

 and through which a co-ordinated series of efferent impulses is 

 sent out to the various muscles has not been satisfactorily deter- 



