306 DIGESTION. 



more or less remote, when the phenomena take place in the 

 order just described. 



The act of vomiting is undoubtedly induced by causes 

 which operate through the nervous system, and a full con- 

 sideration of these properly belongs to another division of 

 this work. The impression produced by the irritant emetics 

 is conducted to the nervous centre and gives rise to a stimu- 

 lus which is sent to the muscles concerned in the act. The 

 transmission takes place in part through the pneumogastrics, 

 though this is not the only channel, for vomiting may be 

 thus induced after division of these nerves. It is by no 

 means necessary that an impression should be made upon 

 the stomach, as is shown by efforts at vomiting after the 

 stomach has been extirpated. Irritation of some of the sym- 

 pathetic nerves, particularly the abdominal ganglia, will pro- 

 duce vomiting. Traction upon the oesophagus, the irritation 

 produced by biliary calculi, uterine disorders, etc., may pro- 

 duce the same result. Yomiting is very common in cerebral 

 disturbances ; and this is undoubtedly the first cause of or- 

 dinary sea-sickness. When vomiting results from titillation 

 of the fauces, the impression is conveyed to the nervous 

 centre and is reflected to the muscles concerned, in the same 

 way. These facts show that there are many avenues for the 

 passage of these impressions to the nervous centres. The 

 action of emetics which operate through the blood is not re- 

 flex, but the vomiting is probably induced by the direct im- 

 pression made by these substances on the nervous centres. 



Eructation. The discharge of gases from the oesopha- 

 gus by the mouth, accompanied with a peculiar and charac- 

 teristic sound, is very common. This is usually accomplished 

 without any marked effort of the muscles concerned in vom- 

 iting, and evidently requires very little force. Usually, the 

 cardia is so effectually closed as to prevent the passage even 

 of gases ; and in eructation, there 'must be a temporary relax- 

 ation of this opening. When thus relaxed, the act is accom- 



