CHAP, i.] DIGESTION, 291 



is full, a copious flow of bile into the duodenum accompanies the 

 act of vomiting. Part of this may find its way into the stomach, as 

 in bilious vomiting, the pylorus then having evidently been opened. 



The nervous mechanism of vomiting is complicated and in 

 many aspects obscure. The efferent impulses- which cause the 

 expiratory effort must come from the respiratory centre in the 

 medulla ; with these we shall deal in speaking of respiration. The 

 dilation of the cardiac orifice is caused, in part at least, by efferent 

 impulses descending the vagi, since when these are cut real 

 vomiting with discharge of the gastric contents is difficult, through 

 want of readiness in the dilation. The sympathetic abdominal 

 nerves coming from the coeliac ganglia and the splanchnic nerves 

 seem to have no share in the matter. The efferent impulses 

 which cause the flow of saliva in the introductory nausea descend 

 the facial along the chorda tympani branch. These various 

 impulses may best be considered as starting from a vomiting 

 centre in the medulla, having close relations with the respiratory 

 centre. This centre may be excited, may be thrown into action, 

 in a reflex manner, by stimuli applied to peripheral nerves, as 

 when vomiting is induced by tickling the fauces, or by irritation 

 of the gastric membrane, or by obstruction due to ligature, hernia, 

 etc., of the intestine. That the vomiting in the last instance is 

 due to nervous action, and not to any regurgitation of the intestinal 

 contents, is shewn by the fact that it will take place when the 

 intestine is perfectly empty and may be prevented by section of 

 the mesenteric nerves. The vomiting attending renal and biliary 

 calculi is apparently also reflex in origin. The centre however 

 may be affected directly, as probably in the cases of some poisons, 

 and in some instances of vomiting from disease of the medulla 

 oblongata. Lastly, it may be thrown into action by impulses 

 reaching it from parts of the brain higher up than itself, as in 

 cases of vomiting, produced by smells, tastes and emotions, and by 

 the memory of past occasions, and in some cases of vomiting due 

 to cerebral disease. 



Many emetics, such as tartar emetic, appear to act directly on 

 the centre, since, introduced into the blood, they will produce 

 vomiting even when a bladder is substituted for the stomach. 

 Others again, such as mustard and water, act in a reflex manner 

 by irritation of the gastric mucous membrane. With others, again, 

 which cause vomiting by developing a nauseous taste, the reflex 

 action involves parts of the brain higher than the centre itself. 



192 



