CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 461 



somewhat suddenly dilated. This dilation opens a way for the 

 contents of the stomach, which, pressed upon by the contraction 

 of the abdomen, and to a certain but probably only to a slight 

 extent by the contraction of the gastric walls, are driven forcibly 

 up the oesophagus. The mouth being widely open, and the neck 

 stretched to afford as straight a course as possible, the vomit is 

 ejected from the body. At this moment there is an additional 

 expiratory effort which serves to prevent the vomit passing into 

 the larynx. In most cases too the posterior pillars of the fauces 

 are approximated, in order to close the nasal passage against the 

 ascending stream. This however in severe vomiting is frequently 

 ineffectual. 



Thus in vomiting there are two distinct acts ; the dilation of 

 the cardiac orifice and the extrinsic pressure of the abdominal 

 walls in an expiratory effort. Without the former the latter, even 

 when distressingly vigorous, is ineffectual. Without the latter, as 

 in urari poisoning, the intrinsic movements of the stomach itself 

 are rarely sufficient to do more than eject gas, and, it may be, a 

 very small quantity of food or fluid. Pyrosis or waterbrash is 

 however probably brought about by this intrinsic action of the 

 stomach. 



During vomiting the pylorus is generally closed, so that but 

 little material escapes into the duodenum. When the gall-bladder 

 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 impulses 

 descending the vagi, since when these are cut real vomiting with 

 discharge of the gastric contents, if it takes place at all, becomes 

 difficult through want of readiness in the dilation. Such intrinsic 

 movements of the stomach as do take place, and the movements of 

 the oesophagus appear to be carried out by the usual nerves. The 

 efferent impulses which cause the flow of saliva in the introductory 

 nausea also descend along the usual nerves such as the chorda 

 tympani. 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 of the 

 intestine due to ligature, hernia, etc. That the vomiting in the 

 last instance is due to nervous action, and not to any regurgita- 

 tion of the intestinal contents, is shewn by the fact that it will 



