CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 379 



generally succeeded at first by ineffectual retching' in which a deep 

 inspiratory effort is made, so that the diaphragm is thrust down 

 as low as possible against the stomach, the lower ribs being at 

 the same time forcibly drawn in; since during this inspiratory 

 effort the glottis is kept closed, no air can enter into the lungs ; 

 but some is drawn into the pharynx, and thence probably descends 

 by a swallowing action into the stomach. When retching passes 

 on to actual vomiting this inspiratory effort is succeeded by a 

 sudden violent expiratory contraction of the abdominal walls, the 

 glottis still being closed, so that the whole force of the effort is 

 spent, as we shall see it is in defsecation, in pressure on the 

 abdominal contents. The stomach is therefore forcibly compressed 

 from without. At the same time, or rather immediately before 

 the expiratory effort, by a contraction of its longitudinal fibres 

 the oesophagus is shortened and the cardiac orifice of the stomach 

 brought close under the diaphragm, while apparently by an 

 inhibition of the circular sphincter, aided perhaps by a contraction 

 of the fibres which radiate from the end of the oesophagus over 

 the stomach, the cardiac orifice, which is normally closed, is 

 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 



