THE MUSCULAR MECHANISMS OF DIGESTION. 299 



ward. It has not, however, been definitely shown that the increasing acidity 

 is the efficient stimulus giving rise to the movements. 



The movements of even a full stomach are said to cease during sleep. 

 The nervous mechanism of the gastric movements had better be considered 

 in connection with that of the intestinal movements. 



233. Vomiting. In a conscious individual this act is preceded by feel- 

 ings of nausea, during which a copious flow of saliva into the mouth takes 

 place. This being swallowed carries down with it a certain quantity of air, 

 the presence of which in the stomach, by assisting in the opening of the car- 

 diac sphincter, subsequently facilitates the discharge of the gastric contents. 

 The nausea is 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 defecation, in pressure on the abdominal contents. The stomach is, there- 

 fore, forcibly compressed from without. At the same time, or rather imme- 

 diately 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 dilatation opens the way 

 for the con tents' 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 dilatation 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 move- 

 ments of the stomach itself are rarely efficient to do more than eject gas, 

 and, it may be, a very small quantity of food or fluid. Pyrosis or water- 

 brash is, however, probably brought about by this intrinsic action of the 

 stomach. 



During vomiting the pylorus is generally closed, so that but little mate- 

 rial 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 dilatation 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, be- 



