380 MOVEMENTS OF THE SMALL INTESTINE. [BOOK n. 



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 start- 

 ing 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 periph- 

 eral 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 

 take place when the intestine is perfectly empty and may be pre- 

 vented by section of the mesenteric nerves. The vomiting attend- 

 ing renal and biliary calculi is apparently also reflex in origin. 

 Vomiting in fact as a rule is a reflex action, the afferent impulses 

 passing along one or other nerves, but most frequently along those 

 connected with the alimentary canal, that is along afferent fibres 

 running in the vagus or in the splanchnic nprves. 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 im- 

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

 in cases of vomiting produced by smells, tastes or emotions, or by 

 the recollection of past events, 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 vom- 

 iting after a bladder has been substituted for the whole 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 

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



223. Movements of the Small Intestine. These, as we have 

 already said, are the typical peristaltic movements, simple except 

 in so far as they are complicated by the existence of the pendent 

 loops, the peculiar oscillating movements of which appear to be 

 produced chiefly by the longitudinal fibres. 



The peristaltic movements, as a rule, take place from above 

 downwards, and a wave beginning at the pylorus may be traced a 

 long way down. But contractions may, and in all probability 



