462 VOMITING. [BOOK n. 



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. 

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

 passing along one or other nerves, but most frequently along 

 those connectedt-with the alimentary canal, that is along afferent 

 fibres running in the vagus or in the splanchnic nerves. 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 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 

 vomiting after a bladder has been substituted for the whole 

 stomach. Others again, such as mustard and water, act in a re- 

 flex 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. 



273. 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 

 occasionally do, begin at various points along the length of the 

 intestine. A movement started by artificial stimulation some 

 way down the intestine, may travel not only downwards but also 

 upwards ; it has been disputed however, whether in the living 

 body any natural backward peristaltic movement really takes 

 place. In the living body the intestines have periods of rest, 

 alternating with periods of activity, the occurrence of the periods 

 depending on various circumstances; the intensity of the move- 

 ments also varies very considerably. 



274. Movements of the Large Intestine. These are funda- 

 mentally the same as those of the small intestine, but distinct in 

 so far as the latter cease at the ileo-ca3cal valve, at which spot the 

 former normally begin; they are simpler, in as much as the 

 pendent loops are absent, and not so vigorous, since relatively to 

 the diameter of the tube, the amount of muscular fibre is less. 

 Along the colon where the sacculi are well developed ( 266) the 

 movement may perhaps be described as almost intermittent from 

 sacculus to sacculus, the contents of one sacculus being driven 



