300 THE TISSUES AND MECHANISMS OF DIGESTION. 



comes difficult through want of readiness in the dilatation. 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 im- 

 pulses 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 ex- 

 cited, 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 regurgitation of the intestinal contents, is shown by 

 the fact that it will take place when the intestine is perfectly empty and 

 may be prevented by section of the mesenteric nerves. The vomiting at- 

 tending renal and biliary calculi is apparently also reflex in origin. Vom- 

 iting 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 nerves. The centre, however, may be affected directly, as prob- 

 ably 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 reflex manner by irritation of the gastric mucous mem- 

 brane. With others, again, which cause vomiting by developing a nauseous 

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



234. 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 downward, 

 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 downward but 

 also upward ; 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 movements also varies very considerably. 



235. Movements of the large intestine. They are fundamentally the 

 same as those of the small intestine, but distinct in so far as the latter cease 

 at the ileo-csecal valve, at which spot the former normally begin ; they are 

 simpler, inasmuch 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 the movement may 

 perhaps be described as almost intermittent from sacculus to sacculus, the 

 contents of one sacculus being driven by the peristaltic contractions of its 

 circular fibres into the next sacculus, which prepares to receive them by a 

 relaxation of its circular and a contraction of its longitudinal fibres. 



Since the lips of the ileo-caecal valve are placed transversely across the 



