526 THE MECHANICAL PROCESSES OF DIGESTION. [CH. xxxv. 



(as in vomiting produced by tickling the fauces), and the vagus 

 (as in vomiting produced by gastric irritants) ; but vomiting 

 may occur from stimulation of other sensory nerves, e.g. those 

 from the kidney, uterus, testicle, &c. The centre may also be 

 stimulated by impressions from the cerebrum and cerebellum, 

 producing so-called central vomiting occurring in diseases of those 

 parts. 



The centre for vomiting is in the medulla oblongata, and coin- 

 cides with the centres of the nerves concerned. 



The efferent (motor) impulses are carried by the vagi to the 

 stomach, by the phrenics to the diaphragm, and by various other 

 spinal nerves to the abdominal muscles. 



Emetics. Some emetics produce vomiting by irritating the 

 stomach ; others, like tartar emetic, apomorphine, &c., by stimu- 

 lating the vomiting centre. 



MOVEMENTS OF THE INTESTINES. 



The movement of the intestines is peristaltic or vermicular, and 

 is effected by the alternate contractions and dilatations of successive 

 portions of the muscular coats. The contractions, which may 

 commence at any point of the intestine, extend in a wave- 

 like manner along the tube. They are similar to what we 

 have described in the oesophagus. In any given portion, the 

 longitudinal muscular fibres contract first, or more than the 

 circular ; they draw a portion of the intestine upwards, over 

 the substance to be propelled, and then the circular fibres of the 

 same portion contracting in succession from above downwards, 

 press the substance into the portion next below, in which at once 

 the same succession of actions next ensues. These movements 

 take place slowly, and, in health, commonly give rise to no sensa- 

 tion ; but they are perceptible when they are accelerated under 

 the influence of any irritant. 



The movements of the intestines are sometimes retrograde ; and 

 there is no hindrance to the backward movement of the contents 

 of the small intestine, as in cases of violent vomiting just referred 

 to. But almost complete security is afforded against the passage 

 of the contents of the large into the small intestine by the ileo- 

 caecal valve. Besides, the orifice of communication between the 

 ileum and ceecum (at the borders of which orifice are the folds 

 of mucous membrane which form the valve) is encircled with 

 muscular fibres, the contraction of which prevents the undue 

 dilatation of the orifice. 



Proceeding from above downwards, the muscular fibres of the 

 large intestine become, on the whole, stronger in direct proportion 



