Cn. XXXV.] MOVEMENTS OF THE INTESTINES 531 



by itself, cannot expel its contents. They by no means show that in 

 ordinary vomiting the stomach is passive, for there are good reasons 

 for believing the contrary. In some cases of violent vomiting the 

 contents of the duodenum are passed by anti-peristalsis into the 

 stomach, and are then vomited. Where there is obstruction to the 

 intestine, as in strangulated hernia, the total contents of the small 

 intestine may be vomited. 



Nervous mechanism.-^-Some few persons possess the power of 

 vomiting at will, or the power may be acquired by effort and practice. 

 But normally the action is a reflex one. 



The afferent nerves are principally the fifth, and glosso-pharyngeal 

 (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, etc. The centre may also be stimulated by im- 

 pressions 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 coincides 

 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, etc., by stimulating 

 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 

 oasophagus. 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 sensation; 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. 



