CH. XXXVII.] INTESTINAL MOVEMENTS 557 



only show that the contraction of the abdominal muscles alone is 

 sufficient to expel matters from an unresisting bag through the 

 oesophagus; and that, under certain circumstances, the stomach, 

 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 medullary centres may also be stimulated 

 by impressions from the cerebrum and cerebellum, producing so-called 

 central vomiting occurring in diseases of those parts. 



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. 



It is very doubtful whether there is any separate centre for 

 vomiting ; the centre for the reflex coincides with those of the nerves 

 mentioned in the medulla oblongata. 



Emetics. Some emetics produce vomiting by irritating the 

 stomach ; others, such as tartar emetic, apomorphine, etc., by stimu- 

 lating the medullary centres. 



MOVEMENTS OF THE SMALL INTESTINE. 



The intestinal movements, like those of the stomach, take place 

 independently of our volition or consciousness. When, however, 

 they become excessive, as they do under the influence of irritants or 

 the presence of obstruction, they produce pain which is usually intense. 



The object of these movements is to force the contents along the 

 tube, and to thoroughly mix them with the digestive juices. But 

 the peristalsis which drives the intestinal contents along does so 

 more slowly than that which occurs in the oesophagus; otherwise 

 the mechanism is much the same. There may occur in the small 

 intestine peristaltic waves in the opposite direction (retro-peristalsis) 

 and this probably never occurs in the oesophagus. .Retro -peristalsis 

 is most marked when obstruction is present, as in the cases of 

 violent vomiting just referred to. 



