& 9 2 A MANUAL OF PHYSIOLOGY 



faeces begin to enter it through the sigmoid flexure, and the 

 Sensations caused by their presence give rise to the desire to 

 "empty the bowels. This desire may for a time be resisted 

 by the will, or it may be yielded to. In the latter case the 

 Abdominal muscles are forcibly contracted, and the glottis 

 being closed, the whole effect of their contraction is ex- 

 pended in raising the pressure within the abdomen and pelvis, 

 and so driving the faeces from the colon to the rectum. The 

 sphincter ani is now relaxed by the inhibition of a centre in 

 'the lumbar portion of the spinal cord, through the activity 

 of which the tonic contraction of the sphincter is normally 

 maintained. This relaxation is partly voluntary, the im- 

 pulses that come from the brain acting probably through the 

 medium of the lumbar centre ; but in the dog, after section 

 of the cord in the dorsal region, the whole act of defaeca- 

 tion, including contraction of the abdominal muscles and 

 relaxation of the sphincter, still takes place, and here the 

 process must be purely reflex. The contraction of the 

 levatorcs ani helps to resist over-distension of the pelvic 

 floor and to pull the anus up over the faeces as they escape. 



Vomiting. We have seen that under normal conditions 

 the movements of the alimentary canal always tend to carry 

 the food in one definite direction, along the tube from the 

 mouth to the rectum. The peristaltic waves generally run 

 only in this direction, and, further, regurgitation is prevented 

 at three points by the cardiac and pyloric sphincters of the 

 stomach and the ileo-caecal valve. But in certain circum- 

 stances the peristalsis may be reversed, one or more of the 

 guarded orifices forced, and the onward stream of the 

 intestinal contents turned back. In obstruction of the bowel, 

 the faecal contents of the large intestine may pass up beyond 

 the ileo-cascal valve, and, reaching the stomach, be driven 

 by an act of vomiting through the cardiac orifice ; in what is 

 called * a bilious attack,' the contents of the duodenum may 

 pass back through the pylorus and be ejected in a similar 

 way ; or, what is by far the most common case, the contents 

 of the stomach alone may be expelled. 



Vomiting is usually preceded by a feeling of nausea and a 

 rapid secretion of saliva, which perhaps serves, by means of 

 the air carried down with it when swallowed, to dilate the 



