288 MOVEMENTS OF THE LARGE INTESTINE. [BOOK n. 



as a general rule, while stimulation of the splanchnic tends to 

 check, that of the vagus tends to excite them ; but much has 

 probably yet to be learnt about the exact manner in which these 

 nerves act. It is probably through the vagus that peristaltic 

 movements can be effected in an indirect manner, as in that in- 

 crease of the movements of the intestine in consequence of 

 emotions, which has given rise to the phrase ' my bowels yearned/ 



When the vagus is stimulated, peristaltic contraction is seen to 

 begin at the pylorus of the stomach and so to descend along the 

 intestine. When however the duodenum is mechanically stimu- 

 lated, both a peristaltic and an antiperistaltic wave, that is, a 

 wave of contraction passing upwards instead of downwards, may 

 be observed, the former passing downward and ceasing at the 

 ileo-csecal valve if not before, the latter passing up and ceasing 

 at the pylorus. And when in the exposed intestines a wave, as 

 occasionally happens, begins spontaneously in the duodenum, it 

 may sometimes be seen to pass both upwards and downwards. It 

 is worthy of notice that stimulation of the small intestine is said 

 not to cause movement either in tke stomach or large intestine, 

 .and stimulation of the large intestine or of the stomach causes 

 no movement of >the small intestine, the ileo-csecal valve and the 

 pylorus barring the .progress of the waves. 



Certain drugs, such as nicotin, induce strong peristaltic action ; 

 the modus operandi of these and of the more specific purgative 

 drugs is at present uncertain. 



Movements of the large Intestine. These are fundametally 

 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 said, moreover, not to be inhibited by 

 stimulation of the splanchnics. 



The fa3ces in their passage through the colon are lodged in the 

 sacculi during the pauses between the peristaltic waves. Arrived 

 at the sigmoid flexure, they are supported by the bladder and the 

 sacrum, so that they do not press on the sphincter ani. 



Defsecation. This is a mixed act, being superficially the result 

 of an effort of the will, and yet carried out by means of an involun- 

 tary mechanism. Part of the voluntary effort consists in produ- 

 cing a pressure-effect, by means of the abdominal muscles. These 

 are contracted forcibly as in expiration, but the glottis being 

 closed, and the escape of air from the lungs prevented, the whole 

 force of the pressure is brought to bear on the abdomen itself, and 

 so drives the contents of the descending colon onward into the 

 rectum. The sigmoid flexure is by its position sheltered from this 

 pressure ; a body introduced per anum into the empty rectum is 

 not affected by even forcible contractions of the abdominal walls. 



The anus is guarded by the sphincter ani, which is habitually 

 in a state of normal tonic contraction, capable of being increased or 



