LARGE INTESTINE. 



These substances are carried, by slow, peristaltic contrac- 

 tions, into the sigmoid flexure. Here they apparently pause, 

 and are carried into the rectum, in an intermittent manner 

 only, under the influence of stronger contractions; they 

 here tend to produce the reflex phenomenon which we shall 

 study under the name of defecation: if this attempt at 

 evacuation, however, does not succeed, and the passage is 

 closed to them the fa3ces return to the sigmoid flexure. These 

 movements are all extremely slow and of such a character as 

 to produce considerable compression throughout the length 

 of the lower end of the gut. As in the case of the small 

 intestine, the form and mode of production of these move- 

 ments are not yet perfectly known; they are peristaltic 

 movements, that is, movements in which the circular fibres 

 of the muscular membrane contract, proceeding in a down- 

 ward direction, causing the substances to pass through the 

 intestinal tube ; thus any substance being compressed above, 

 is forced into the lower part of the intestine, the fibres of 

 which are still relaxed. Those movements called anti- 

 peristaltic^ which take place in the contrary direction, and 

 thus have the effect of forcing back the contents of the intes- 

 tine, do not appear to exist in the living animal, when in its 

 normal condition. 1 They are evidently produced in certain 

 pathological cases. Those, observed in the intestinal canal 

 of an animal in which the abdomen is opened immediately 

 after it has been killed, appear to be owing to an interruption 

 in the abdominal circulation, causing ultimate excitation of 

 the smooth fibres, at the instant of death. We have scarcely 

 any means of deciding on the nature of the reflex mechanism 

 by which the nervous system influences or produces these 

 movements. The solar plexus may, perhaps, serve as the 

 centre of these reflexes ; embryology, indeed, shows that 

 this abdominal nerve centre appears to be developed inde- 

 pendently of the spinal cord. The solar plexus is, however, 

 united to the cord by two large nerve commissures, if they 

 may be so called, the pneumo-gastric and the splanchnic 

 nerves ; it is remarkable that excitation of the former pro- 

 duces or increases the movements in the intestines, while 

 excitation of the latter (great splanchnic nerves) appears to 

 render the viscera motionless, and paralyzes their muscular 



1 See Braam-Honckgeest, *' Untersuchungen liber Peristaltik 

 des Magens und Darmkanals." (Pfliiger's Archiv., September, 

 1872.) 



