776 - PHYSIOLOGY 



The large intestine is supplied with nerves from the central nervous 

 system. These run partly in the sympathetic system along the colonic 

 and inferior mesenteric nerves, partly in the pelvic visceral nerves or nervi 

 erigentes, which come off from the sacral cord and pass direct to the pelvic 

 viscera. In addition it possesses a local nervous system, presenting the 

 same structure as that found in the small intestine. The movements of the 

 large intestine differ considerably in various animals, as has been shown by 

 Elliott, according to the nature of the food and the part played by this 

 portion of the gut in the processes of absorption. In the dog absorp- 

 tion is almost complete at the ileocolic valve, whereas in the herbivora a very 

 large part of the processes of digestion and absorption occurs in the colon and 

 caecum. Man takes an intermediate position as regards his large intestine 

 between these two groups of animals. Elliott and Barclay Smith divide 

 the large intestine into four parts, according to their functions, viz. the 

 caecum, and the proximal, intermediate, and distal portions of the colon. 

 Of these the dog possesses practically only the distal colon. We may take 

 Elliott's account of the movements as they probably occur in man. They 

 agree very closely with those observed by Cannon under normal circum- 

 stances in the cat by means of the Rontgen rays. The food as it passes from 

 the ileum first fills up the proximal colon. The effect of this distension is to 

 cause a contraction of the muscular wall at the junction between the ascend- 

 ing and transverse colon. This contraction travels slowly over the tube 

 in a backward direction towards .the caecum, and is quickly succeeded by 

 another, so that the colon may present at the same time several of these 

 advancing waves. These waves are spoken of as anti-peristaltic ; but as 

 they do not involve also an advancing wave of inhibition, they must not 

 be regarded as representing the exact antithesis of a peristaltic wave, as we 

 have defined it. The effect of these waves is to force the food up into the 

 caecum, regurgitation into the ileum being prevented partly by the obliquity 

 of the opening, partly by the tonic contraction of the ileocolic sphincter. 

 As the whole of the contents cannot escape into the caecum, a certain portion 

 will slip back in the axis of the tube, so that these movements have the same 

 effect as the similar contractions in the pyloric end of the stomach, causing a 

 thorough churning up of the contents and its close contact with the intestinal 

 wall. The movements are rendered still more effective by the sacculation of 

 the walls of this part of the large intestine. The distension of the caecum 

 caused by this anti-peristalsis excites occasionally a true co-ordinated 

 peristaltic wave which, starting in the caecum, drives the food down the 

 intestine into the transverse part. These waves die away before they reach 

 the end of the colon, and the food is driven back again by waves of anti- 

 peristalsis. Occasionally more food escapes through the ileocolic sphincter 

 from the ileum, so that the whole ascending and transverse colon may be filled 

 with the mass undergoing a constant kneading and mixing process. The 

 result of this process is the absorption of the greater part of the water of the 

 intestinal contents, as well as of any nutrient material ; and the drier part of 

 the intestinal mass collects towards the splenic flexure, where it may be 



