822 PHYSIOLOGY 



food is thoroughly mixed with the digestive juices, and the greater 

 part of the products of digestion are brought into contact with the 

 intestinal wall and absorbed. What is left a proportion varying 

 in different animals according to the nature of the food is passed 

 on by occasional peristaltic contractions through the lower end of 

 the ileum into the colon, or large intestine. The lowest two centi- 

 metres of the ileum present a distinct thickening of the circular 

 muscular coat, forming the ileocolic sphincter. This sphincter relaxes 

 in front of a peristaltic wave and so allows the passage of food into 

 the colon. On the other hand, it contracts as a rule against any 

 regurgitation which might be caused by contractions in the colon. 

 Although thus falling into line with the rest of the muscular coat, 

 as concerns its reaction to stimuli arising in the gut above or below, 

 it presents a marked contrast to the rest of the gut in its relation 

 to the central nervous system. It is unaffected by stimulation 

 of the vagus. Stimulation of the splanchnic, however, which causes 

 complete relaxation of the lower part of the ileum with the rest of 

 the small intestine, produces a strong contraction of the muscle- 

 fibres forming the ileocolic sphincter (Elliott). 



MOVEMENTS OF THE LARGE INTESTINE 



By means of the occasional peristaltic contractions, accompanied 

 by relaxation of the ileocolic sphincter, the contents of the small 

 intestine are gradually transferred into the large. In man these 

 contents are considerable in bulk, are semi-fluid, and probably fill 

 the ascending as well as the transverse colon. 



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 con- 

 siderably 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 the process of absorp- 

 tion is almost complete at the ileocolic valve, whereas in the herbi- 

 vora 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 



