ascending colon, which passes upward on the 

 right side of the abdominal cavity; (4) the 

 transverse colon, which slants diagonally 

 across the abdomen, from the right to left; 

 and (5) the descending colon, which passes 

 downward toward the rectum, on the left 

 side of the body. Just prior to the point 

 where the large intestine joins the rectum, 

 the descending colon displays a marked bend, 

 the sigmoid flexure. One muscular valve, the 

 ileocaecal sphincter, guards the opening of 

 the ileum into the caecal portion of the large 

 intestine; and two valves, the anal sphincters, 

 close off the anal passage except when eges- 

 tion is about to occur. 



When the residual food mass reaches the 

 colon, most organic compounds, as well as 

 considerable quantities of water, have been 

 absorbed (p. 310). However, the mass is still 

 very fluid when it reaches the ileocaecal valve 

 — where it tends to accumulate, pending one 

 of the periodic peristaltic rushes that sweep 

 the material into the colon. This fluidity is 

 not surprising when it is realized that the 

 quantity of water absorbed from the food 

 mass, while it passes through the small in- 

 testine, is just about counterbalanced by the 

 water content of the digestive juices that are 

 secreted into the small intestine. 



The main function of the colon, aside from 

 conducting the residual food mass to the 

 rectum and anus, is to absorb water from the 

 mass, conserving this water for the use of 

 the body. Aquatic vertebrates, in which water 

 conservation is not a serious problem, do not 

 possess a very well-developed colon, but in 

 terrestrial forms the colon is usually more 

 conspicuous. The contents of the large bowel 

 move very slowly, and usually about 12 to 14 

 hours are required for a given sample of the 

 food mass to traverse the colon. Thus time 

 is allowed for water absorption; and usually 

 the fecal mass, when it reaches the descend- 

 ing colon, displays a plastic semisolid con- 

 sistency. 



This lag in the passage of the food resi- 

 dues through the colon accounts, at least 

 partially, for the very large growth of bac- 



T he Digestive Sysiem - 315 



teria that are always found in the large in- 

 testine. In fact, these bacteria represent as 

 much as 50 percent of the dry weight of the 

 normal feces. The colonic bacteria thrive — 

 under ideal conditions of warmth, moisture, 

 and darkness — upon such remnants of pro- 

 teins, carbohydrates, and fats as may escape 

 absorption in the small intestine. Ordinarily 

 the bacteria inhabiting the colon are non- 

 pathogenic; that is, these bacteria are neutral 

 parasites for which man has been a host for 

 many centuries. 



The proteolytic, or putrefactive, bacteria 

 of the colon produce a variety of nitrogenous 

 compounds in the course of their digestive 

 and metabolic activities, and some of these 

 products are highly toxic and odoriferous. 

 Formerly it was thought that these nitroge- 

 nous compounds might be responsible for the 

 unpleasant symptoms of "autointoxication"; 

 but more recent studies indicate that these 

 substances never reach toxic concentrations 

 in the general circulation (see p. 343). 



Movements of the Large Intestine. Two 

 types of colonic movement have been de- 

 scribed: (1) feeble, sluggish, peristaltic waves, 

 which gradually advance the food residue 

 through the ascending and transverse loops; 

 and (2) vigorous peristaltic rushes, which 

 periodically evacuate the contents of the 

 whole colon into the rectum. Ordinarily the 

 rectum remains empty of feces until a few 

 moments before defecation is to occur. 



The various movements of the large and 

 small bowels are coordinated by autonomic 

 reflexes (p. 465). Peristaltic waves may be 

 initiated by any local distention of the gut — 

 as by an accumulation of the digesting food 

 mass, or by artificially stretching the wall by 

 the inflation of a rubber balloon placed in 

 the lumen. A particularly vigorous peristaltic 

 rush, involving a large part of the small in- 

 testine, as well as the transverse and descend- 

 ing loops of the large intestine, frequently 

 accompanies the ingestion of a new meal. 

 This reflex, which is called the gastrocolic 

 reflex, sweeps the food remnants from the 

 colon into the rectum. In fact, this filling of 



