Chap. XV] DIGESTIVE PROCESSES 299 



will pass to a certain extent through a filter paper, kept wet with 

 a solution of bile salts, whereas it will not pass, or passes with 

 extreme difficulty, through one kept wet with distilled water. 



(3) It has a feeble and questioned antiseptic action upon the 

 intestinal contents, and its presence limits putrefaction to some 

 extent. 



. (4) In addition to being a secretion, about one-sixteenth of the 

 bile is an excretion, as it furnishes the channel by which the products 

 of the disintegration of haemoglobin are carried from the body. 

 (5) It acts as a mild laxative by stimulating peristalsis.^ 

 Action of bacteria in small intestine. — Bacteria are constantly 

 present in the small intestine, but only those capable of fermenting 

 carbohydrate food show any activity. If the products of protein 

 digestion are promptly absorbed, there is no fermentation of protein 

 material. Various theories are offered to explain this protection 

 of protein, but opinions differ, even among investigators. 



CHANGES THE FOOD UNDERGOES IN THE LARGE 

 INTESTINE 



Movements of the large intestine. — After the food passes from 

 the small intestine into the large intestine, its regurgitation is pre- 

 vented by the closure of the ileo-csecal valve. When the caecum 

 becomes filled, strong contractions of the walls exert pressure upon 

 the contained food and force it into the ascending colon. The 

 waves of contraction which pass over the walls of the ascending 

 colon are described as antiperistaltic because they pass in two 

 directions, (1) from the small intestine, and (2) toward the small 

 intestine. This delays the food, keeps it moving backward and 

 forward, and helps absorption. It has been estimated that it 

 requires about two hours for the food to pass from the ileo-csecal 

 valve to the hepatic flexure, and about four and one-half hours to 

 reach the splenic flexure. 



Secretion of the large intestine. — The secretion of the large 

 intestine is alkaline, contains much mucus, and does not contain 

 any enzymes. Wlien the contents of the small intestine pass 

 the ileo-caecal valve, they still contain a certain amount of 

 unabsorbed food material. This remains a long time in the 



1 Slow peristalsis will cause constipation, and is often associated with a torpid 

 liver. As bile is a natural stimulant to the muscles of the bowel, an insufficient 

 quantity may result in slow peristalsis. 



