314 - Mulficellular Animals, Especially Man 



pounds also exert a detergent action upon 

 cholesterol and help to keep it in solution. 



The Flow of Bile and Pancreatic Juice. Cutting 

 the nerves to the gall bladder and pancreas 

 results in only a small reduction in the flow 

 of bile and pancreatic juice. Moreover, intra- 

 venous injections of extracts of the duodenal 

 mucosa are followed by a copious flow of 

 these juices, even after all nerves to the 

 glands are cut. These and other experiments 

 indicate that a hormonal control predom- 

 inates for both the gall bladder and the pan- 

 creas; and in fact, two distinct hormones are 

 involved: (1) cholecystokinin, for the gall 

 bladder; and (2) secretin for the pancreas. 



In the normal course of events, both secre- 

 tin and cholecystokinin are liberated by the 

 mucosa cells of the duodenum as soon as 

 chyme flows from the stomach into the duo- 

 denum. The hormones pass in the blood 

 stream to all parts of the body; but only the 

 gall bladder is stimulated by cholecystokinin; 

 and only the pancreas is activated by secretin. 



The Intestinal Juice. The final stages of 

 digestion proceed in the jejunum and upper 

 coils of the ileum under the auspices of the 

 enzymes of the intestinal juice, of which 

 there are at least five: two peptidases and 

 three disaccharases. Together the two pep- 

 tidases, formerly designated as erepsin, com- 

 plete the digestion of protein foods by 

 splitting off individual amino acids from the 

 end position of the peptide chains. On this 

 account they are called exopeptidases (Table 

 5-1). Each is very specific, however. Carboxy- 

 peptidase splits off amino acids at one end of 

 the chain, where a carboxyl ( — COOH) group 

 is exposed; whereas aminopeptidase acts at 

 the other end, where an amino ( — NH 2 ) 

 group is free. 



As to the disaccharases, maltase, sucrase, 

 and lactase act respectively on maltose, su- 

 crose, and lactose, hydrolyzing these disac- 

 charides into their monosaccharide constitu- 

 ents (p. 80). Recent reports indicate some 

 lipase activity in the intestinal juice; but the 

 potency is not very significant. The intes- 

 tinal juice also contains sodium bicarbonate, 



which maintains a slightly alkaline reaction 

 in the intestinal chyme, fostering a maximal 

 activity of the intestinal enzymes. 



By the time the chyme reaches the lower 

 coils of the small intestine, practically all 

 digestion is finished and absorption is pro- 

 ceeding apace. At the end point of digestion 

 the various foodstuffs are entirely converted 

 to readily absorbable compounds; the pro- 

 teins to amino acids; the carbohydrates to 

 monosaccharides; and the fats to glycerol and 

 fatty acids. 



Movements of the Small Intestine. Some 

 5 to 10 hours are usually required for the 

 transmission of a meal through the small 

 intestine. During this time the chyme is 

 gently churned and gradually moved from 

 coil to coil along the length of the small gut. 



The churning movements of the small in- 

 testine, as revealed by x-ray studies, are 

 mainly of a type called pendular movements. 

 These, essentially, are gentle peristaltic waves 

 which sweep back and forth without progress- 

 ing beyond the limits of a single loop of 

 the bowel. Pendular movements assure a 

 complete mixing of the intestinal chyme with 

 the digestive juices. 



To move the chyme forward toward the 

 large intestine, there are two kinds of peri- 

 staltic movements: (1) ordinary peristaltic 

 waves (p. 303); and (2) peristaltic rushes. 

 The waves are relatively slow (about 1 cm 

 per sec), and transient, since they tend to 

 fade away after traveling a meter along the 

 bowel; but peristaltic rushes are faster (about 

 12 cm per sec), and they sweep the bowel 

 for considerable distances, sometimes all the 

 way from the duodenum to the colon. 



The Colon, Rectum, and Anus. The rela- 

 tions of the colon, or large intestine, to the 

 other parts of the digestive tract are shown 

 in Figure 16-24. This figure also shows the 

 positions of the subsidiary parts of the colon, 

 which are: (1) the caecum, a large blind 

 pocket, which hangs down below the point 

 where the small bowel enters the large one; 

 (2) the vermiform appendix, a small finger- 

 like outpocketing of the caecum; (3) the 



