GENERAL ZOOLOGY 



reappear at alternate levels, they produce a very thorough mixing of the 

 intestinal contents. 



Food in the small intestine is acted upon by three digestive juices: bile, 

 secreted by the liver and stored in the gall bladder, enters the intestine by 

 way of the bile duct; pancreatic juice, secreted by the pancreatic acini (see 

 Fig. 4.25, p. 120), comes from the pancreas through the pancreatic duct; and 

 intestinal juice comes from intestinal glands in the lining of the duodenal 

 portion of the small intestine. It is well to realize that food entering the 

 small intestine is exposed to the simultaneous action of the various enzymes 

 of these juices, all maximally effective in the alkaline medium of this region. 

 Thus, digestion is a continuous process, not so neatly subdivided as our dis- 

 cussion may suggest. 



Bile, although it does not contain any digestive enzymes, is an important 

 aid to digestion and absorption. It serves chiefly as an emulsifying agent for 

 lipids, which in its presence become divided into very fine droplets; these offer 

 a large surface for the action of the fat-splitting enzyme. In addition, bile 

 stimulates motility of the intestine and is an effective neutralizer of the acid 

 food mass coming from the stomach. 



Pancreatic juice, which is also strongly alkaline, contains three types of 

 digestive enzymes, acting on proteins, carbohydrates, and lipids, respectively. 

 Because of its wide spectrum of action, pancreatic juice is by far the most 

 important single digestive juice. In man approximately 750 ml. is secreted 

 each day. Several enzymes in pancreatic juice act on proteins. Of these, the 

 proteinases trypsin and chymotrypsin reduce protein, or proteoses and peptones 

 resulting from prior pepsin digestion, to smaller fragments known as poly- 

 peptides. Chymotrypsin also clots milk and renders its digestion more effec- 

 tive. Both trypsin and chymotrypsin are secreted in the form of inactive 

 precursors known, respectively, as trypsinogen and chymotrypsinogen. These 

 are converted to the active enzymes in the small intestine. Another protein- 

 splitting enzyme, the peptidase called carboxypeptidase, acts on certain poly- 

 peptide linkages, freeing some of the amino acids. The most effective splitter 

 of the complex carbohydrates, pancreatic amylase or amylopsin, is present in 

 pancreatic juice. It changes starch, glycogen, or the dextrins formed during 

 the action of salivary amylase to maltose, a disaccharide. Finally, a pancreatic 

 lipase, sometimes called steapsin, is found in pancreatic juice. This enzyme 

 is an effective splitter of emulsified lipid into fatty acids and glycerol. 



Intestinal juice is not such a well-defined entity as the other digestive 

 juices. It is known to contain certain peptidases, specifically aminopeptidase 

 and dipeptidase, which act on polypeptide fragments and reduce them to 

 amino acids. Also, there are several enzymes which complete the digestion of 

 carbohydrates. These are disaccharidases, specifically maltase, sucrase, and 

 lactase which split the disaccharides, yielding monosaccharides or simple 

 sugars. 



The final stages of digestion of all foods occur in the small intestine. Diges- 

 tion is normally so efficient that there is very little usable food discharged into 



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