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EVOLUTION OF ANIMALS 



Part V 



by the muscles of the stomach, the cardiac valve at the upper end and pyloric 

 valve at the lower end keep its contents from escaping in either direction. In 

 the meantime, the gastric juice flows into it. This contains acid that softens 

 shell and bone, and the enzyme pepsin which begins the digestion of proteins, 

 converting them into proteoses and peptones. When a sufficient stage of soft- 

 ness and acidity has been attained, the food mass is passed through the relaxed 

 pyloric ring into the intestine. 



This is divided into the relatively long small intestine, in which digestion is 

 completed and digested food absorbed, and the shorter large intestine, in which 



Fig. 34.15. Action of the tongue when a frog catches a fly. (Courtesy, Tinbergen: 

 Study of Instinct. London, England, Oxford University Press, 1951.) 



water is absorbed from the residue of indigestible matter. Like other parts of 

 the alimentary canal, both intestines are attached to the dorsal wall by mesen- 

 tery. The acid food mass entering the small intestine immediately stimulates 

 glandular cells in the lining to produce the hormone, secretin. This soon enters 

 the circulation, reaches the pancreas and stimulates it to produce its digestive 

 secretion, the pancreatic juice. The pancreas and liver pour their secretions 

 through the common bile duct opening into the first loop of the small intestine, 

 the duodenum. The pancreas performs two functions; the bulk of it produces 

 the digestive fluid called pancreatic juice, and islets of cells within it form the 

 hormone, insulin. The pancreatic juice, able to act in the alkaline conditions 

 within the intestine, affects all classes of foods and virtually completes diges- 

 tion. It does this mainly by three enzymes; trypsin that breaks proteins into 

 peptones; amylase that changes starches into sugars; and lipase that separates 

 fats into fatty acids and glycerol. Cells in the intestinal lining also secrete diges- 

 tive enzymes, the most important of these (erepsin) breaks peptones to amino 

 acids, the basic constituents of proteins. 



In all these processes, molecules of the food substances become smaller and 



