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278 AMPHIBIA 



sac between the lobes of the liver. The bile duct leads to the 

 duodenum, one-half inch below the pylorus. The distal end reaches 

 the pancreas. The pancreas is a whitish irregularly lobed mass lying 

 in loop between stomach and duodenum. Pancreatic ducts are 

 numerous and open into the bile-duct which passes through the 

 pancreas to reach the duodenum. 



Layers of Stomach. — id) Serous, flattened and derived from 

 peritoneum; sub-serous, consisting of a few longitudinal muscles and 

 connective tissue, {b) Circular muscle layer, {c) Submucous, 

 composed of connective tissue and blood vessels with a muscularis 

 mucosae made up of inner circular and outer longitudinal muscles. 

 {d) Mucous, consisting of glands embedded in a matrix of connective 

 tissue. 



At the cardiac end of the stomach, the glands are long and their 

 mouths deep. The pyloric glands are less deep with the mouths 

 of the glands deeper in proportion. 



Gastric Digestion. — As in higher vertebrates, the gastric juice 

 contains HCl and the ferment pepsin and acts on protetlfs, converting 

 them into soluble peptones. There is no action on carbohydrates 

 and fats. The secretion of the esophagus is alkaline and lubricates 

 the esophagus. It must be made acid by the gastric juice before 

 it can digest a substance. Holmes states that the pepsin content 

 of the esophageal glands of the frog is greater than an equal area of 

 the stomach. The frog digests a piece of worm in about twenty- 

 four hours. 



The small intestine has folds but no true villi or glands in the 

 mucous layer. Goblet cells and absorptive cells are found. The 

 small intestine receives the bile and pancreatic juice and is important 

 in absorption. The intestinal juice is primarily amylotic. 



Pancreatic secretion is alkaline from Na2C03 and contains three 

 ferments: (i) Trypsin, which changes proteins to amino-acids to 

 peptones, completing the action of the digestive juices. Unlike 

 pepsin it acts in an alkaline or neutral medium. (2) Amylopsin, 

 which converts starch to sugar. (3) Steapsin or lipase, which 

 splits fats into fatty acids and glycerin and then emulsifies and 

 saponifies them. 



The liver secrets bile, an alkaline fluid of complex composition. 

 The fatty substance, cholesterin, and the bile pigments are waste 

 products. In higher vertebrates it has been proved that bile has a 

 weak ferment that acts on fats, aiding in their emulsification. 



