50 



GENERAL BIOLOGY 



empties into the common bile duct. The three-lobed liver, already men- 

 tioned, also secretes an alkaline digestive fluid, known as bile. This is 

 carried by the little bile capillaries into the gall bladder where it is stored 

 until food enters the intestine, when it passes into the duodenum through 

 the common bile duct. Digestion begins in the stomach. 



According to Latter, "the alkaline fluid secreted by the mucosa 

 layer of the oesophagus and the acid gastric juice secreted by the glan- 

 dular walls of the stomach digest out the proteid portion of the food by 

 means of a ferment, ( ) called pepsin, which changes 



proteids into soluble peptones. The food then passes through the pyloric 

 constriction into the intestine. Here it is attacked by the pancreatic 

 juice and the bile. The pancreatic juice contains three ferments: (1) 

 trypsin, which converts proteids into peptones ; (2) amylopsi^ which 

 converts starch into sugar; and (3) steapsin, which splits up fats into 

 fatty acid and glycerin. The bile emulsifies fats and converts starch into 

 sugar. The intestinal wall produces a secretion which probably aids in 

 converting starch into sugar. 



"Absorption begins in the stomach, but takes place principally in 

 the intestine. The food substances which have been dissolved by the 

 digestive juices are taken up by the mucosa layer, passed into the blood 

 and lymph, ( ) and are transported to various parts 



of the body (C, Fig. 8). The undigested particles of food pass out 



Fig. 8. Diagrams of Important Relationships. 



A. The relation of the hepatic portal system to the stomach, intestine, 

 pancreas and liver. 



B. Diagrammatic transverse section through the abdominal region of a frog. 



C. Diagram of the two main channels by which food enters the general circulation 

 in mammals, e, intestine with villi ; v, v, in its walls ; r a, right auricle of the 

 heart ; m, postcava ; n, precava ; o, thoracic lymph duct ; p, pancreas ; q, pancreatin 

 duct ; r, hepatic vein ; s, portal vein ; t, bile duct from I, liver ; arrows indicate the 

 course of secretions entering the intestine, and of the absorbed food departing 

 therefrom. 



(A, after Howes; B, after Parker; C, from Needham's General Biology," by 

 permission of The Comstock Publishing Co.) 



of the intestine into the cloaca and are then discharged through the anus 

 as faeces." 



The absorbed food is used by the frog to build up new protoplasm 

 to take the place of that consumed in the various life activities, and to 

 increase the size of the body. Food is stored up in the liver-cells as 



