XIX] ALIMENTARY CANAL 429 



wall of the canal. Pepsin is active only in an acid medium, and 

 free hydrochloric acid is found in the contents of the stomach in 

 small quantities, produced by special cells in the walls of the 

 pouches just mentioned. 



An organ called the liver is very conspicuous (Fig. 226). 

 It consists of a ventral outgrowth of the gut, arising just behind 

 the stomach, which extends forwards and branches into an immense 

 tree-like mass of tubes welded together by connective tissue into a 

 solid mass extending forwards and nearly obliterating the front part 

 of the body cavity. The branches of the tree unite with one 

 another in such a way as to constitute a network of rods of cells. 

 Whether this organ really performs the same function as the so- 

 called liver in Amphioxus is doubtful. It has been proved that 

 the function of the Craniate liver is largely the elaboration of an 

 alkaline fluid called the bile. This is partly excretory in nature, 

 but has an important influence upon the processes of digestion and 

 absorption in the intestine. It neutralises the acid of the stomach 

 and causes the food when it enters the intestine to be alkaline. 

 The main stem of the liver tubes is called the bile-duct; there is 

 often a lateral outgrowth from this which acts as a reservoir for the 

 bile, called the gall-bladder. Besides this, the liver cells can 

 extract from the blood with which they are bathed, sugar and con- 

 vert it into a substance called glycogen, which is allied to starch in 

 composition, and which acts as a reserve of carbohydrate material 

 available for the system as needed. Among other influences which 

 the liver exercises on the chemical processes of the body is the very 

 important one of transforming the nitrogenous waste products into 

 a suitable form (urea or uric acid) for excretion by the kidneys. 



Another outgrowth from the intestine arises sometimes just 

 behind the opening of the bile-duct, sometimes from the duct 

 itself. This outgrowth, like the liver, branches into a tree of tubes 

 which are bound together by connective tissue to form a solid mass, 

 though one of much smaller size than the liver. This organ is 

 called the pancreas and it produces a secretion called pancreatic 

 juice, by which the process of digestion is completed. This juice 

 contains three ferments: these are amylopsin, which converts 

 starch into soluble sugar; trypsin, which, acting only in an 

 alkaline medium, converts proteid into peptone and simpler de- 

 rivatives ; and steapsin, which splits up fat into soluble fatty 

 acids and glycerine. The fatty acids unite with the alkalis present 

 in the mixed contents of the intestine to form soluble soaps, and 



