viii THE FUNCTIONS OF THE BODY 147 



are grist, (4) those that depend on minute living organ- 

 isms, and (5) those that feed on organic detritus. 



The second step is digestion, dissolving the food and 

 making it readily soluble. For the fuel of the living fire 

 requires a good deal of treatment before it is really 

 available. In some of the lower animals, such as sponges, 

 the food-particles are engulfed by internal cells and 

 digested within them. This is the Protozoon method 

 and is called intracellular digestion. In most cases, how- 

 ever, the food is digested in the food-canal and then 

 absorbed. This is called extracellular digestion. Both 

 modes may occur together, as in Coelentera and some 

 simple worms. But above the level of worms the intra- 

 cellular mode of digestion is dropped, and extracellular 

 digestion occurs in the food-canal bv the action of fer- 



^ 



ments or enzymes secreted by cells in the wall of the 

 canal or in associated glands. Among these ferments 

 may be noted the ptyalin of the salivary glands which 

 changes starch into sugar, the pepsin of the stomach 

 which turns proteids into peptones, the trypsin and 

 other ferments of the pancreas. One of the peculiarities 

 of these ferments is that a small quantity goes a long 

 way and can act on a large mass of material without 

 itself undergoing much change. 



The third step is absorption., the digested food has to 

 be carried to the various parts of the body and there 

 incorporated into the tissues that need recuperation, or, 

 it may be, stored for subsequent use. In backboned 

 animals the digested carbohydrates and proteids are 

 absorbed by mesenteric veins which combine into a 

 portal vein that goes to the liver and breaks up there 

 (the hepatic-portal system). The blood is re-collected 

 from the liver after it has undergone some changes and 

 passes by the hepatic veins to the heart, whence it is 

 distributed throughout the body. The digested fat 

 follows another course ; it passes from the food-canal 

 into a system of lymph-vessels, w r hich communicate in 

 various ways with the venous system and thus eventually 

 with the heart. 



5. Functions of the Liver. The liver of higher animals 



