MULTICELLULAR ANIMALS 165 



by the contraction of the walls of the rectum. In the very young 

 these contractions are not under control of the will. 



The Liver. Digested foods, except the fats, pass through the 

 wall of the intestine into fine veins, which drain into a single large 

 vein, and are then passed on into the liver (Fig. 108). Here the 

 large vessel, termed the hepatic portal vein, breaks up again into 

 fine vessels. In the liver bacteria when present in the blood are taken 

 up by the liver cells. But the chief function of the liver is the con- 

 version of the carbohydrates after digestion back into large mole- 

 cules of a substance known as glycogen. It is said to have the 

 formula (CGHio05)n> which, it will be noted, is also the formula of 

 ordinary vegetable starch. It is therefore sometimes called animal 

 starch. Glycogen is apparently not identical with ordinary starch, 

 but the chemical differences do not concern us here. The essential 

 point for consideration here is the fact that the large molecules of 

 crude starch, which are taken in as food, are by digestion converted 

 to a soluble form of sugar that passes through the walls of the 

 digestive canal and in the liver is converted back again into large 

 molecules that are stored there. Glycogen is the important source of 

 energy in muscles and other activities of the body. It is released 

 from the liver by being again converted by liver enzymes to a 

 simpler form and is re-converted to glycogen again by enzymes in 

 the body cells that eventually receive it. 



The liver also acts upon blood proteins that have resulted from 

 the breakdown of cell substances, and converts them into urea. The 

 urea molecule has the formula (NH2)2CO; it passes through living 

 membranes rather freely, and is the chief nitrogen-containing com- 

 pound excreted by the body. Thus, in addition to its role in diges- 

 tion the liver has an important function in excretion. 



The BILE, which is a complex mixture of various substances, is 

 secreted by the liver cells. In Man it is stored in a sac known as the 

 GALL BLADDER and is excretcd into the intestine through the bile 



