24 THE FUNCTIONS OF ANIMALS. 



hydrates, a little further. The starch, we know, is converted 

 into sugar, and this, with the sugar of the food, passes into 

 the capillaries of the villi, and is carried to the liver. Dur- 

 ing digestion there is an increase of sugar in the blood vessel 

 going to the liver from the intestine, that is, in the portal 

 vein, but no increase in the hepatic veins, the vessels leav- 

 ing the liver. The increase must, therefore, be retained in 

 that organ, and we recognise as one of the functions of the 

 liver, the regulation of the amount of sugar in the blood. 

 There is no special organ for the regulation of the amount 

 of fat ; the drops pass through the capillary walls, and are 

 retained in the connective tissue. 



We must remember that all the products of digestion, 

 except the fat, pass through the liver, which receives every- 

 thing before it is allowed to pass into the general circula- 

 tion. Thus, many poisons, especially metals, are arrested 

 by the liver, and many substances which result from digestive 

 processes and would be harmful, are there altered into harm- 

 less compounds. The excess of sugar, we have already 

 noted, is stored in the liver. It is converted there into a 

 substance called glycogen, which can be readily retrans- 

 formed into sugar according to the needs of the system. 

 Glycogen is stored in the muscles also, and is the material 

 chiefly useful as the fuel for the supply of muscular energy 

 and of the warmth of the body. Thus, if an animal be 

 subjected to a low temperature, the glycogen of the liver 

 disappears just as it does during the performance of muscular 

 work. 



Another of the many functions of the liver is that in it 

 nitrogenous waste products begin to be prepared for their 

 final elimination by the kidneys. 



Respiration. 



There is another most important food stuff to be noticed, 

 namely, the oxygen which is absorbed from the air by the 

 lungs. We may picture a lung as an elastic sponge-work of 

 air chambers, with innumerable blood capillaries in the 

 walls, enclosed in an air-tight box, the chest, the size of 

 which constantly and rhythmically varies. When we take 

 in a breath the size of the chest is increased, the air pressure 

 within is lowered, and the air from without rushes down 



