CHIEF FUNCTIONS OF THE ANIMAL BODY. 29 



which they pass into the blood; but we must follow the 

 starchy material, or carbohydrates, a little further. The 

 starch, we know, is converted into sugar, and this, with the 

 sugar of the food, pass'es into the capillaries of the villi, 

 and is carried to the liver. During 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 leaving 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 walls of the capillaries, and are stored in 

 connective tissue cells. 



All the products of digestion, except the fat, pass through 

 the liver, which receives everything before it is allowed to 

 pass into the general circulation. Thus many poisons, 

 such as metals, are arrested by the liver, and various 

 harmful substances which are formed in the course of 

 digestion are changed by the liver into harmless com- 

 pounds. The excess of sugar, we have already noted, is 

 stored in the liver. It is synthesised there into a substance 

 called glycogen, which can be readily retransformed into 

 sugar according to the needs of the system. Glycogen is 

 stored in the muscles also, and forms an important part of 

 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 foodstuff 

 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 the windpipe until the pressure is equalised. 



