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, passes 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. 
