30 THE FUNCTIONS OF ANIMALS. 



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. 

 The oxygen of this air combines with a substance called 

 haemoglobin, contained in the red corpuscles of the blood, 

 and is thus carried to all parts of the body. The proto- 

 plasm of the tissues having a stronger affinity for oxygen 

 than the haemoglobin has, removes as much as it requires. 

 The carbon dioxide formed as a waste product is ab- 

 sorbed by the serum of the blood, and so in time reaches 

 the lungs. But as the partial pressure of the carbonic 

 acid in the air is lower than it is in the serum, the gas 

 escapes from the latter into the air chambers of the lungs. 

 When the size of the chest is decreased, the pressure 

 is increased, and the gas escapes by the mouth or nose until 

 the pressure is equalised. 



Excretion. We have seen that the blood carries the 

 digested food to the various parts of the body, and that it is 

 also the carrier of oxygen and of the waste carbon dioxide. 



But there is much waste resulting from tissue changes, 

 which is not gaseous. It is cast into the blood stream by 

 the tissues, and has to be got rid of in some way. This is 

 effected by the kidneys, which are really filters introduced 

 into the blood stream. But they are the most marvellous 

 filters imaginable, and give us a good example of the in- 

 tricacy of life processes. For the kidneys not only take 

 out of the blood all the waste products that result from 

 the metabolism of proteids, and contain nitrogen, but they 



