RESPIRATION AND EXCRETION. 19 



noted, is stored in the liver. It is converted 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 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 

 2& it does during the performance of muscular work. 



There is another most important food-stuff to be noticed. 

 This is the oxygen which is absorbed from the air by the 

 lungs. The lungs we may picture as a sort of 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 hsemoglobin, 

 contained in the red corpuscles of the blood, and is thus 

 carried to all parts of the body. The protoplasm of the 

 tissues having a stronger affinity for oxygen than has the 

 haemoglobin, takes as much as it requires. The carbonic 

 acid gas formed as a waste-product is absorbed by th e serum 

 of the blood, and so in time reaches the lungs. But' as thq"' 

 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 until the pressure is equalised. By the constant ' 

 repetition of the breathing movements, oxygen is Constantly 

 being taken in, and carried to the tissues which are in a 

 marvellous way " hungry " for it, while the waste carbonic 

 acid gas is as constantly being removed. 

 J Thus the gaseous waste of animal life is got rid of 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 for the purpose of purifying it. But 

 they are the most marvellous filters imaginable, and give us 

 a good example of the intricacy of life prodesses. Tor the 



