EXCRETION 



35 



there is a regular relation between metabolism and the 

 partial pressure of oxygen. 



It has been explained that muscle cells derive energy 

 from the splitting of glycogen, without oxidation, though 

 they effect an economy by using oxygen when they can. 

 Many bacteria derive all their energy from such reactions ; 

 and it has been suggested that some Invertebrates, such 

 as intestinal worms, which normally live in a medium 

 extremely poor in oxygen, do the same. But the most 

 recent researches (of Slater and others) make it unlikely 

 that any of the Metazoa have so fundamentally adapted 

 their physiology that they can thrive all their lives without 

 oxidations ; though undoubtedly many Invertebrates have 

 an amazing power of surviving the absence of oxygen for 

 long periods. 



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 

 intricacy of life processes. For the kidneys not only take 

 out of the blood all the waste products that result from 

 the metabolism of proteins and contain nitrogen, they also 

 maintain the composition of the blood at its normal, 

 rejecting any stuffs that vary from that normal, either 

 qualitatively or quantitatively, doing this work according to 

 laws quite different from the simple laws of diffusion or 

 solubility : thus sugar and urea are about equally soluble, 

 and yet the sugar is kept in the body, while the urea 

 is cast out. Even substances as insoluble as resins 

 are removed from the blood by the living cells of the 

 kidneys. 



A considerable quantity of water, plus traces of salts, fats, 

 etc., leaves the body by the skin, but its chief use is to 

 protect, and to regulate the temperature by variations in 

 the size of its blood vessels. Some special substances are 



