4 INTRODUCTION [CH. 



Phosphorus is a constituent of nucleic acid, a substance which 

 in combination with proteid is characteristic of the nucleus (see 

 p. 18). Proteids have a percentage composition which varies 

 somewhat, though not widely, in different cases. 



Carbon from 50 to 55 per cent. 



Hydrogen ,, 6*5 to 7 '3 ,, 



Nitrogen 15 to 17'6 



Oxygen 19 to 24 



Sulphur '3 to 2'4 



The size of the molecules of which proteids are composed is un- 

 doubtedly a large one. It is difficult if not impossible to determine 

 exactly how many atoms are contained in a molecule of a particular 

 proteid because it is difficult to obtain one such substance in a pure 

 condition free from admixture with others. The best determinations 

 which have been made show however that at least 1000 atoms must 

 be contained in the molecule. But the proteids known to the 

 chemist are of course taken from the dead bodies of animals and 

 are themselves to be regarded as products of the decomposition of 

 the molecules which existed during life. The proteid as the seat 

 of life has probably a decidedly different composition from the dead 

 substance, and so to avoid confusion, we may call the living molecules 

 biogens. 



The biogen molecule is continually absorbing oxygen from the 

 outside. This process is called respiration or breathing. It 

 decomposes and some of the products are no longer capable of being 

 built up again into other biogen molecules and are therefore got rid 

 of, since otherwise they would interfere with the chemical action, 

 just as accumulating ashes will eventually put out a fire. The 

 process of ejecting these waste products is called excretion, the 

 waste substances themselves, excreta, and the chemical changes 

 which lead to their production, katabolism (Gr. Kara/3oAry, deposi- 

 tion). The commonest excreta are water, carbon dioxide, urea, 

 and uric acid ; the last two substancQS contain nitrogen. But 

 it is not necessary that in all cases excreta should be ejected. 

 They may remain within the bounds of a mass of protoplasm ; 

 if they are removed from the sphere of the chemical action going 

 on in the protoplasm this is sufficient. In some animals uric acid 

 is stored up in this way. Many of the excreta, though injurious 

 if they remain in the protoplasm, are indirectly useful to the 

 animal after ejection. Such useful excreta are called secre- 



