156 THE STUDY OF ANIMAL LIFE CHAP. 



these functions, the blood is the main medium of the 

 wandering amoeboid cells or phagocytes which have many 

 a role in the economy of the body, e.g. in dealing at 

 strategic places with intruding microbes. 



10. Modern Conception of Protoplasm. Our know- 

 ledge of protoplasm the chemical and physical basis of 

 life is still very incomplete. Perhaps our most certain 

 knowledge of it is that in our brains its activity is asso- 

 ciated with consciousness. Protoplasm appears to be a 

 mixture of many substances among which various chemi- 

 cal reactions occur. Just as a firm may owe its success 

 to the effective wav in which the various members w r ork 



& 



into one another's hands, so part of the potency of proto- 

 plasm may be in the possibilities of interaction between 

 the component substances. The most important of these 

 substances are proteids or proteins, extremely complex 

 compounds of carbon, hydrogen, oxygen, and nitrogen, 

 with frequently a small quantity of sulphur. The 

 characteristic percentage composition of proteids is 

 indicated approximately by the numbers : carbon 53, 

 oxygen 22, hydrogen 7, nitrogen 16, and sulphur 1-2. 



The chemical changes that go on in the body are 

 summed up in the word metabolism, and the most im- 

 portant of these may be ranked as either disruptive or 

 constructive, katabolic or anabolic. There is a con- 

 tinuous twofold process of waste and repair, discharge 

 and restitution, expenditure of energy and recuperation, 

 running down and winding up. There is a synthesis of 

 nutritive substances into complex molecules, such as 

 those of proteids, and an analysis or explosion of these, 

 with liberation of energy. 



To this conception, however, there has to be added 

 another, that each kind of living matter has an organisa- 

 tion, or in any case a substratum, of a colloid nature 

 which is at once a product of the characteristic meta- 

 bolism, and a physical system in which this continues to 

 occur. Many of the characteristics of vital activities 

 become more intelligible when the presence of the colloid 

 substratum is recognised. Prof. Child has defined a 

 living organism as "a specific complex of dynamic 



