THE MECHANISM OF LIFE 



577 



becomes the dependence of the cells upon one another, the stronger 

 IS the necessity of bringing into relation with one another the more 

 outlying cells, tissues and organs of the cell-community, in order 

 that uniiied co-operation may take place ; selection must make this 

 relation constantly more intimate, the more complex the structure 

 of the cell-community becomes. Along 

 with this there arises in the community 

 a tendency toward centralisation. 



The first step in the direction of cen- 

 tralisation is really taken by means of the 

 division of labour, when certain cell- 

 groups or organs undertake a definite 

 function for the whole community. Thus 

 the function in question becomes cen- 

 tralised for the whole body in one place, 

 and as many centres arise as there are 

 organs differentiated for definite func- 

 tions. This first step toward centralisa- 

 tion of administration is met with in the 

 cell-community of the plant. Here the 

 synthesis of starch, upon which the 

 nutrition of the whole plant depends, 

 is centralised in the green cells of the 

 leaf Further, the function of taking up 

 water, without which life cannot continue 

 to exist, is localised in the roots alone. 

 Corresponding localisations are present 

 in the animal cell-community. Thus, in 

 the higher animals the nutrition and 

 respiration of the individual tissue-cells 

 are centralised in the heart, which drives 

 the blood, rich in food and oxygen, to 

 all the cells of the various tissues and 

 organs (Fig. 278). 



In the animal cell-community the 

 second important step toward centralisa- 

 tion is taken, namely, the union of all 

 individual centres or organs of function 

 with one another by the appearance of 



a central nervous system with its paths of conduction. This 

 principle, in greater and greater perfection, leads finally in the 

 animal series to a far-reaching centralisation, such as is met with 

 in the complex cell-community of the vertebrates, and especially 

 of man. We have in the central nervous system a central organ 

 which alone has the function of uniting cells, tissues, and organs 

 with one another, so that an advantageous co-operation of them 

 becomes possible ; and the farther we ascend in the animal series, 



P P 



Fig. 278. — Scheme of the circula- 

 tion of blood in man. Centralisa- 

 tion of the nutrition of all cells 

 in the blood-current. The darker 

 half is the venous, the lighter the 

 arterial system. The two are 

 united by the capillary network 

 of the lungs (above) and of the 

 other tissues (below). In the 

 capillaries the blood - current 

 bathes all the tissues, the cells 

 of which take food from it and 

 give off to it their useless sub- 

 stances. (From Ranke.) 



