148 BACTERIA 



already been built up by vegetable life. Again, the com- 

 plementary functions of animal and vegetable life are seen 

 in the absorption by plants of one of the waste materials of 

 animals, viz., carbonic acid gas. Plants abstract from this 

 gas carbon for their own use, and return the oxygen to the 

 air, which in its turn is of service to animal life. 



By animal activity some of these foods supplied by the 

 vegetable kingdom are at once decomposed into carbonic 

 acid gas and water, which goes back to nature. Much, 

 however, is built up still further into higher and higher com- 

 pounds. The proteids are converted by digestion into 

 alburnoses and peptones, ultimately entirely into peptones; 

 these in their turn are reconverted into proteids, and be- 

 come assimilated as part of the living organism. In time 

 they become further changed into carbonic acid, sulphuric 

 acid, water, and certain not fully oxidised products, 1 which 

 contain the nitrogen of the original proteid. In the table 

 these bodies have been represented by one of their chief 

 members, viz., urea. 



It is clear that there is in all animal life a double process 

 continually going on ; there is a building up (anabolism, as- 

 similation), and there is a breaking down (katabolism, dis- 

 similation). These processes will not balance each other 

 throughout the whole period of animal life. We have, as 

 possibilities, elaboration, balance, degeneration ; and the 

 products of animal life will differ in degree and in substance 

 according to which period is in the predominance. These 

 products we may subdivide simply into excretions during 

 life and final materials of dissolution after death, both of 

 which may be used more or less immediately by other forms 

 of animal or vegetable life, or mediately after having passed 

 to the soil. We may shortly summarise the final products 

 of animal life as carbonic acid, water, and nitrogenous rem- 

 nants. These latter will occur as urea, new albumens, com- 



1 E. A. Schafer, Text-book of Physiology, vol. i., p. 25 (W. D. Halliburton). 



