CH. XIl] THE CIRCULATION OF NITROGEN 275 



into somewhat simpler compounds. The proteids are converted 

 into peptones, or other substances ; the fats are dissociated ; and 

 the carbohydrates are hydrolysed to form sugars. All these 

 products of digestion pass through the walls of the alimentary 

 canal into the blood stream and ultimately reach the cells of the 

 bodily tissues. There they become assimilated to form the living 

 protoplasm, or perhaps they become lodged among the particles of 

 the latter as stored-up food material. This is the constructive meta- 

 bolism of the animal body. But very soon the living tissue, or its 

 included food-materials, are oxidised, that is, burned up by the 

 inspired oxygen, or otherwise broken down ; and as a result of this 

 destructive metabolism, the mechanical energy and psychical charac- 

 ters, which are the manifestations of the life-activity of the animal, 

 are exhibited. The products of this destructive metabolism again 

 pass into the blood stream, and are finally thrown out from the 

 body as waste substances, or excretions. This elimination of the 

 effete products of the body takes place in various organs, the lungs 

 and kidneys in the mammal, from w^hich carbon dioxide, water, and 

 some nitrogenous residue, such as urea, are discharged ; or in 

 analogous structures in the lower animals. Thus life is maintained 

 by the continual death of parts of the tissues of the living body ; 

 and just as continually the latter are renewed by the assimilation 

 of food materials, which assuming the organic phase become part 

 and parcel of the living organism. 



We cannot speak so precisely of the elimination of waste sub- 

 stances from the plant organism. There the energy requirements 

 are far less than in the case of the animal, and we find that 

 constructive metabolism is immensely greater, in the plant, than 

 destructive metabolism. Therefore during life the mass of a plant 

 continually increases, until, as in the case of great trees, an enormous 

 bulk of organised substance is formed. In these cases life continues 

 throughout centuries and the body of the plant is always becoming 

 greater because comparatively little organic matter is being shed. 

 In the case of long-lived animals, such as some reptiles, the body 

 ceases to grow after a certain age has been attained, and the mass 

 of food assimilated is exactly balanced by the mass of w^aste sub- 

 stances eliminated. But there are no definite excretory organs in 

 the plant, and it is only by the removal, from season to season, of 



18—2 



