216 GENERAL PHYSIOLOGY 



substances with a rearrangement of their atoms. Free affinities 

 come into the body with oxygen especially ; and it is well known 

 that in the combination of oxygen with other substances, or, in 

 other words, in combustion, a great quantity of energy is liberated. 

 Hence the process of oxidation plays an extremely important role 

 in all life ; and, as has already been seen, the comparison of life 

 with fire is a very happy one. Complex compounds come into 

 the organism, especially in the case of animals, with the organic 

 food ; there they undergo a long series of transformations, which 

 thus far have not been followed, in which decompositions and 

 syntheses proceed together to the construction of the living 

 proteid molecule. Living proteids may be classed with explosive 

 bodies. They tend toward decomposition ; and out of the com- 

 plexes of atoms set free there arise synthetically by rearrangement, 

 partly immediately after the decomposition and partly later in 

 combination with substances newly introduced, chemical com- 

 pounds the origin of which under certain circumstances is again 

 associated with the evolution of energy. 



In the present condition of our knowledge it is not possible 

 to follow in detail the intricate series of chemical processes, 

 the decompositions and syntheses and the transformations of 

 energy associated with them, from the first cleavage of carbonic 

 acid and the synthesis of the first product of assimilation in the 

 plant to the decomposition of the living proteid in the plant 

 and the animal. It is known, however, that the final products of 

 metabolism, such as carbonic acid, water, urea, etc., are extremely 

 poor in chemical energy. The larger quantity of chemical energy 

 introduced into the body with the food must, therefore, have been 

 transformed into other forms of energy upon its way through 

 metabolism, and thus results the work of the organism. 



2. The Introduction of Light and Heat 



It has been said that the main quantity of all the energy that 

 is introduced comes into the body as chemical energy. For the 

 animal organism this statement holds good without limitation; 

 for the plant, however, it needs a correction. It is true that in 

 the plant the energy at the expense of which its work goes on 

 is likewise pre-eminently chemical ; but a part of this potential is 

 not introduced into the body as free, available energy, i.e., in the 

 form of free affinities, such as oxygen possesses ; another form of 

 energy must first be introduced in order to create free affinities 

 in the former. It is well known that carbonic acid and water are 

 necessary for the synthesis of the first product of assimilation. 1 

 But carbonic acid and water as such are poor in chemical energy 



1 Cf. p. 158. 



