466 BIOLOGY. [Book vii. 



As to the organic aliments, all are at once respiratory and 

 plastic. The ternary or hydro-carbonised aliments themselves 

 can be fixed, under the form of fat, in the animal tissues, but it 

 is certain that a large proportion of them burn, leaving as the 

 residuum of their complete combustion water and carbonic acid. 

 They evidently represent the great source of the calorific move- 

 ment. As to the azotised quaternary aliments, they burn for the 

 most part incompletely, it is true, furnishing at once assimilable 

 materials, which lodge for a time in the organism, and also 

 sensible heat and dynamic movement. Moreover, there is no 

 impassable barrier between these two groups of aliments, since 

 CI. Bernard and Lehmann have proved that the albuminoidal 

 aliments can he partially metamorphosed into sugar, and thus 

 form substances totally oxydable, — respiratory aliments. Ordi- 

 narily, however, these substances only undergo incomplete 

 oxydations, evolvements, isomeric metamorphoses, catalyses. 

 These varied reactions, which, however, produce heat, leave as 

 excrementitious residua azotised compounds much more oxydised 

 than the quaternary substances whence they are derived. These 

 azotised wastes are composed of leucine, tyrosine, of creatine, of 

 uric acid, lastly of urea, which is the most azotised product of the 

 series. Thus for 1 equivalent of oxygen, albumine contains 3| 

 equivalents of carbon, creatine 2 equivalents, uric acid 1^, and 

 urea only 1 equivalent. ^ 



The value of aliments depends therefore, on the one hand, on 

 the quantity of movement and of force : on the other, on the 

 quantity of assimilable molecules which they can furnish. 



A ternary aliment has the more value, the less oxygen, and 

 the more hydrogen and carbon it contains. 



A quaternary aliment has the more value, the richer it is in- 

 azote, carbon and hydrogen, the poorer it is in oxygen. 



Every substance completely oxydised can, from the alimentary 

 point of view, play in the economy only a secondary part (water, 



^, G. See, JDe V Alimentation (Eevue Scientifique, 1866, B"o. 35). 



