250 FUNCTIONS OF NUTRITION. 



Starch. Carbonic acid. Water. 



6 H 10 6 + O u = 6(C0 2 ) + 5(H 2 0). 



In an animal supported on this food, the whole of the oxygen taken 

 in by respiration would reappear in the expired carbonic acid. But 

 in an animal also consuming fatty substances, the proportions would 

 be changed. As these matters do not contain enough oxygen to form 

 water with their hydrogen, more oxygen must be taken in with the 

 breath than is needed to convert their carbon into carbonic acid ; and a 

 part of it will consequently disappear from the gaseous products of 

 respiration. The change in this instance is as follows : 



Oleine. Carbonic acid. Water. 



57 H 1(H 6 + 1GO = 57(C0 2 ) + 52(H 2 0). 



In effecting, therefore, the complete disappearance of a fatty sub- 

 stance, 160 parts of oxygen will be absorbed, and only 114 parts 

 returned in the carbonic acid. This will also take place where albu- 

 minous matters are used as food, since all the nitrogen of these sub- 

 stances is excreted in the form of urea ; and after the separation of 

 urea from albumen, what is left must be analogous in composition to 

 fat ; that is, containing less oxygen than would be required to convert 

 its hydrogen into water. 



It is no doubt for these reasons that, in herbivorous animals, feeding 

 largely on carbo-hydrates, the oxygen exhaled in the carbonic acid is 

 nearly equal to that taken in with the breath; while in carnivora, 

 which consume only fats and albuminous matters, a larger proportion 

 of oxygen disappears from the products of respiration. 



Finally, some kinds of vegetable food, as fruits and green tissues, 

 contain substances, the oxygen of which is more than sufficient to form 

 water with their hydrogen. Such are the salts of vegetable acids, like 

 oxalic, citric, gallic, malic, and tartaric acid. The result of the internal 

 consumption of tartaric acid, for example, would be as follows : 



Tartaric acid. Carbonic acid. Water. 

 C 4 H 6 O 6 + O 5 = 4(00.,) -f- 3(H 2 0). 



In this instance more oxygen will be exhaled, in the carbonic acid 

 produced, than was absorbed from the atmosphere ; because a super- 

 abundance already existed in the material used as food. 



The proportions of oxygen and carbonic acid, absorbed and expired 

 in respiration, will therefore vary, as shown by Mayer,* not only with 

 the nature of the food, but also according to the transformations, within 

 the living organism, of one nutritive substance into another, as of a 

 carbohydrate into a fat, or of either into an organic acid. In the fer- 

 mentation of glucose (p. 57) there is even an elimination of carbonic 

 acid without any absorption of oxygen whatever ; this being a process, 

 not of direct oxidation, but of the rearrangement of elements already 

 present in the sugar, a portion being exhaled as carbonic acid, while 

 the rest remain in the form of alcohol. 



*Lehrbuch der Agrikultur-Chemie Heidelberg, 1871, p. 101. 



