1234 PHYSIOLOGY 



temperature, but in the animal body the processes of oxidation have 

 to go on at a temperature varying between 5 and 40 C., and in a 

 solution which is almost neutral in reaction. It might be said that 

 at the temperature of an ordinary flame the combustion of the food- 

 stuffs is immediate and complete, whereas in the body the oxidation 

 takes place by stages. Recent research has tended to remove this 

 point of distinction by pointing out that even in an explosion of a 

 mixture of methane and oxygen there is a series of intermediary 

 products, and that the whole process, if analysed, is made up of stages 

 in which hydrolysis and oxidation go on simultaneously, so that 

 on this account it is difficult to cause a combination, even of hydrogen 

 and oxygen, in the complete absence of any watery vapour. The 

 oxidations in the body are strictly limited both in nature and extent. 

 The mere fact that a substance is readily or even spontaneously 

 oxidisable (autoxidi sable) affords no guarantee that it will undergo 

 oxidation in the animal body. Thus phosphorus or pyrogallol taken 

 by the mouth can be recovered in an unoxidised form from the urine. 

 Carbon monoxide is excreted unchanged. There must apparently be 

 some definite relationship between the molecular structure of the food- 

 stuff and that of the cells of the body. Thus ordinary proteins which 

 undergo complete oxidation contain large quantities of leucine. This 

 substance is Isevorotatory and is designated Z-leucine. If Z-leucine 

 be administered to rabbits it is completely oxidised. If its isomer 

 (Z-leucine, resembling it in every particular so far as we can see except 

 in its relation to polarised light, be administered to a rabbit, the greater 

 part of the substance passes through the body unchanged. In the 

 same way there are sixteen sugars of the formula C 6 H 12 6 . Of these 

 only four, namely, glucose, fructose, galactose, and mannose, can be 

 oxidised in the animal body. Other sugars differing in so slight a 

 degree from these four as, e.g., Z-glucose or /-fructose, cannot be 

 utilised by the body. Not only must there be a distinct relation 

 between the structure of the cell and the molecular structure of the 

 food-stuff supplied, but there must be different mechanisms for the 

 food -stuffs and their derivatives. Thus in certain cases of disease or 

 of abnormal nutrition the body may lose absolutely the power of 

 utilising, i.e. of oxidising, a whole class of food-stuffs. In severe 

 diabetes, or after destruction of the pancreas, glucose behaves in the 

 body as if it were one of the artificial unassimilable sugars. The 

 normal oxidation of fats probably proceeds by stages in each of which 

 two atoms of carbon undergo oxidation. The penultimate stage in 

 the oxidation of any of the higher fatty acids is thus oxybutyric acid. 

 In complete carbohydrate starvation, for some reason or other, the 

 body loses its power of completing this last stage, so that the oxy- 

 butyric acid undergoes no further oxidation, and either accumulates 



