1106 PHYSIOLOGY 



complete, whereas in the body the oxidation takes place by stages. Eecent 

 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 (autoxidisable) 

 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 laevorotatory and is designated Meucine. If 

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

 ^-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., I- glucose or I- 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 oxybutyric acid undergoes no further oxidation, and either accumu- 

 lates in the body or is excreted combined with bases in the urine. In the 

 normal individual tyrosine, whether administered separately or in combina- 

 tion in protein, is completely oxidised, the benzene ring being broken up. 

 In certain rare cases of disordered metabolism the patient, who is otherwise 

 apparently well, is unable to effect the total oxidation of tyrosine, which 

 is therefore excreted as homogentisic acid, after undergoing only the first 

 stage of its normal transformation in the body. These various mechanisms 

 are adjusted in each case to the functional activity of the cell and are limited 

 therefore, not by the supply of oxygen or of food-stuff to be oxidised, but 

 by the necessities of the cell, i.e. the adaptations induced in it by its environ- 



