METABOLISM OP CARBO-HYDRATES 54! 



without the appearance of sugar in the urine is sometimes called the 

 assimilation limit for that carbohydrate. The attsmpt has been 

 made to fix the limit of tolerance of dextrose for normal persons 

 and persons suffering from incipient diabetes, with the object of 

 aiding in early diagnosis of that condition. But the limit varies with 

 so many conditions, only some of which can be controlled, that such 

 observations are not easily interpreted. 



Except as an occasional phenomenon, glycosuria other than ali- 

 mentary is inconsistent with health; and therefore in the normal 

 body the sugar of the blood must be either destroyed or transformed 

 into some more or less permanent constituent of the tissues. The 

 transformation of sugar into fat we have already mentioned, and 

 shall have again to discuss; it only takes place under certain con- 

 ditions of diet, and no more than a small proportion of the sugar 

 which disappears from the body in twenty-four hours can ever, in 

 the most favourable circumstances, be converted into fat. The 

 dextrose which is taken up from the blood by the tissues and there 

 condensed to glycogen suffers sooner or later the converse change, 

 in all probability under the influence of diastases or glycogenases 

 produced in the cells, and reappears as dextrose to take its place 

 in the cellular katabolism and begin the series of cleavages and 

 oxidations by which its chemical energy is set free. Accordingly, 

 it is the destruction of sugar which concerns us here, and there is 

 every reason to believe that this takes place, not in any particular 

 organ, but in all active tissues, especially in the muscles, and to a 

 less extent in glands. 



It has been asserted that the blood which leaves even a resting 

 muscle, or an inactive salivary gland, is poorer in sugar than that 

 coming to it ; and the conclusion has been drawn that in the metabo- 

 lism of resting muscle and gland sugar is oxidized, the carbon 

 passing off as carbon dioxide in the venous blood. This is indeed 

 extremely likely, for we know that, when the skeletal muscles of a 

 rabbit or guinea-pig are cut off. from the central nervous system by 

 curara, the production of carbon dioxide falls much below that of 

 an intact animal at rest ; and the carbon given off by such an animal 

 on its ordinary vegetable diet can be shown, by a comparison of 

 the chemical composition of the food and the excreta, to come 

 largely from carbo-hydrates. But, considering the relatively feeble 

 metabolism of muscles and glands when not functionally excited, 

 the large volume of blood which passes through them, the difficulty 

 of determining small differences in the proportion of sugar in such 

 a liquid, the possibility that even in the blood itself sugar may be 



The greatest quantity of cane-sugar recovered from the urine was 8 grammes 

 (7.92 grammes by Fehling's method and 8-29 grammes by the polarimeter) ; 

 the highest proportion of the quantity taken which appeared in the urine was 

 2-5 per cent. When dextrose was found, cane-sugar was always present as 

 well. 



