THE METABOLISM OF CARBOHYDRATES 903 



tion in the blood-sugar, pointing to the liver being the chief, if not the 

 sole, source of the blood-sugar. In some animals, e.g. the carnivora, it 

 would seem that the liver can continue to supply sugar to the blood 

 on a diet which includes only proteins and fats, and we have already 

 seen that in such animals glycogen itself can be stored up at the 

 expense of protein. It is doubtful whether a perfectly normal exist- 

 ence is possible in man in the total absence of carbohydrates from the 

 food, though there is no doubt that in the northern nations, e.g. the 

 Eskimos, the amount of carbohydrate consumed is very small in 

 comparison with the fats and proteins. During muscular exercise the 

 increased output of energy is associated with a corresponding 

 increase in the absorption of oxygen and in the output of carbon 

 dioxide, pointing to a consumption of carbohydrate and fat in the 

 contracting muscles. We might therefore assume that sugar is being 

 normally released by the liver into the blood stream so as to maintain 

 the proportion of this substance in the blood at a certain level, and that 

 the sugar is as constantly being taken up and oxidised in the muscles, 

 where it serves as a source of energy. According to Chauveau and 

 Kauf mann the venous blood flowing from a contracting muscle contains 

 less sugar than the arterial blood flowing to the muscle. A similar 

 consumption of glucose has been described as occurring in the isolated 

 contracting mammalian heart when fed with Ringer's fluid containing 

 a small trace of glucose. It has long been maintained by Pavy that 

 the sugar which exists -free in the blood is unavailable for the nutrition 

 of the tissues and that it must undergo some further process of assimi- 

 lation or synthesis before it can become available as a source of energy 

 to the tissues or serve for the building up of protoplasm. It has been 

 lately shown by Professor Knowlton, working with the author, that 

 a heart, fed with blood and performing a normal amount of work, is 

 able to use up sugar, the consumption of sugar amounting to about 

 4 mg. per gramme of heart muscle per hour. That the question of 

 utilisation of sugar by the tissues is highly complex is shown by a 

 study of the conditions under which sugar may appear in the urine. 

 We learn thereby to appreciate to some extent the significance of 

 carbohydrates both as sources of energy and as foods for the tissues, 

 though we are still a long way from unravelling all the changes which 

 the sugar must undergo in the cell before it appears once again in the 

 oxidised products, carbon dioxide and water. 



GLYCOSURIA 



Normal urine always contains a small proportion of sugar, about 

 1 part per 1000, i.e. in about the same proportion as the blood itself. 

 For the detection of these small traces of sugar in the urine special 

 methods are necessary. The term glycosuria is not employed unless 



