THE METABOLISM OF THE CARBOHYDRATES 709 



of one leg in the frog are tetanized, the glycogen content, compared with 

 that of the other leg, will be found to be diminished. 



At first sight it might appear that the easiest way to study the utiliza- 

 tion of glucose in the muscles would be to compare its concentrations 

 in the blood flowing to and coming from them. The muscle that has been 

 most successfully employed in studies of this kind has been the heart. 

 Some years ago Starling and Knowlton 24 sought to measure the consump- 

 tion of glucose by the excised mammalian heart, by comparing the per- 

 centage in the perfusion fluid at various periods during the observation. 

 Patterson and Starling 25 showed later, however, that the results obtained 

 by such a method can furnish no criterion of the actual consumption of 

 glucose by the tissue on account of the fact that the heart itself may 

 store away large quantities of carbohydrate in an unused state, i.e., as 

 glycogen. After allowing for the glycogen as well as for the glucose 

 consumption by the lungs, Cruikshank and Patterson 64 computed that 

 the heart (of the cat) uses 1.5 mg. glucose per gram per hour. 



Other investigators have thought to study the utilization of glucose 

 by observing the rate at which it disappears from drawn blood kept in 

 a sterile condition at body temperature for some hours after death. 

 This process is called glycolysis, and it has been assumed that the process 

 is similar to that which occurs in the tissues themselves an assumption, 

 however, for which there is no warranty. Indeed, it may readily be 

 shown that the glycolysis occurring in blood has very little if anything 

 to do with the utilization of glucose in the tissues, for it has been found 

 that glucose disappears from drawn blood very slowly indeed when 

 compared with the rate at which it disappears from the blood of animals 

 in which the addition of glucose from the liver has been prevented by 

 removal of this viscus (Macleod). 26 



A third method for studying the utilization of glucose consists in 

 observing the respiratory exchange of animals. In normal animals the 

 injection of glucose causes an increase in the carbon-dioxide excretion 

 and a rise in the respiratory quotient, which it will be remembered is 

 a ratio expressing the relationship between the amount of carbon dioxide 

 excreted and of the oxygen retained in the organism. When carbohy- 

 drate is undergoing combustion, the quotient is nearly 1, whereas with 

 that of protein it is about 0.7 (see page 582). By observing the quotient 

 under given conditions one can compute the proportions of carbohydrate 

 and of fat and protein that are undergoing metabolism. In the hands 

 of Murlin and others, 27 this method has proved of some value in settling 

 certain questions concerning the utilization of glucose in normal and 

 diabetic animals ; but the results must be interpreted with great care on 

 account of the fact that temporary changes in the blood may cause a 



