METABOLISM 209 



of the carbohydrate to C0 2 and H 2 is complete. If a fatty 

 acid is burned in the same way, the respiratory quotient is less 

 than one. It does not contain sufficient oxygen to take care of 

 even the hydrogen. Thus part of the oxygen consumed is ex- 



CO 



creted as water, and in the expression yr- 2 , the denominator is 



^2 



larger, and the ratio is less than one (about 0.7). In the case of 

 ammo acids (from the proteins) the value lies between those for 

 carbohydrates and fats (about 0.8). By actual measurement of 

 the amount of C0 2 excreted by an animal, and the amount of 

 2 consumed it is possible to draw conclusions as to the kind of 

 material which the body is burning. 



Further evidence of the fate and history of glucose in the 

 body has been obtained by a study of the various experimental 

 glycosurias described above. 



Although much labor has been expended to clear up the 

 exact mechanism of the burning of glucose in the muscles, very 

 little is known of the steps in the process. We are sure that 

 glucose serves as fuel for the muscles from evidence of various 

 sorts but only in the presence of a substance produced in the 

 pancreas. It has been suggested that glyceric aldehyde COH 

 CHOH CH 2 OH is an intermediate stage in the process of 

 burning sugar, and that this is converted next into alcohol CH 3 

 CH 2 OH and C0 2 , but this has not been proven. The diabetic 

 is unable to use glucose, but the exact reason for the failure of 

 the tissues to use sugar is unknown. It is probable that in some 

 cases at least, the necessary internal secretion of the pancreas is 

 wanting, but this only brings us a step nearer the solution with- 

 out actually furnishing it. The diabetic is still capable of per- 

 forming oxidations, for many substances other than glucose still 

 are oxidized with ease. It has been suggested that the difficulty 

 is in the first attack of cleavage on the sugar molecule, but there 

 is some evidence which does not bear out this idea. We shall 

 have to await the final solution of the problem. 



Sources of Glycogen. The study of the sources from which 

 glycogen can be built up in the body has been greatly facilitated 



