616 GENERAL METABOLISM [CH. XLI. 



them and carbohydrate so easy to understand. Glycerin is another 

 substance the conversion of which into carbohydrate appears to be 

 possible; glyceric aldehyde is isomeric with lactic acid, so here 

 again we have a feasible explanation. 



Turning now to the other side of the picture, what information 

 have we about carbohydrate katabolism ? The final products of com- 

 bustion are carbonic acid and water, but what are the intermediate 

 steps ? Just as lactic acid has been assumed to be sometimes a 

 stage in the formation of sugar, so also there is evidence that 

 it is a stage in its breakdown. We know that certain micro- 

 organisms possess the power of transforming sugar into lactic acid, 

 and even still further into butyric acid (see formulae, p. 411), and 

 Buchner has recently asserted that lactic acid is a stage in the 

 formation of alcohol and carbonic acid from sugar by means of yeast. 

 The atoms in lactic acid (C 3 H 6 3 ) are in the same proportion as in 

 sugar (C 6 H 12 6 ), but of course they are very differently arranged, 

 and the rearrangement involved in the conversion of the one into the 

 other, or vice versd, is differently explained by different chemists. 

 Lactic acid undoubtedly occurs in the body, but whether it all comes 

 from sugar is extremely doubtful. The principal lactic acid found 

 is the dextro-rotatory variety (sarco-lactic acid), whereas that formed 

 in fermentative processes, as in milk, is the optically inactive variety. 

 Now, there is a good deal of evidence that sarco-lactic acid originates 

 from proteins; for instance, in the birds from which Minkowski 

 removed the liver, the giving of protein food increased the lactic acid 

 (which was not synthesised in the absence of the liver into uric acid) 

 of their urine, and we have further seen that alanine and other 

 protein cleavage products are possible parent substances of lactic 

 acid. Still, if we admit that some of the lactic acid is of carbohydrate 

 origin, and the admission is quite justifiable, we must remember that 

 such a breakdown of sugar yields no heat; the calorific value of 

 sugar and lactic acid being equal. The formation of lactic acid 

 involves no transformation of energy ; there is no formation of animal 

 heat, or of its equivalent in work, and so the change is merely 

 preliminary to a further change into carbonic acid and water, in 

 which there will be that liberation of energy which it is the main 

 object of carbohydrate breakdown to accomplish. (G-lycolysis in the 

 blood and tissues, and the importance of glycuronic acid as an inter- 

 mediate substance in carbohydrate cleavage, are discussed on p. 540.) 



Metabolism of Pat. 



The most conspicuous use of the fat in the body is to act as 

 a reserve fund of fuel. The storage of 100 calories in the form of 

 fat may be effected in the space of about 12 c.c. of tissue, weighing 



