138 NUTRITIONAL PHYSIOLOGY 



over is a still more marked phenomenon when a vegetable 

 oil is eaten and transformed into animal fat. 



The Metabolism of Carbohydrates. The sugar of the 

 blood is usually called dextrose or glucose. As a matter of 

 fact, it cannot be strictly correct to speak of a single sugar 

 in the plasma, for there are probably three at least. 

 Glucose, however, is undoubtedly the principal one, and 

 so far as we know the possible services to the body are prac- 

 tically the same for all. Reference has already been made 

 to the fact that the quantity of sugar in the blood is small, 

 but singularly constant. It is now time to explain how this 

 constancy is maintained. 



It has been stated that the body contains relatively 

 little carbohydrate in spite of its large supply. When it 

 is considered that the entire volume of blood contains less 

 than 10 grams of sugar, though the amount absorbed after 

 a single meal may be as much as 100 grams, it appears 

 strange that there should be, as a rule, no significant in- 

 crease in the percentage circulating during the period of 

 digestion. The solution of this problem was achieved in 

 great measure by the French physiologist Bernard, near 

 the middle of the last century. Knowing that the incom- 

 ing sugar passes to the liver, he anticipated that this organ 

 might have the power to take the surplus from the passing 

 stream and store it temporarily in some form. 



Glycogen. Investigation showed that there could be 

 obtained from the liver of a well-fed animal (rabbit) con- 

 siderable quantities of a carbohydrate resembling starch. 

 This substance is called glycogen. Its presence within the 

 cells of the liver can be demonstrated in microscopic prep- 

 arations. Its molecule is of unknown size, and it is 

 capable of undergoing digestion in the same manner as 

 vegetable starch with the formation of sugar. The amount 

 may be strikingly large, reaching, in the rabbit, one-fourth 

 the total weight of the liver, deducting the weight of the 

 blood usually contained in it. In the human liver it does 

 not attain to such a high percentage, but may still equal 

 something like 10 per cent, of the net weight of the organ. 



