444 THE HUMAN BODY 



percentage, about 0.15 per cent, of sugar, whether the sugar con- 

 tent of the portal vein is high or low. 



It is evident that the liver must be able to store within itself the 

 excess sugar that comes to it during active absorption from the 

 intestine, and to give this out again between times. The sugar 

 is retained in the liver, not as such, but in the form of glycogen or 

 animal starch. The conversion of sugar into glycogen is a simple 

 dehydration (C 6 H 12 O 6 H 2 O=C 6 H 10 O 5 ), and is doubtless easily ef- 

 fected by the liver-cells. The purpose of the change from sugar 

 to starch seems to be to make the retention by the liver easier; 

 sugar is too soluble to be held readily, whereas the liver can hold 

 the glycogen without trouble. The liver is said to be able to hold 

 10 per cent of its weight of glycogen. 



The use of the sugar is, as we have already seen, for fuel for the 

 Body. Oxidations are constantly going on in the living-tissues, 

 therefore there is a steady withdrawal of sugar from the blood, 

 and the liver must be continually making good the depletion by 

 reconverting some of its glycogen into sugar. That the sugar 

 content of the blood is kept up at the expense of liver-glycogen is 

 proven by observations on fasting animals. A comparatively 

 short period of starvation results in the complete disappearance 

 of glycogen from the liver. That in fact is the first fuel supply to 

 be drawn upon in the absence of food. 



Just how the chemical process of converting glycogen to sugar 

 is performed is not certain; although an enzym capable of effect- 

 ing the transformation is said to be present in the liver. If the 

 process is carried on by an enzym it is under closer control than 

 the enzym reactions we have studied in connection with diges- 

 tion, for it does not go on rapidly till all the glycogen is used up, 

 but only so fast as is necessary to make good the loss of sugar from 

 the blood. 



Storage of Glycogen in the Muscles. These organs, as we learned 

 when studying them (Chap. VII), perform their work through the 

 oxidation of sugar, and since they are likely to be called upon for 

 prolonged activity need to have immediately available a supply of 

 their special fuel. Such a supply they have, in the form of glyco- 

 gen, which makes up about 1 per cent of the weight of muscle 

 tissue. This glycogen is, of course, derived from the sugar of the 

 blood, so that the muscle-cells must have the same power that 



