THE ABSORPTION AND USE OF FOODS 493 



sugars from the intestinal cavity into the blood-capillaries of the 

 villi. These capillaries all drain, as previously stated (Chap. XIX), 

 into the portal vein, which in turn passes to the liver and breaks up 

 therein into the liver-capillaries (Chap. XXVI) ; so that all blood 

 from the intestine, with whatever it may have taken up there, is 

 forced to traverse the liver, and to come into intimate contact with 

 the liver-cells, before it reaches any of the other living tissues of 

 the Body. 



The amount of sugar present in the blood of the portal vein is, of 

 course, variable, there being a higher concentration at times when 

 sugar is being actively absorbed from the intestine than at other 

 times. Curiously, the blood flowing away from the liver, in the 

 hepatic vein, is always found, normally, to contain a certain small 

 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 Hi 2 O 6 H 2 O = C 6 Hi O & ), 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 



