METABOLISM 521 



part into a form of sugar, termed glycose, glucose, or dextrose, the chemic 

 formula for which is CeH^Oe- Under this form 1 the carbohydrate mate- 

 rial is absorbed by the epithelial cells covering the villi, and by them trans- 

 mitted to the blood in their contained capillary blood-vessels. It is then 

 transported by the blood stream, by way of the portal vein, to the liver. 

 That this is the case is shown by the results of chemic analysis. Thus the 

 blood obtained from the portal vein after the ingestion of a meal rich in 

 sugar, shows a higher percentage of sugar than the blood obtained from 

 any other portion of the circulatory apparatus. 



On reaching the liver a large percentage of the sugar will in the course 

 of the day be carried onward into the blood of the general circulation, and 

 in a short time be delivered to the capillaries of the muscle tissues. On 

 reaching this situation, the sugar being a highly diffusible substance will 

 pass across the capillary wall into the surrounding lymph spaces, thence 

 across the sarcolemma into the interior of the muscle cell, where it comes 

 into direct contact with the cell material. Here through the action of an 

 intracellular enzyme, the sugar is deprived of a molecule of water and is 

 thereby transformed into a non-diffusible substance which resembles in its 

 chemic relations vegetable starch. Under this form the carbohydrate 

 material in part is retained until the necessity arises for its release and 

 oxidation. 



It is generally admitted that though starch is the source of much of 

 the energy expended by the body, it can not be disrupted and oxidized as 

 such, but that it must first be transformed into sugar, glycose or glucose, 

 and for this purpose the assumption is made that a special intracellular 

 enzyme is present and active. Inasmuch as the stored starch is subse- 

 quently transformed into glucose it has been termed glycogen, a generator 

 of sugar. For this reason the muscle may be said to have a starch-forming 

 or an amylogenic or a glycogenic function, and a sugar-forming or a 

 glycogenetic function. 



The .muscle tissue is therefore characterized by two closely related 

 processes, amylo genesis and glyco genesis. 



Muscle Glycogen. Chemic analysis has shown that muscles contain 

 normally from 0.5 to i.o per cent, of glycogen, and as these organs con- 

 stitute about 40 per cent. (28 kgm.) of the body weight (70 kgm.), they 

 collectively generally contain from 140 to 280 grams (5 to 10 ounces) of 

 glycogen. As chemic analysis has failed to detect the presence of glyco- 

 gen in the blood the inference is that it arises in the muscle cell itself in the 

 manner already indicated. From these facts it is apparent that the 

 muscles are organs in which carbohydrate material is stored in very con- 

 siderable quantity, and hence it is readily accessible when the need for 

 energy is pressing. 



During the periods of prolonged activity of the muscles the percentage 

 of glycogen rapidly diminishes, a fact that leads to the inference that it is 

 the source in large part of the energy expended by the muscle. During the 

 period of rest the percentage of glycogen rapidly increases until the normal 

 is regained. 



1 Other sugars such as levulose and galactose, formed to some extent during digestion, 

 are left out of consideration. They probably undergo after absorption the same transforma- 

 tion that glucose does. 



