522 TEXT-BOOK OF PHYSIOLOGY 



Liver Glycogen. Coincidently with the storage of sugar in the 

 muscles a certain portion (10 to 20 per cent.) of the sugar absorbed from 

 the intestine is stored in the liver. As the blood flows through the capil- 

 laries of the liver, some of the sugar passes across the capillary wall into 

 the surrounding lymph spaces, where it comes into direct contact with 

 the liver cells. Here also, through the agency of an intracellular enzyme, 

 the sugar is dehydrated (deprived of a molecule of water), converted into 

 starch, and stored in the liver cells, where it presents itself in the form of 

 hyaline masses which can be readily seen with the aid of the microscope. 

 Inasmuch as this starch is subsequently transformed into sugar (glu- 

 cose) it, too, has been termed glycogen. For this reason the liver may be 

 said to possess a glycogen-forming or a glyco genie function. Chemic 

 analysis has shown that the liver contains glycogen to the extent of from 

 1.5 to 4 per cent, of its entire weight, and as this organ weighs on an average 

 about 1600 grams it contains from 24 to 64 grams of glycogen. On a diet 

 rich in carbohydrates the glycogen may be increased to 10 per cent. The 

 liver glycogen thus represents an excess of carbohydrate material not 

 immediately needed for oxidation purposes. See page 490. 



Though the muscles and the liver are the main organs concerned in 

 the storage of sugar, it is highly probable that many other organs are 

 engaged in the same process, especially those in which growth and nutri- 

 tion are active. 



Blood Sugar. The blood plasma also contains a portion of the ab- 

 sorbed sugar under the form of glucose, and is to be regarded as one of its 

 normal constituents. The quantity present varies from o.io to 0.15 

 per cent. To this condition of the blood the term glycemia has been given. 

 If the statement be accepted that the amount of blood in a body weighing 

 70 kilos is 3684 grams (one-nineteenth of the body weight), the amount of 

 sugar in the entire volume is at most about 5.5 grams. 



The storage or the assimilation of sugar is influenced to some extent 

 by the activity of the posterior lobe of the pituitary body (see page 502) . 

 Thus it has been shown by Gushing that mechanical and electric stimula- 

 tion of the posterior lobe is followed by glycosuria, the amount of sugar 

 excreted depending on the amount of glycogen in the liver. This is 

 attributed to a discharge, in amounts greater than normal, of its internal 

 secretion into the cerebrospinal fluid and subsequently into the blood. 

 The effect of the excess of the secretion is to lower the normal assimilating 

 power or storage capacity, and hence leads to an insufficient oxidation and 

 elimination. If the stalk of the posterior lobe be clamped so as to prevent 

 the discharge of the secretion the assimilation capacity rises. As a 

 result of this procedure a condition of nutrition is established, charac- 

 terized by a general deposition of fat, suggesting a conversion of the sugar 

 into fat. The assimilation of sugar is apparently regulated by the pos- 

 terior lobe of the pituitary, since it is lowered by a hypersecretion and 

 raised by a hyposecretion. The manner in which this is brought about is, 

 however, unknown. 



The assimilation of sugar is apparently influenced in a similar manner 

 by the parathyroids (page 498). Thus when three of the four bodies are 

 removed the tolerance for sugar is diminished, as shown by the appearance 

 of glycosuria. The thyroids appear to be without influence in this relation. 



