THE METABOLISM OF CARBOHYDRATES. 117 



The reason why the sugar in the systemic blood tends to fall is 

 that the tissues, especially the muscles, are using it up as fuel. 

 If so much sugar is taken that the storage capacity of the liver 

 is overstepped, the excess of sugar is carried by the systemic 

 blood to the tissues, where much of it may be changed into fat. 

 The glycogenic function of the liver, as the above process is 

 called, is analogous to the starch-forming function of many 

 plants, such as potatoes. Of the sugar which is formed in the 

 green leaves, some is immediately used for building up other 

 substances, the remainder being converted into starch, which be- 

 comes deposited in the roots, etc., until it is required (as during 

 the second year's growth), when it is gradually reconverted into 

 sugar. 



Besides carbohydrates it is known that proteins form glyco- 

 gen; fats, however, cannot form it. In severe cases of diabetes 

 it is therefore usual to find that although carbohydrate foods 

 are entirely withheld, dextrose continues to be eliminated in the 

 urine. It may come partly from the protein of the food and 

 partly from that of the tissues. 



The adjustment between the rate at which the glycogen of the 

 liver becomes converted into dextrose and the percentage of 

 sugar in the systemic blood is effected partly through the nervous 

 system and partly by means of substances called chemical mes- 

 sengers or hormones (see p. 124) secreted into the blood from the 

 ductless glands, such as the pancreas and the adrenals. The 

 very first symptoms of diabetes, which we have seen, consist in 

 an excessive postprandial rise in the systemic blood-sugar and a 

 consequent glycosuria, must therefore be due to defects in one 

 or other of these regulatory mechanisms, so that it is of great 

 interest to know that glycosuria can be induced in the lower 

 animals by stimulation of the nerves of the liver or by interfer- 

 ing with the function of the pancreas or the adrenal glands. The 

 nerves of the liver may be stimulated either directly or through 

 ;i nerve center located in the medulla oblongata (see p. 246). 

 Complete removal of the pancreas is followed in a few hours 

 by a very acute form of diabetes, which is invariably fatal in a 

 few weeks, whatever the treatment may be. Injection of extract 



