THE METABOLISM OF CARBOHYDRATE 427 



ing to one view the least accepted sugar is broken down by the body 

 in the same way as it is by the yeast cell that is, through inter- 

 mediary stages to alcohol, and then to carbon dioxide and water. 

 There is some proof of this taking place in plants, but the evidence 

 for such a katabolism in the animal tissues is scanty. 



According to a second view, dextrose breaks down to glycuronic 

 acid, and then to carbon dioxide and water. Until recently, this 

 view has received wide acceptance. Glycuronic acid is closely related 

 to dextrose, and is found in the blood and in the urine. 



All the recent evidence, however, tends to show that dextrose is 

 normally broken down to lactic acid, and then to carbon dioxide and 

 water. The muscles are the chief seat of dextrose katabolism. In 

 the absence of oxygen, lactic acid is not broken down, and may be 

 detected in the urine and in the sweat e.g., after hard muscular 

 exercise, or when an excised muscle is tetanized in the absence of 

 oxygen. 



(1) Dextrose (2) Dextrose (3) Dextrose 



Alcohol Glycuronic acid Lactic acid 



CO, H 2 C0 2 H 2 CO 2 H 2 



We must now inquire with a little more detail into (1) the glyco- 

 genic function of the liver ; (2) the conditions necessary for the breaking 

 down of sugar in the body. 



The Glycogenic Function. By this function, the supply of dextrose 

 in the blood is regulated, and kept at a constant amount (0-01 per 

 cent.) adequate to the needs of the body. When the supply of sugar 

 in the blood is large, as at the height of digestion, the dextrose is 

 abstracted and stored as glycogen; when the supply of sugar in the 

 blood is poor, then glycogen is converted into dextrose, and passes 

 into the blood. Normally, the liver restrains any surplus supply 

 of dextrose from reaching the systemic blood, and causing what is 

 termed a hyperglyccemia. When hyperglycsemia results, the kidneys 

 immediately eliminate the excess sugar, and dextrose appears in the 

 urine, forming the condition known as glycosuria. The efficiency with 

 which the liver performs this function is judged by the sugar 

 content of the blood and urine. 



As a general rule, the urine only is tested, but in special conditions 

 the sugar content of the blood is also tested, for a small excess of sugar 

 may exist in the blood without causing glycosuria. Moreover, all 

 glycosurias are not due to inefficiency of the liver and hyperglycaemia . 

 To estimate the amount of dextrose, the blood is laked and diluted, 

 shaken with colloidal ferric hydroxide (dialyzed iron), and then a 

 small amount of sodium sulphate added. By this means the blood - 

 proteins are precipitated. The sugar in the filtrate may then be 

 estimated by Bertrand's process (see p. 468). 



The Sources of Glycogen Carbohydrate. That glycogen is largely 

 formed from dextrose may be experimentally proved by starving 

 a rabbit, and subsequently feeding it with carrots a diet rich in 



