THE METABOLISM OF THE CARBOHYDRATES 



691 



It will be observed that, so far as can be judged from changes in the concentration 

 of sugar in the blood, the sugar-retaining power of the liver is about equal to that of 

 the muscles. One objection which may properly be raised to these observations is 

 that the animals on which they were made were under anesthesia, and that the anesthetic 

 may have* had a paralyzing effect on the sugar-retaining power of the liver. In view of 

 this criticism it is important to examine the results obtained on animals that are not 

 under the influence of anesthesia. By collecting blood from the ear veins of rabbits, it 

 has been found, after giving from two to ten grams of glucose by stomach, that the 

 glucose concentration of the systemic blood begins to vise in fifteen minutes, attaining a 

 maximum in about an hour and then returning to the normal level in about three hours. 



Similar results have been obtained by examination of the venous blood 

 in man. After giving 100 grams of glucose by mouth, for example, there 

 is commonly an increase in blood sugar amounting to from 30 to 34 per 

 cent of the normal and lasting for from one to four hours. The existence 

 of this postprandial hyperglycemia, as we may call it, indicates that the 



/ / / 



Fig. 190. Curves showing the percentage of glucose in blood after a constant injection of 

 an 18 per cent solution into a mesenteric vein. V.C., vena cava, continuous line; P.O., pan- 

 creaticoduodenal vein, broken line; I, iliac, dotted line. 



sugar-retaining powers of the liver and muscles are not sufficiently de- 

 veloped to prevent the accumulation of some of the absorbed sugar in the 

 systemic blood. Whenever this increase exceeds a certain limit, some of 

 the sugar begins to escape through the kidney into the urine, producing 

 glycosuria postprandial glycosuria. The concentration to which blood 

 sugar must rise before glycosuria occurs in the case of man is about 

 0.17 gm. per cent. After damage to the kidney, as in nephritis, or 

 in long-standing cases of mild diabetes, the percentage may probably 

 rise considerably higher in the blood without evidence of glycosuria. 



Value of Blood Examination.!!! Diagnosis of Diabetes. The determina- 

 tion of the amount of ingested carbohydrate required to bring about post- 

 prandial glycosuria constitutes, as we have already seen, the so-called 



