492 TEXT-BOOK OF PHYSIOLOGY 



ever, through oxidation (glycolysis) it yields heat and contributes to the 

 production of muscle energy. Should there be a failure on the part of the 

 liver cells to store up its usual percentage of the absorbed sugar, 10 to 20 

 per cent., by reason of impaired nutrition, disturbance of the portal circu- 

 lation, or a larger excess of sugar in the blood of the portal vein, it would 

 pass through the liver into the blood of the general circulation and increase 

 the percentage amount of sugar above the normal (o.i to 0.15 per cent.) 

 establishing the condition of hyperglycemia. This would soon be followed 

 by its elimination from the blood by the kidneys and its appearance in 

 the urine, giving rise to a glycosuria. 



In opposition to this view, Dr. Pavy, after years of accurate experimentation, 

 states that the blood on the cardiac side of the liver never under normal circum- 

 stances contains a larger percentage of sugar than is to be found in any part of 

 the circulation, except in the portal vein. He states that glycogen is never re- 

 converted into sugar, and denies that the liver produces sugar, to be discharged 

 into the blood; the function of the liver is merely to arrest the passage of sugar, 

 and so to shield the general circulation from an excess; the sugar which arises 

 in the liver after death is a post-mortem product and not an illustration of what 

 takes place during life. Dr. Pavy, having apparently demonstrated the glucosid 

 constitution of protein material in general, accounts for the presence of glycogen 

 in muscles and other tissues on the assumption that during the cleavage of the 

 protein molecule the carbohydrate element is set free and temporarily stored as 

 glycogen. He thus accounts for the production of sugar in the body, even in the 

 absence of all sugar and starch from the food. Pavy believes that the glycogen 

 produced in the liver is utilized in the formation of fat and the synthesis of complex 

 proteins necessary to the construction of the tissues. 



The Formation of Urea. It is now generally believed that the liver 

 is the most active of all the organs which may be engaged in the production 

 of urea. This belief is based on numerous physiologic and pathologic 

 data. The compounds out of which the hepatic cells construct urea have 

 been for chemic reasons asserted to be the ammonium salts, e.g., the car- 

 bonate, carbamate, and lactate, which are constantly present in the blood. 

 These salts, which result from protein metabolism, may be absorbed from 

 the tissues or from the intestines, carried to the liver, and there synthesized to 

 urea. This supposition is supported by an experiment as follows: The 

 liver of an animal recently living is removed from the body and its vessels 

 perfused continuously with blood (the urea content of which is known) 

 containing the ammonium salts. An analysis of this blood shows, after a 

 time, a diminution of these salts, and a large increase in the amount of the 

 urea. After the establishment of an Eck fistula (the union of the portal 

 vein with the ascending vena cava whereby the liver is largely excluded from 

 acting on products absorbed from the intestines) there is a marked diminu- 

 tion in the production of urea while the ammonia content of the urine 

 largely increases. One large source for the ammonium which is trans- 

 formed into urea by the liver cells, is that split off from the protein molecule 

 during digestion by the action of the gastric and pancreatic juices. The 

 ammonia is then absorbed and combined with carbon dioxid with formation 

 of ammonium carbonate. When this compound is transported to the liver 

 by the portal blood, the cells convert it into urea in a manner shown in the 

 following formula: 



(NH 4 ) 2 CO 3 - 2H 2 O = CON 2 H 4 . 



