EXTERNAL SECRETIONS 489 



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 C0 3 - 2 H 2 = CON 2 H 4 . 



Destructive diseases of the liver e.g., acute yellow atrophy, suppuration, 

 cirrhosis largely diminish the production of urea, but increase the quanti- 

 ties of the ammonium salts in the urine. The same is true when the liver 

 cells are destroyed during acute phosphorus poisoning. 



The Conjugation of Products of Protein Putrefaction. One of the 

 important functions of the liver is the conversion of toxic compounds, the 

 products of the putrefaction of proteins, into non-toxic compounds. These 

 compounds are formed in the intestine, are absorbed and carried by the blood 

 of the portal vein to the liver. In their passage through the capillaries of 

 the liver they are conjugated for the most part with potassium sulphate by 

 the action of the liver cells and thus deprived of their toxicity. Among the 

 substances thus conjugated are indol, skatol, phenol, and cresol. After 

 absorption indol and skatol are oxidized to indoxyl and skatoxyl and then 

 combined with potassium sulphate giving rise to potassium mdoxyl sul- 

 phate and potassium skatoxyl sulphate. Phenol and cresol are apparently 

 directly combined with potassium sulphate. All of these compounds then 

 pass into the blood of the general circulation and finally are eliminated by 

 the kidneys. Potassium indoxyl sulphate or indican is the source of the 

 indigo-forming substance found in the urine. Other compounds ^like- 

 wise reduced in toxicity by the liver cells though the methods by which tl 

 is accomplished vary with the nature of the compound. The liver t 

 presents a chemic defense against the entrance of more or less toxic agents 

 into the blood of the general circulation. 



