360 VETERINARY PHYSIOLOGY 



Urea is chiefly formed in the Liver. — This is indicated — 

 (1) By the fact tliat when an ammonium salt, such as the 

 carbonate, dissolved in blood, is streamed through the organ, 

 it is changed to urea ; (2) by the evidence that the liver 

 stores the surplus amino-acids, and that, as the}' again 

 disappear from the liver, urea increases in the blood ; (3) by 

 the observation that, when the liver is cut out of the 

 circulation, the urea in the urine rapidly diminishes, and 

 ammonia and lactic acid take its place. 



The exclusion of the liver from the circulation in 

 mammals is difficult, because, when the portal vein is 

 ligatured, the blood returning to the heart tends to accumu- 

 late in the great veins of the abdomen. But this difficulty 

 has been overcome by Eck, who devised a method of connect- 

 ing the portal vein with the inferior vena cava, and afterwards 

 occluding the portal vein, and of thus allowing the blood to 

 return from the abdomen to the heart without passing 

 through the liver. 



That it is not produced in the kidneys was first shown by 

 the French chemist Dumas. He found that when these 

 organs are excised, urea accumulates in the blood. Later 

 investigators found that when ammonium carbonate is 

 added to blood artificially circulated through the kidney of 

 an animal just killed, no urea is formed. 



That it is not formed to any marked extent in the muscles 

 is shown — (1) By the absence of a definite increase in urea 

 formation during muscular activity ; (2) by the fact that 

 when the blood, containing ammonium carbonate, is streamed 

 through the muscles, urea is not produced. 



The Sources of Urea. — (1) The source of urea from the 

 amino-acids formed in the digestion of proteins in the food 

 has already been discussed (p. 349). (2) But urea is also 

 formed during starvation, and it must therefore be derived 

 from the proteins of the tissues. It has been found that 

 in starvation there is an increase of the amino-acids in such 

 tissues as muscle, and it would thus seem that they are pro- 

 ducts of the disintegration of the muscle proteins, and that 

 they are carried to the liver to be converted to urea. 



The fate of haemoglobin tends to show that the whole 



