804 PHYSIOLOGY 



Confirmatory evidence of this function of the liver was supplied by 

 Schroder's experiments on birds. In these animals the chief nitrogenous 

 excretion is not urea, but ammonium urate, 60 per cent, of the nitrogen of 

 the semi-solid urine appearing in the form of uric acid. In birds there is 

 naturally a communication between the portal system and the general venous 

 system by means of the vein of Jacobson, which connects the lower branches 

 of the portal vein with/as a rule, the left renal vein (Fig. 365) . On this account 

 the liver can be cut out of the body or of the circulation without entailing 

 the rapid death of the bird, which may live for three or four days, and pass 

 urine after the operation. The urine is however fluid, and the uric acid, 

 instead of accounting for 60 per cent, of the total nitrogen, now forms only 



I- Inf. Vena Cava 



Iliac vein- 

 Kidney- 



V. of Jacobson 

 Inf. mes.v. 



Caudal v. 



Rectum 



FIG. 365. Diagram to show the arrangement of the veins in the bird, 

 with the communication of the renal and portal veins. (After 

 MORAT.) 



5 per cent. The place of the greater part of the uric acid has been taken 

 by ammonium lactate, which therefore seems to be the chief immediate 

 precursor of the uric acid in the urine of birds. We shall have occasion 

 to consider the method of transformation of ammonium lactate to uric 

 acid more fully when dealing with the origin of the latter body. 



It is by no means easy to perform similar experiments in mammals, 

 since it is difficult to* cut off the flow of blood through' the liver without 

 altogether stopping the circulation through the abdominal viscera. Ligature 

 of the portal vein, which would be a necessary step in the extirpation of the 

 liver, causes the blood to be dammed up behind the ligature in the portal 

 area. The intestinal wall gets full of effused blood, the blood pressure falls 

 steadily, and the animal dies within a few hours, being bled to death, so to 

 speak, into its portal vessels. A way of obviating this difficulty was sug- 

 gested by a Russian surgeon, Eck, and was successfully carried out by Pawlow. 

 Before ligature of the portal vein, this vessel was joined to the vena cava 



