THE SECRETION OF URINE 



1193 



arteries leaving the aorta enter the kidney and supply the whole of the 



glomeruli, the vasa efferentia from which pass, as in the mammalian kidney, 



into the intertubular capillaries. These are also supplied with blood of 



venous character by the renal portal vein. If all the renal arteries be divided 



or ligatured, the glomeruli, as was shown by Nussbaum, are entirely cut out 



of the circulation, though the tubules continue to receive venous blood 



through the renal portal vein. Nussbaum stated that ligature of all the 



renal arteries caused cessation of the urinary secretion, which could be 



reinduced by injection of urea. 



He concluded that urea with 



water was secreted by the 



tubules, whereas peptone, sugar, 



and haemoglobin were turned 



out by the glomeruli. Beddard 



showed that these results of 



Nussbaum must have been due 



to the fact that he had not 



obstructed the whole of the renal 



arteries. One or two of these 



small vessels will suffice to 



supply blood to a considerable 



number of the glomeruli. After 



complete obstruction of the 



Test-is 

 kidney 



Renal porhal 



Anr.abdom.v- 



-Aorte 



Vena cava 



Renal arteries 



The vascular supply to the kidney 

 in the frog. 



-Femoral v. 

 FIG 546. 

 arteries, no urinary flow could be 



induced even with subcutaneous 



injection of urea. But the cutting off of the arterial blood supply from the 

 tubules caused a rapid destruction of the tubular epithelium, so that the 

 result of the experiment could not be taken as negativing the possibility of 

 this epithelium having, when in a normal state of nutrition, some secretory 

 power. He therefore carried out, with Bainbridge, another series of ex- 

 periments of the same description, in which the frogs, after ligature of the 

 renal arteries, were kept in an atmosphere of pure oxygen. Under these 

 circumstances sufficient oxygen diffused into the blood of the renal portal 

 vein to maintain an adequate supply of this gas to the tubules. No desquam- 

 ation of the epithelium resulted, and injection of urea produced a small 

 flow of urine even when, by subsequent injection of the blood vesselsj it was 

 proved that every glomerulus had been cut out of the circulation. 



In the cells of the convoluted tubules various kinds of granules and of vatfuoles may 

 be distinguished. Gurwitsch divides these vacuoles into three classes; 



(1) Large granules staining densely with osmic acid, and probably rich in lecithin. 



(2) Smaller very numerous granules consisting of some form of protein material. 



(3) Large vacuoles lying close to the free margins of the cells, whose contents do 

 not undergo coagulation with the ordinary fixing reagents, and therefore are free from 

 protein, fat, or mucin. These vacuoles are especially marked in kidneys which are 

 secreting at a great rate, in consequence of the injection of saline diuretics or of large 

 quantities of normal salt solution. They have been regarded as excretory vacuoles, and 

 as containing water or saline fluids which have been collected by the cells and are 

 being passed on by them to the lumen of the tubules. 



