THE SECRETION OF URINE 



1145 



tion of the 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 



body 



\&~,-Aorta 



Vena cava 

 Renal arteries 



Tesl-is 



Kidney 



Renal portal 



Anr. abdom.v- 



- Femoral v. 

 FIG. 535. The vascular supply to the kidney in the frog. 



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-vessels, it was 

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



The view that a portion of the tubules is secretory in function is further supported 

 by histological examination of these structures under various conditions of activity. 

 In the cells of the convoluted tubules various kinds of granules and of vacuoles 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 may be regarded as excretory vacuoles, and 

 consist of water or saline fluids which have been collected by the cells and are being passed 

 on by them to the lumen of the tubules. 



As a rule it is impossible to trace any definite constituent of the urine 

 on its way through the cells of the tubules. But if a solution of uric acid in 

 piperazin be injected intravenously into a rabbit, the kidneys, taken twenty 

 to sixty minutes after the injection, present tubules full of uric acid con- 

 cretions. In the medullary portion of the kidney this uric acid precipitate is 

 confined to the lumen of the tubules, but in the convoluted tubules granules 



