EXCRETION OF WASTE PRODUCTS 179 



scopic examination after injecting the blood vessels he showed 

 that the blood supply of all the glomeruli was cut off. ;With 

 such preparations he showed that diuresis resulted after the 

 injection of urea, therefore the tubules which are supplied by 

 the renal-portal system must be able to form urine ; that is, 

 the tubules excrete even when the glomeruli are deprived of 

 their circulation.* This has been confirmed by later experi- 

 ments and the only possible fallacy is that some glomerular 

 nitration can occur by back pressure from the anastomosis 

 with the renal-portal system. 



FIG. 41. Diagram of circulation in frog's kidney. 



The arterial blood supply (A) enters the glomerulus (G), from which it passes to the tubule 

 (T), where it mixes with the venous blood of the renal-portal vein (P), and finally leaves by 

 the renal vein (V). 



In the mammalian kidney the circulation is the same, with the exception that there is no 

 renal-portal vein to join with the capillaries surrounding the tubule. 



(Copied from " The Secretion of Urine," Cushny. Longmans.) 



COMPOSITION OF URINE 



The composition of the excretion of the kidney varies 

 according to the preponderance of the processes of nitration 

 and secretion. It varies in different species and at different 

 times in the same animal. The substances found in the urine 

 indicate the end products of metabolism. 



Birds and reptiles furnish a solid excretion. This may 

 possibly be due to economy of water. Reptiles frequently 

 live in dry places where there is not a large supply of water. 

 Less water means less weight for birds to carry when flying. 

 We also find that the substances excreted by birds and reptiles 

 are relatively insoluble in water, whilst in mammals where a 

 fluid excretion is the rule water-soluble substances predomi- 

 nate. The presence of insoluble substances is a great advantage 

 * M. Nussbaum, Arch. /. d. ges. Physiol., 1878, vol. 16, p. 139. 



