646 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



increased and filtration through the kidneys facilitated. As a conse- 

 quence, the urine in cold weather is of low specific gravity from the large 

 amount of water forced through the glomeruli. In warm weather, on the 

 other hand, the capillaries of the skin are relaxed, general blood pressure 

 is, therefore, reduced and filtration to that extent interfered with, and 

 the urine is now scanty and of a higher specific gravity from the decrease 

 in its percentage of water. 



Filtration is not, however, the only process concerned in the forma- 

 tion of urine. This is evident from the examination of the composition 

 of urine ; for if it were merely a filtrate from the blood it could con- 

 tain no soluble constituent in greater proportion than it exists in the 

 blood; for a solution cannot by filtration through a moist animal mem- 

 brane become more concentrated. Yet the urine contains many sub- 

 stances, especially urea, in much larger amount than in the blood, and it 

 must, therefore, be assumed that the fluid which is removed from the 

 blood by filtration through the glomeruli differs in important respects 

 from the urine. It must be almost identical with transudations from 

 the blood in other localities. It is, however, probable that it is free from 

 albumen. For the fluids which leave the blood must traverse not only 

 the walls of the capillaries of the glomeruli, but also the walls of the 

 capsule, and it is well known that such relatively thick membranes offer 

 difficulty to the filtration of albumen. 



It is well known that normal urine is free from albumen, and this is to 

 be explained either by the statement above mentioned, or by the assump- 

 tion that albumen does pass through the glomeruli into the interior of 

 the urinary tubules, but is again absorbed by the epithelial cells lining the 

 looped portion of the tubules. This, however, is perhaps supported by the 

 fact that in kidney diseases, where the epithelium of the tubules is 

 diseased or absent, and absorption, presumably, thus interfered with, 

 albumen is then constantly found in the urine. 



It is readily conceivable that various constituents may be added to 

 the urine in its passage through the different portions of the uriniferous 

 tubules. That such is the case is rendered probable by the histological 

 structure of the kidney. The fluid removed by the glomeruli from the 

 blood passes through a series of convoluted tubules lined with epithelial 

 cells similar to those found in other secreting organs and surrounded by 

 a second net-work of capillaries. The epithelial cells lining the tubules 

 may be regarded as specific secretory cells which are concerned in 

 removing the specific constituents of the urine from the blood. That 

 they are capable of removing substances from the blood circulating in 

 the capillaries may be demonstrated by the injection into the blood-cur- 

 rent of indigo carmine, after previous section of the spinal cord, thus 

 preventing the formation of the urinary secretion by filtration. If ani- 



