XIX. 



THE URINARY TRACT. 



THE essential parts of the urinary tract are those portions of 

 the kidneys which secrete the urine, while all other mem- 

 branous tubules and sacs connected with them i. e., the pelves, 

 the ureters, the bladder, and the urethra are merely a collecting 

 and conducting apparatus. The investigations of R. Heidenhain 

 have corroborated the views of Bowman that, from the capillary 

 blood-vessels of the tuft, which carry arterial blood, only a 

 watery liquid, destitute of salts, is exuding into the capsular 

 space, while the saline constituents of the urine are the excretory 

 product of those uriniferous tubules which are richly supplied 

 with capillary blood-vessels, and whose epithelia are closely con- 

 nected with the walls of the vessels. The inspissated blood con- 

 tained in these vessels reabsorbs a portion of the liquid from 

 the tubule and supplies the liquid in the tubules with a certain 

 amount of its salts. Obviously, the whole process is accom- 

 plished through the agency of the living epithelia, and is not to 

 be considered as a simple process of osmosis. The differences in 

 the structure of the epithelia in certain portions of the urinifer- 

 ous tubules, and the striking differences in the distribution of 

 the blood-vessels, strongly point toward a difference in the func- 

 tion of portions of the tubules ; but no exact demonstration of 

 these functions, concerning the constituent elements of urine, 

 has as yet been furnished. The Greek denomination, " dialysis," 

 serves to conceal our lack of knowledge of the process concerned 

 in urinary excretion in about the same manner that the word 

 " catalysis " disguises our ignorance of the process of digestion 



