420 THE SECRETION OF URINE. 



warty, hard "mulberry calculi") is not affected by acetic acid, is dissolved by mineral acids 

 without effervescence, and again precipitated by ammonia. Heated on platinum foil it chars 

 and blackens, then it becomes white, owing to the formation of calcic carbonate, which 

 effervesces on the addition of an acid. 



(c) Calcic carbonate (chiefly in whitish-grey, earthy, chalk-like calculi, somewhat rare) 

 dissolves'with effervescence in hydrochloric acid. When burned it first becomes black, owing 

 to admixture with mucus, and then white. 



(d) Ammonio-magnesic phosphate and basic calcic phosphate usually occur together in soft, 

 white, earthy stones, which occasionally are very large. These stones show that the urine has 

 been ammoniacal for a very long time. The first substance when heated gives the odour of 

 ammonia, which is more distinct when heated with caustic potash ; is soluble in acetic acid 

 without effervescence, and is again precipitated in a crystalline form from this solution on the 

 addition of ammonia. When heated it fuses into a white enamel-like mass ; [hence, it is called 

 "fusible calculus"]. Basic calcic phosphate does not effervesce with acids. The solution in 

 hydrochloric acid is precipitated by ammonia. When ammonium oxalate is added to the acetic 

 acid solution, it yields calcic oxalate. 



(c) Neutral calcic phosphate is rare in calculi, while it is frequent in the form of gravel. 

 Physically and chemically, these concretions resemble the earthy phosphates, only they do not 

 contain magnesia. 



273. THE SECRETION OF URINE. [The functions of the kidney are 



1. To excrete waste products, chiefly nitrogenous bodies and salts; 



2. To excrete water ; 



3. And perhaps also to reabsorb water from the uriniferous tubules, after it has 

 washed out the waste products from the renal epithelium. 



The chief parts of the organs concerned in 1, are the epithelial cells of the con- 

 voluted tubules ; the glomeruli permit water and some solids to pass through them, 

 while the constrictions of the tubules may prevent the too rapid outflow of water, 

 and thus enable part of it to be reabsorbed.] 



Theories. The two chief older theories regarding the secretion of urine are the 

 following : 1. According to Bowman (1842), through the glomeruli are filtered 

 only the water and some of the highly diffusible and soluble salts present in the 

 blood, while the specific urinary constituents are secreted by the activity of the 

 epithelium of the urinary tubules, and are extracted or removed from the epithelium 

 by the water flowing along the tubules. This has been called the " vital " theory. 

 2. C. Ludwig (1 844) assumes that very dilute urine is secreted or filtered through the 

 glomerulus. As it passes along the urinary tubules it becomes more concentrated, 

 owing to endosmosis. It gives back some of its water to the blood and lymph of 

 the kidney, thus becoming more concentrated, and assuming its normal character. 

 [This is commonly known as the " mechanical " theory.] 



The secretion of urine in the kidneys does not depend upon definite physical 

 forces only. A great number of facts force us to conclude that the vital activity 

 of certain secretory cells plays a foremost part in the process of secretion (R. 

 Heidetihain). 



The secretion of urine embraces (1) The water, and (2) the urinary constitu- 

 ents therein dissolved j both together form the urinary secretion. The amount of 

 urine depends chiefly upon the amount of water which is filtered through or 

 secreted by the glomeruli ; the amount of solids dissolved in the urine determines 

 its concentration. 



(A) The amount of urine, which is secreted chiefly within the Malpighian 

 capsules, depends primarily upon tlie blood-pressure in the area of the renal artery, 

 and follows, therefore, the laws of filtration ( 191, II.) (Ludwig and Goll). [In 

 this respect the secretion of urine differs markedly from that of saliva, gastric juice, 

 or bile. We may state it more accurately thus, that the amount of urine depends 

 very closely upon the differences of pressure between the blood in the glomeruli 

 and the pressure within the renal tubules. If the ureter be ligatured, the secretion 

 of urine is ultimately arrested, even although the blood-pressure be high. The 

 secretion may also be arrested by ligature of the renal vein ; and in some cases of 



