PHYSIOLOGICAL PROCESS OF URINARY SECRETION. 509 



fused, the specimen melts to a white, porcelain-like mass. Basic calcium phos- 

 phate does not effervesce with acids. The solution in hydrochloric acid is pre- 

 cipitated by ammonia. The solution in acetic acid yields calcium oxalate on 

 addition of ammonium oxalate. In order to isolate calcium and magnesium from 

 such stones, the process described in paragraph (a) should be followed. 



(e) Neutral calcium phosphate is rarely found in calculi, but, on the other 

 hand, not rarely in urinary sand. Such concretions resemble the earthy phos- 

 phates in physical and chemical properties, except that they contain no magnesia. 



THE PHYSIOLOGICAL PROCESS OF URINARY SECRETION. 



The two older and most important theories of secretion will be 

 mentioned: (i) Bowman held that the glomeruli secrete only water, 

 and that the epithelial cells of the urinary tubules through their glandu- 

 lar activity furnish the specific urinary elements, which the onflowing 

 urinary water washes out of the cells. (2) C. Ludwig assumed that a 

 dilute urine is secreted in the capsules. Passing through the urinary 

 tubules, this, by endosmosis, returns water to the blood, which is more 

 deficient therein, and to the lymph of the kidney, and thus becomes 

 reduced to normal consistence. 



The secretion of the urine in the kidneys depends, however, not alone 

 upon physically definable influences, but it must rather, in accordance 

 with a series of acquired facts, be assumed that in addition the vital 

 activity of special secretory cells plays a prominent role. The physical 

 or chemical forces obviously underlying the latter have not as yet been 

 determined. The secretion includes (i) the urinary water, and (2) the 

 urinary elements dissolved therein. Both together constitute the 

 totality of the secretion. The amount of urinary water secreted in the 

 glomeruli determines principally the amount of urine, while the amount 

 of substances dissolved in the urinary water determines the concen- 

 tration of the urine. 



The amount of urinary water, which is secreted principally in the 

 capsules, depends, in the first place, upon the blood-pressure in the 

 distribution of the renal artery; and, accordingly, is governed by the 

 laws of filtration. The amount of urinary water furnished is, however, 

 not dependent upon the hydrostatic pressure alone, but the functional 

 activity of the cells lining the glomerulus is also of influence. In ad- 

 dition to the water, a certain amount of the salts occurring in the urine 

 is secreted in the glomerulus; albumin, however, is retained. In 

 consideration of the functional activity of the cells, the amount of 

 urinary water must depend also in part upon the rapidity with which 

 new blood conveying the material for secretion passes to the glomeruli; 

 and, in part, upon the amount of urinary elements and water contained 

 in the blood. 



The independent activity of the secretory cells is present only when their 

 vitality is intact. It is paralyzed in consequence of transitory occlusion of the 

 renal artery. For this reason, the kidney no longer secretes under such circum- 

 stances, even when the circulation is restored after removal of the compression. 

 The observation that the urine is not rarely found to have a higher temperature 

 than the arterial blood is also indicative of this activity. 



The dependence of the secretion upon the blood-pressure will be 

 made clear by the following observations : 



i. Increase of the total contents of the vessels, in consequence of 

 which the tension in the -vascular system must increase, increases the 



