CHAPTER XII. 



THE URINE. 



THE urine is by far the most important excretory product of the 

 animal body, and the medium through which the end-products of 

 nitrogenous metabolism and soluble mineral salts are almost exclu- 

 sively eliminated under normal conditions. Abnormal products of 

 metabolism also, and many substances that have found their way 

 into the circulation from without, and which are foreign to the body, 

 are likewise removed in this manner, either as such or in a more or 

 less modified form, All these substances are found in the urine in 

 aqueous solution, and it is to be noted that of the total amount of 

 water which is daily excreted at least 50 per cent, appears in this 

 form. 



Formerly, it was supposed that the various elements which are 

 found in the urine, and notably the mineral salts and water, were 

 eliminated by a simple process of osmosis. Later it was shown, 

 however, that in the elimination of the organic constituents at least 

 the renal epithelium of the uriniferbus tubules plays an active part, 

 and it now appears, indeed, that all the substances which occur in 

 the urine, including a certain amount of water even, are removed 

 from the blood, viz., the lymph, through the intervention of the 

 epithelial cells. It is supposed, moreover, that these structures 

 possess certain selective properties, and we can accordingly under- 

 stand why the composition of the blood always remains constant. 



The kidneys cannot be regarded as simple excretory organs, how- 

 ever, for we know that important synthetic processes also take place 

 in them, the object of which is to transform certain substances which 

 may occur in the circulating blood into compounds that can be more 

 readily eliminated. 



The most important synthesis of this kind is that of glycocoll and 

 benzoic acid, which results in the formation of hippuric acid (see 

 page 258). 



GENERAL CHARACTERISTICS OF THE URINE. 



The general appearance of the urine varies in different animals. 

 In man it is perfectly transparent when recently passed, but soon 

 becomes turbid, and on standing deposits a light, flocculent sediment, 

 which consists of a mucinous body and a few epithelial cells and 

 leucocytes that are derived from the urinary passages. 



In addition, a small number of crystals of uric acid or of oxalate 

 of calcium may also be seen. The supernatant fluid is then per- 



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