THE UROGENITAL SYSTEM 143 



grayity of about 1035, and is turbid from the presence of the 

 carbonates of lime which it contains. Bovine urine is clear. 

 A considerable amount of mucus gives fresh urine a sticky feel. 

 The most important constituents are: carbonic, uric, and hip- 

 puric acids, and urea, creatin, creatinin, allantoin, and other 

 organic wastes. Urea and hippuric acid are found in relatively 

 large quantities. The former comes from protein metabolism 

 and the putrefaction of proteins in the intestines, while the latter 

 is formed chiefly from the benzoic acid in the roughage, and is 

 present in larger amounts upon a ration consisting mostly of hay 

 or grass than of concentrates. 



It is obvious that the urine is derived from the blood flowing 

 through the kidneys. By arranging the various substances 

 found in each of these fluids in tabular form for comparison, it 

 is evident that they differ but slightly in qualitative composition, 

 for the only substance found in the urine and not in the blood is 

 hippuric acid. However, the quantitative proportion of nearly all 

 'the substances varies widely, if refined methods of chemical 

 analysis are employed. 



The concentration of the salts of sodium and urea and uric 

 acid in the urine is greater than that of the blood. Normally 

 the kidneys do not allow any sugar to pass out with the urine. 

 When there is injected into the veins some sodium chlorid solu- 

 tion of the same strength of salt content as is contained in blood 

 itself, a curious effect is observed. The kidneys become unable 

 to hold back the sugar in the blood so it filters through them and 

 escapes in the urine. If, however, there is added to the sodium 

 chloride solution some calcium, magnesium, and potassium salts 

 as found in blood, large amounts of the solution can be introduced 

 into the blood stream without causing any disturbance in the 

 function of the kidneys whatsoever. It is not known why this is, 

 but it has been observed many times, so physiologists have given 

 the term "balanced salt solution" to that solution necessary for 

 the proper functioning of the living tissues. 



It is evident from this brief review, that the process of urine 

 secretion is not a simple filtration, as was at one time supposed, 

 but is a vital process involving cellular activity. Moreover, that 

 the flow of urine will be increased (1) by raising the blood-pressure; 

 (2) by increasing the water content of the blood through absorp- 

 tion from the intestines; (3) by retarding the elimination of fluids 

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