380 



EXCRETION BY THE SKIN AND KIDNEYS. 



does not increase the elimination of urea ; but that even less work, when vio- 

 lent and attended with shortness of breath, increases the discharge of urea. 

 According to this view, moderate work draws upon the oxygen supplied to 

 the body and at once largely increases the elimination of carbon dioxide ; but 

 the less active processes which result in the production of urea are not so 

 promptly affected. Violent muscular work, however, or work which is ex- 

 cessively prolonged, consumes those parts of the tissues the destruction of 

 which is represented by the discharge of urea. This view, if accepted, har- 

 monizes the apparently contradictory experiments upon the influence of 

 muscular work on the elimination of nitrogen. 



The daily quantity of urea excreted is subject to very great. variations. 

 It is given in the table as 355 to 463 grains (23 to 30 grammes). This is 

 less than the estimates frequently given ; but when the quantity has been 

 very large, it has generally depended upon an unusual amount of nitrogen- 

 ized food, or the weight of the body has been above the average. Parkes 

 has given the results of twenty-five different series of observations upon this 

 point. The lowest estimate was 286-1 grains (18-24 grammes), and the 

 highest, 688-4 grains (44-61 grammes). 



Uric Acid and its Compounds. Uric acid (CgH^N^Os) seldom if ever 

 exists in a free state in normal urine. It is very insoluble, requiring four- 

 teen to fifteen thousand times its volume of cold water or eighteen to nine- 

 teen hundred parts of boiling water for its solution Its presence uncom- 

 bined in the urine must be regarded as a pathological condition. 



In normal urine, uric acid is combined with sodium, ammonium, potas- 

 sium, calcium and magnesium. Of these combinations, the sodium urate 



FIG. 120. Crystals of uric acid, obtained partly 

 by the solution and subsequent precipita- 

 tion of chemically pure acid, and partly by 

 decomposition of the urates by nitric or 

 acetic acid (Funke). 



FIG. 121. Sodium urate (Funke). 



and ammonium urate are by far the most important, and they constitute the 

 great proportion of the urates, potassium, calcium and magnesium urates 

 existing only in minute traces. Sodium urate is very much more abundant 

 than ammonium urate. The union of uric acid with the bases is very feeble. 



