416 EXCRETION. 



acid, creatine, creatinine, xanthine, hypoxanthine, and some other bodies of similar nature. 

 That certain bodies are mutally convertible by the addition or subtraction of a few elements 

 of water, there can be no doubt. Examples of these simple transformations are, the 

 change of starch, dextrine, etc., into glucose, the change of creatine into creatinine, etc., 

 but the atomic changes necessary for the conversion into urea of the principles from 

 which this substance has been assumed to be produced are much more complicated. 

 There is no positive proof that the proportion of these various principles in the muscles, 

 blood, and urine, bears an inverse ratio to the proportion of urea. Again, the argument 

 that the excrements of reptiles contain an excess of uric acid because the activity of oxi- 

 dation is less than in the mammalia is met by the fact that, in birds, in which the amount 

 of oxygen consumed is greater, the proportion of urates is enormous ; and urea is not 

 generally found in this class, but is contained only in the excrements of the rapacious 

 birds, and here only in small quantity. 



There are no sufficient reasons for regarding urea as the final result of oxidation of cer- 

 tain of the tissues of the body, uric acid, creatine, etc., being substances in an intermediate 

 stage of transformation ; and we are forced to admit that this principle is formed during 

 the general process of disassimilation, probably from the nitrogenized elements of the 

 body, by a destructive action, with the exact nature of which we are as yet imper- 

 fectly acquainted. 



The daily amount of urea excreted is subject to very great variations. It is given in 

 the table as ranging between 355 and 463 grains. This is much less than the estimates 

 frequently given ; but, when the quantity has been very large, it has generally depended 

 upon an unusual amount of exercise or of nitrogenized food, or the weight of the body 

 has been above the average. Parkes gives the results of twenty -five different series of 

 observations upon this point. The lowest estimate is 286 1 ! grains, and the highest, 688*4 

 grains. 



Uric Acid and its Compounds. Uric acid seldom if ever exists in a free state in 

 normal urine. It is exceedingly insoluble, requiring from fourteen to fifteen thousand 

 times its volume of cold water, and from eighteen to nineteen hundred parts of boiling 

 water for its solution. It was at one time supposed to exist in the urine in sufficient 

 quantity to give it its acid reaction ; but it has since been ascertained that its solution 

 does not redden litmus. Its presence in the urine uncombined must be regarded as a 

 pathological condition ; still, it is often found in urinary deposits, where it is interesting 

 to study the peculiar and varied forms of its crystals. Frequently, in tables of the com- 

 position of the urine, the proportion of uric acid is given, but this is simply a matter of 

 convenience, and it has precisely the same signification as the estimates of the proportions 

 of sulphuric or of phosphoric acid. None of these acids constitute, of themselves, prox- 

 imate principles of the urine, but they are always combined with bases. 



In normal urine, uric acid is combined with soda, ammonia, potassa, lime, and mag- 

 nesia. Of these combinations, the urate of soda and the urate of ammonia are by far the 

 most important and constitute the great proportion of the urates, the urates of potassa, 

 lime, and magnesia existing only in minute traces. The urate of soda is very much more 

 abundant than the urate of ammonia. The union of uric acid with the bases is very 

 feeble. If from any cause the urine become excessively acid after its emission, a deposit 

 of uric acid is liable to occur. The addition of a very small quantity of almost any acid 

 is sufficient to decompose the urates, when the uric acid appears, after a few hours, in a 

 crystalline form. 



Uric acid, probably in combination with bases, was found in the substance of the liver 

 in large quantity by Oloetta, and his observations have been confirmed by. recent Ger- 

 man authorities. It is more than probable that the urates also exist in the blood and 

 pass ready -formed into the urine ; but their proportion in the blood is so slight, under 

 normal conditions, that their presence in this fluid has not been definitely determined, 



