THE KIDNEYS. SECRETION OF URINE. 603 



of this substance, and probably, therefore, from the disintegration of the gela- 

 tigenous as well as of the albuminous tissues. Next in importance to urea, 

 among the organic products of the metamorphosis of the azotized constituents 

 of the tissues or of the blood, but ordinarily bearing a very small proportion to 

 it in quantity, is Uric acid. It has been shown ( 57) that the formation of 

 this substance is probably anterior to that of urea; and we shall see that its 

 proportion in the urine is augmented under the same conditions as regards food 

 ( 640). On the other hand, there is reason to think that exercise, by augment- 

 ing the respiration, tends to dimmish the proportion of uric acid in the urine, 

 by converting it into urea. The circumstances that most favor the genesis of 

 uric acid in the system, therefore, and its increased proportion in the urine if 

 there be no obstacle to its elimination, are a highly azotized diet and inactive 

 habits ; whilst the reduction of the azotized portion of the diet to what is really 

 wanted for the nutrition of the system, and the promotion of the respiration by 

 active exercise, tend to the reduction of the proportion of this component. The 

 precipitation of uric acid (usually in combination with alkaline bases), which 

 frequently takes place on the cooling of the urine, must not be regarded as in- 

 dicative of the presence of an unusual amount of this substance ; since it may 

 depend upon a variety of other conditions ( 56). There are many diseases, 

 however, especially those of a febrile nature, in which the presence of an excess 

 of uric acid is a very marked symptom; there is usually, at the same time, a 

 reduction in the proportion of urea; and thus it would seem that, with perhaps 

 an augmented tendency to disintegration of the tissues, there is an incapacity 

 for the performance of that higher process of oxidation which is requisite for 

 the genesis of urea; so that a larger proportion of the products of the "waste" 

 passes off in the state of uric acid, as in animals whose respiration is feeble. 

 This view derives support from the fact that Hippuric acid, which is only to 

 be found in extremely minute proportion in healthy Human urine, and the com- 

 position of which indicates that it is to be regarded as a result of very imper- 

 fect oxidation ( 58, 59), undergoes a marked increase under the same circum- 

 stances, and especially when obstructed action exists in either of the other great 

 emunctories, the lungs, liver, or skin, so that a larger amount of carbonaceous 

 matter is thrown upon the kidneys for elimination ; for in this case, also, there 

 is a deficiency in the normal amount of urea. Although the presence of Crea- 

 tine and Creatinine in the Urine, the former in very small proportion, but the 

 latter in considerably larger amount, is now a well-established fact, the actual 

 quantities ordinarily excreted, and the circumstances which favor their increase 

 and diminution, have not yet been determined. From the considerations for- 

 merly adduced ( 60, 61), it seems likely that Creatine is one of the first pro- 

 ducts of the disintegration of muscular tissue, and that a portion of the urea 

 eliminated in the urine, as well as of the greater part (if not the whole) of the 

 Creatinine, is generated at its expense. The presence of Lactic acid in the 

 Urine, although by no means infrequent, must be regarded as exceptional. We 

 have seen that a constant genesis of this substance is taking place in the body, 

 not merely as a product of the metamorphosis of the saccharine matters em- 

 ployed as food, but also as one of the results of the disintegration of the azotized 

 tissues ( 48-50); but that the respiratory process affords the ordinary channel 

 for its removal; so that it is only when its production is excessive, or when 

 there is some obstruction to its elimination by the lungs, that it makes its ap- 

 pearance in the urine. These conditions are so often present in disease, that 

 Lactic acid is far more commonly present in abnormal than in normal states of 

 the secretion. The Extractive Matters of the Urine, as already pointed out 

 ( 64), are made up of a variety of different compounds, our knowledge of which 

 is gradually being extended. Among the substances which rank under this head 

 in the ordinary analyses of Urine, are ^creatine, creatinine, and hippuric acid ; 



