SECRETION OF URINE. 309 



( 13) and gelatin ( 19), that the amount of azote in propor- 

 tion to that of the other elements is much greater in urea 

 than it is in these substances, which form the materials of 

 the animal tissues. The quantity of Urea which is daily 

 excreted is very considerable, the average in an adult being 

 about an ounce, and in a child of eight years old about half 

 as much. There is another compound which does not usually 

 exist in large amount in the urine of the Mammalia, but 

 which makes up a considerable part of the solid matter of 

 this secretion in Birds and the lower Vertebrata ; this is uric 

 or lithic acid, which consists of 10 equivalents of Carbon, 4 of 

 Hydrogen, 4 of Azote, and 6 of Oxygen. It is almost entirely 

 insoluble in water, unless it be combined with soda or am- 

 monia ; and in this state it ordinarily exists. When formed 

 in too large quantity, however, it may be deposited in an 

 insoluble form, constituting gravel ( 348) ; and the same 

 effect may result from the presence of some other acid, which, 

 combining with the ammonia, precipitates or sets free the 

 lithic acid. In the urine of herbivorous animals, uric acid is 

 replaced by Hippuric acid, which contains a much larger 

 proportion of carbon, its composition being 18 Carbon,. 8 

 Hydrogen, 1 Nitrogen, and 5 Oxygen. Urine also contains 

 a considerable quantity of Saline matter; part of which 

 consists of what has been introduced into the system in the 

 same form, and has to be got rid of as superfluous; whilst 

 another part results from the conversion of the sulphur and 

 phosphorus of the food into sulphuric and phosphoric acids by 

 union with atmospheric oxygen ( 343), and from the com- 

 bination of these acids with alkaline bases furnished by the 

 food. The amount of alkaline phosphates contained in the 

 urine may be considered as in some degree a measure of the 

 expenditure of nervous tissue ; whilst that of alkaline sulphates 

 has some relation to the expenditure of muscular substance. 



368. The Kidney, by which the secretion of Urine is eli- 

 minated from the blood, is an organ whose structure in the 

 higher animals is very peculiar, although in the lower it is a 

 mere aggregation of tubes or of follicles. If we make a ver- 

 tical section of the kidney of Man or any of the higher Mam- 

 malia (fig. 172, A), we find that it seems composed of two 

 different substances, one surrounding the other ; to the outer; 

 a, the name of conical (bark-like) substance has been given ; 



