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 considerabie, 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 wric 
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 
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 cortical (bark-like) substance has been given ; 
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