410 



THE HUMAN BODY. 



average composition of the urine excreted in twenty-four 

 hours expressed in grams; in the second column the same 

 expressed in grains. The third column gives the composi- 

 tion of 1000 parts of urine. 



The urine, however, even in health is subject to consid- 

 erable variation in composition; not only as regards the 

 amount of water in it, but also in its solid constituents; 

 the latter are especially modified by the quantity and nature 

 of the food taken. 



100 volumes of urine contain in solution about 14 vol- 

 umes of gas, consisting of about 13 of carbon dioxide, 1 

 of nitrogen, and mere traces of oxygen. 



Mechanism of the Renal Secretion. The kidneys, as 

 secreting organs, consist of two distinct parts; (1) the glome- 

 ruli through which a filtration of water, probably with salts 

 in solution, takes place; and (2) an actively secretory appa- 

 ratus, formed by all parts of the uriniferous tubules between 

 their terminal capsules and the collecting tubes. Accord- 

 ingly, we find in the urine bodies, as water and common salt, 

 which already exist in the blood and can be removed from it 



