THE METABOLISM OF THE BODY. 447 
Urea may be prepared artificially, as represented by the fol- 
lowing equations: 
1 CO<A #4 = CON.H.+ HO. 
‘ 
Ammonium Urea. 
carbamate. 
2. CN.NH, + H,O = CON,.H,. 
Cyanamide. 
3. CN(ONH,) = CON.H, 
Ammonium 
cyanate, 
Leucin is amido-caproic acid (CH,CH,CH,CH,CH(NH,) 
CO.H). 
Another amido-acid, glycin— 
Amido-acetic acid, 
when introduced into the digestive tract, gives rise to an in- 
crease of the urea of the urine. 
It will be seen that ammonia compounds, both in the labora- 
tory and apparently in the body, have a formative relation to 
urea; but beyond this we can not go very far in furnishing a 
chemical explanation of the formation of urea as a part of a 
series of metabolic processes. Do the kidneys merely pick 
out from the blood and pass on into the urinary tubules the 
already formed urea—i. e., eat, so to speak, and then discharge 
it, Amoesba-like—or do they manufacture it from bodies that 
have gone on the way a certain distance toward urea before 
they reach the kidneys; or, again, do they form urea in some 
such way as the mammary gland constructs fat ? 
If the ureters be tied, the renal arteries ligatured, or the 
kidneys extirpated, urea accumulates in the blood and tissues, 
This might be explained on the supposition that urea formed 
elsewhere was not eliminated; or that some body related to 
urea, and the usual transformations of which are completed 
in the kidneys, under these unwonted circumstances becomes 
urea, either in the tissues in which it arose or elsewhere. 
We can not pronounce with certainty in favor of any one 
or all of these conceivable methods. We may perhaps assume 
that creatin and possibly other allied bodies are antecedents of 
urea; that the leucin and perhaps the tyrosin of digestion in 
some way give rise to urea; and that the liver and possibly 
the spleen are organs in which a portion of the urea is formed ; 
that a part of the urea of urine is simply withdrawn from the 
blood by the kidneys; but, as to whether any part is made by 
