42 PRINCIPLES OF ANIMAL NUTRITION. 
thus far been acquired upon this subject has resulted chiefly from 
attempts to trace back the excretory products to their antecedents. 
The products of the complete breaking down and oxidation of 
proteids in the body are carbon dioxide and water, excreted through 
the lungs, skin, and kidneys, and urea and a number of other com- 
paratively simple crystalline nitrogenous compounds found in the 
urine. To these are to be added the nitrogenous metabolic prod- 
ucts of the feces, the sulphuric and phosphoric acids resulting from 
the oxidation of the sulphur of the proteids and the phosphorus of 
the nucleo-proteids, and the relatively minute amounts of nitroge- 
nous matter found in the perspiration. 
EXcRETION OF FREE Nirrocgen.—The question whether any 
portion of the nitrogen of the proteids is excreted as free gaseous 
nitrogen is one which has been the subject of no little investigation 
and controversy in the past, the especial champions being, on the 
affirmative, Seegen in Vienna and, on the negative, Voit in Munich. 
It would lead us too far aside from our present purpose, however, to 
attempt even to outline the evidence, and it must suffice to say that 
the great majority of physiologists regard it as established that there 
is no excretion of gaseous nitrogen as a result of the katabolism of 
proteids, but that all the proteid nitrogen is excreted in the urine 
and feces with the exception of small amounts in the perspiration. 
In accordance with this view, we shall assume in subsequent pages 
that the urinary nitrogen (together with, strictly speaking, the 
metabolic nitrogen of the feces and perspiration) furnishes a meas- 
ure of the total proteid katabolism of the body. 
A brief consideration of some of the principal nitrogenous 
products of proteid katabolism will serve to indicate some of the 
main features of the process, so far as they have been made out. 
Urea.—Urea, or dicarbamid, CON,H,, is the chief nitrogenous 
product of proteid metabolism in the carnivora and omnivora. In 
the urine of man, e.g., from 82 to 86 per cent. of the nitrogen is in 
the form of urea.* 
Antecedents of Urea.—A vast amount of study has been expended 
upon this question without as yet leading to any general unanimity 
of views. It appears, however, to be fairly well made out that at 
*v. Noorden, Pathologie des Stoffwechsels, p. 45. 
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