438 A MANUAL OF PHYSIOLOGY 



is true of the suggestion of Loew and Bokorny, that the endowments 

 of living protoplasm depend on the presence of the unstable aldehyde 

 group H C = O. Nor do the known differences of chemical com- 

 position in dead organs give any insight into the peculiarities of 

 organization and function which mark off one living tissue from 

 another. For so far as they do not depend upon differences in the 

 dead plasma which interpenetrates the living substance, they only 

 show that the latter does not split up quite in the same way at death 

 in all the tissues, while the general similarity in the elementary 

 composition of excitable structures leaves us free to imagine as great 

 or as small a similarity as we please in the grouping of the atoms 

 in the living combinations. Be this as it may, the living proteid 

 molecule, whatever function it may have been fulfilling in the 

 organized elements of the body, has certainly a much greater 

 tendency to fail to pieces than the dead proteid molecule. And 

 it falls to pieces in a fairly definite way, the ultimate products, 

 under the influence of oxygen,* being carbon dioxide, water, and 

 comparatively simple nitrogen-containing substances, which after 

 further changes appear in the urine as urea, uric acid, and 

 ammonia. We have no definite information as to the production 

 of water from the hydrogen of the tissues, except what can be 

 theoretically deduced from the statistics of nutrition (p. 472). A 

 few words will be said a little farther on about the production of 

 carbon dioxide from proteids ; we have now to consider the seat and 

 manner of formation of the nitrogenous metabolites. And since in 

 man and the other mammals urea contains by far the greater part of 

 the excreted nitrogen, it will be well to take it first. 



Formation of Urea. The starting-point of all inquiries into 

 the formation of urea is the fact that it occurs in the blood 

 in small amount (4 to 6 parts per 10,000 in man ; 3 to 12 

 parts per 10,000 in the dog), the largest quantity being found 

 when the food contains most proteid, and at the height of 

 digestion. Evidently, then, some, at least, of the area 

 excreted in the urine may be simply separated by the kidney 

 from the blood ; and analysis shows that this is actually the 

 case, for the blood of the renal vein is poorer in urea than 

 that of the renal artery, containing only one-third to one-half 

 as much. If we knew the exact quantity of blood passing 

 through the kidneys of an animal in twenty-four hours, and 



* It is a remarkable fact that this combustion of proteids, as well as of 

 the other constituents of the tissues and the food, takes place at a tem- 

 perature so low as that of the body. Some have supposed that this is due 

 to the action of ozone derived from a portion of the oxygen taken in ; others 

 that it is due to the presence in the organs of oxidizing ferments. As a 

 matter of fact, extracts of various tissues contain substances, with the pro- 

 perties of globulins or nucleo-proteids, which act as ' oxygen-carriers' and 

 cause rapid oxidation of suctf bodies as the guaiaconic acid of guaiacum, 

 salicylic aldehyde, glucose, etc.. on which atmospheric oxygen has either 

 no action or a feeble and tardy one. The activity of these oxygen-carriers 

 is destroyed by boiling. 



