588 THE URINE [CH. XXXIX. 



in urea formation. Similar results were obtained by Nencki with 

 ammonium carbamate. 



The importance of ammonia is accentuated when we remember 

 that ammonia is one of the products of pancreatic digestion, and 

 probably also of endogenous protein metabolism. The ammonia, 

 whether it is formed directly or through the intermediate stage of 

 amino-acid, will combine with the carbonic acid of the blood to form 

 ammonium carbonate or carbamate, and the following structural 

 formulae exhibit the close relationship between these substances and 

 urea. The loss of one molecule of water from ammonium carbonate 

 produces ammonium carbamate; the loss of a second molecule of 

 water produces urea 



/O.NH 4 r /NH 2 



O - C \O.NH* '\O.NH 4 



[Ammonium carbonate.] [Ammonium carbamate.] [Urea or carbamide.] 



Urea is absent, or nearly so, from the muscles, and its place there 

 is taken by the substance called creatine. It is, however, doubtful 

 whether creatine is a precursor of urea in the body. The fact that 

 muscular work does not appreciably increase protein katabolism is 

 intelligible, when, in light of recently acquired knowledge, we realise 

 that protein katabolism, in so far as its nitrogen is concerned, is 

 independent of the oxidations which give rise to heat, or to the 

 energy which is converted into work. 



Urcemia. The older authors considered that urea was formed in the kidneys, 

 just as they also erroneously thought that carbonic acid was formed in the lungs. 

 Prevost and Dumas were the first to show that after complete extirpation of the 

 kidneys the formation of- urea goes on, and that it accumulates in the blood and 

 tissues. Similarly, in those cases of disease in which the kidneys cease work, urea 

 is still formed and accumulates. This condition is called uraemia, and unless the 

 products of nitrogenous breakdown are discharged from the body the patient dies 

 in a condition of coma preceded by convulsions. 



This term was originally applied on the erroneous supposition that it is urea or 

 some antecedent of urea which acts as the poison. There is no doubt that the 

 poison is not any constituent of normal urine ; if the kidneys of an animal are 

 extirpated, the animal dies in a few days, but there are no uraemic convulsions. 

 In man, also, if the kidneys are healthy, or approximately so, and suppression of 

 urine occurs from the simultaneous blocking of both renal arteries by clot, or of both 

 ureters by stones, again uraemia does not follow. On the other hand, uraemia may 

 occur even while a patient with diseased kidneys is passing a considerable amount 

 of urine. What the poison is that is responsible for the convulsions and coma, is 

 unknown. It is doubtless some abnormal katabolic product, but whether this is 

 produced by the diseased kidney cells, or in some other part of the body, is also 

 unknown. 



Ammonia. 



A small quantity of ammonia always slips through into the urine, 

 because a portion of the ammonia-containing blood passes through the 

 kidney before reaching the organs that are capable of converting it into 

 urea. In man the daily amount of ammonia excreted varies between 



