CHAP, v.] -NUTRITION. 439 



normal state of things, contain urea. We are thus led to the view 

 that among the numerous metabolic events which occur in the 

 hepatic cells, the formation of urea out of leucin or out of other 

 antecedents may be ranked as one. And in support of this view it 

 may be urged that a large quantity of urea seems to be present in 

 the liver of mammals, and of urates in the liver of birds. More- 

 over when a stream of fresh blood is passed several times through 

 the liver of an animal recently killed, the percentage of urea in 

 the blood so used is found to be decidedly increased. This however 

 is not conclusive, for the increased quantity in the blood which had 

 been circulated might have been simply urea which had been 

 washed out from the liver, where it had previously been staying. 

 Probable, therefore, as this view may seem, it has not as yet been 

 established as a fact. A strong presumption however in favour of 

 urea arising through the hepatic metabolism, from leucin as an 

 antecedent, is afforded by the fact that in cases of acute atrophy of 

 the liver, where the hepatic cells lose their functional activity, the 

 urea of the urine is replaced by leucin and tyrosin. And, lastly, it 

 may be remarked that not only are leucin and tyrosin found in 

 nearly all the tissues after death, especially in the glandular tissues, 

 but they also appear with striking readiness in almost all de- 

 compositions of proteids, and leucin is also a product of decompo- 

 sition of gelatiniferous substances. 



The view that leucin is tranformed into urea lands us however 

 in very considerable difficulties. Leucin, as we know, is amido- 

 caproic acid; and, with our present chemical knowledge, we can 

 conceive of no other way in which leucin can be converted into 

 urea than by the complete reduction of the former to the ammonia 

 condition (the caproic acid residue being either elaborated into a 

 fat or oxidized into carbonic acid) and by a reconstruction of 

 the latter out of the ammonia so formed. We have a somewhat 

 parallel case in glycin. This, which is amido-acetic acid, when 

 introduced into the alimentary canal, also reappears as urea ; here 

 too a reconstruction of urea out of an ammonia phase must take 

 place. Moreover when ammonium chloride is given to a dog a 

 very large portion reappears as urea, i.e. there is an increase in the 

 urea of the urine corresponding to a large portion of the nitrogen 

 contained in the ammonium chloride. And there is a certain 

 amount of evidence into which we cannot enter here, leading to 

 the conception that the immediate antecedent of urea is ammonium 

 carbamate which by dehydration (and this it is stated may be 

 effected by electrolysis with rapidly alternating currents) is trans- 

 formed into urea. Or the antecedent which is dehydrated may be not 

 ammonium carbamate but ammonium carbonate (see Appendix); and 

 on the other hand, seeing how readily ammonium cyanate is trans- 

 formed into urea, it may be that the immediate antecedent is some 

 cyanogen compound. Leaving these matters for the present, and 

 indeed we have ventured to call attention to them, chiefly because 



