754 FORMATION OF UREA. [BOOK 11. 



of nitrogenous metabolism taking place in connective tissue, 

 cartilage, bone, and the skin is probably still less, and for our 

 present purposes needs no special discussion. 



487. The nitrogenous metabolism of the glands however, 

 more particularly that of the liver, does deserve special con- 

 sideration ; and we may at once turn to a quite different aspect of 

 the question in hand. 



When the rate of discharge of urea from the body is observed 

 during a period of some length, especially under varied circum- 

 stances, the direct effect of nitrogenous food becomes most striking. 

 We have already said, and shall again return to the point, that 

 muscular contraction does not directly increase the output of 

 urea ; the discharge of urea for instance is not necessarily increased 

 by even great bodily labour. The introduction however of even a 

 small quantity of proteid material into the alimentary canal at 

 once increases the urea of the urine ; and in the curve of the 

 discharge of urea in the twenty-four hours each meal is followed 

 by a conspicuous rise. The absorption of proteid material from 

 the alimentary canal is followed by an immediate proportionate 

 increase in the quantity of urea which is secreted by the kidneys, 

 and that as we have seen means an increase in the urea brought 

 to the kidney by the renal artery. What is the origin of this 

 additional urea ? 



Two views present themselves. On the one hand since some 

 portion of the proteid material of every meal, at all events of every 

 necessary meal, goes to repair the proteid waste continually going 

 on in the parts of the body where proteid metabolism is taking 

 place, we may suppose that the presence of an extra quantity of 

 proteid material thrown upon the blood from the food acts as a 

 stimulus to the tissues, to the muscles for instance as well as others, 

 stirs them up to increased nitrogenous metabolism and thus 

 produces an increase of energy, chiefly if not exclusively in the 

 form of heat, accompanied by an increase of the antecedents of 

 urea and so of urea. In other words the increase of urea in 

 question is the result of an increase in the general nitrogenous 

 metabolism of the body. 



On the other hand we may suppose that in order to prevent 

 the whole body being encumbered with it, this excess of proteid 

 food material is, in some special part of the body, split up into a 

 nitrogenous and a non-nitrogenous moiety, and that, while the 

 latter is stored up as fat or glycogen, the former is at once 

 converted into urea and got rid of. We have already ( 249) seen 

 that a step in this direction may take place while the food is as 

 yet in the alimentary canal ; we have seen that pancreatic juice 

 may carry part of the proteids on which it acts beyond the stage 

 of albumose and peptone, and reduce that part into leucin, tyrosin, 

 and other bodies. We do not know, as we have already said, to 

 what extent this more profound digestion by pancreatic juice does 



