Chap, iv.] METABOLIC PROCESSES OF THE BODY. 593 



than that set free by the metabolism of an equal mass of muscle, 

 or in other words its metabolism is less rapid. Hence we may 

 probably consider the metabolism of the nervous system as a 

 mere addition to that of the muscular system, at least as regards 

 the point on which we are now dwelling. The amount of nitro- 

 genous metabolism taking place in connective tissue, cartilage, 

 bone, and the skin is probably still less, and for our present pur- 

 poses needs no special discussion. 



§ 385. 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 neces- 

 sarily increased by even great bodily labour. The introduction 

 however of even a small quantity of proteid material into the ali- 

 mentary 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 pro- 

 teid material from the alimentary canal is followed by an imme- 

 diate 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 ha^nd since some 

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

 every necessary meal, goes to repair the proteid waste continu- 

 ally 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 metabo- 

 lism 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 con- 

 verted into urea and got rid of. We have already (§ 210) seen 

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



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