460 THE METABOLIC PROCESSES OF THE BODY. 



400. Our knowledge of the metabolism of the nervous tissues is, as 

 we have seen, very imperfect ( 70), but the presence of the kreatin in the 

 central nervous system leads us to infer that the nitrogenous metabolism of 

 the living substance of nerve-cells and of the axis-cylinder of nerve-fibres 

 is, in its broad features, identical with that of muscle substance. The mass, 

 however, of the nerve-cells and axis-cylinder of the body, all put together, 

 is small, compared with the mass of skeletal muscle ; moreover, the energy 

 set free by the metabolism of a mass of nervous matter though " higher " 

 in quality is less in quantity 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 nitrogenous 

 metabolism taking place in connective tissue, cartilage, bone, and the skin 

 is probably still less, and, for our present purposes, needs no special dis- 

 cussion. 



401. The nitrogenous metabolism of the glands, however, more par- 

 ticularly that of the liver, does deserve special consideration ; 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 circumstances, the direct 

 effect of nitrogenous food becomes more striking. We have already said, 

 and shall again return to the point, that muscular contraction does not di- 

 rectly increase the output of urea ; the discharge of urea, for instance, is 

 not necessarily increased by even great bodily labor. 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 dis- 

 charge of urea in the twenty-four hours each meal is followed by a con- 

 spicuous 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, accom- 

 panied 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-nitro- 

 genous 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 

 ( 218) 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 pep- 

 tone, 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 actually take place in the living body ; it may take 



