788 ORIGIN OF UREA. [BOOK n. 



during starvation, is very markedly increased by a diet of flesh 

 which contains kreatin, and is not increased either by muscular 

 exercise (which however would only indirectly affect the nitro- 

 genous metabolism of muscle) or by such conditions, fever for 

 instance, as notably increase the urea of urine by increasing the 

 nitrogenous metabolism of muscle. We infer therefore that the 

 normal presence of kreatinin in urine is due to the direct adminis- 

 tration of kreatin present in a (normal) flesh diet and has nothing 

 to do with the muscular metabolism of the individual who is 

 secreting the kreatinin in his urine. 



The fact however that the kreatin present in the muscle of the 

 food and absorbed from the alimentary canal does not undergo a 

 change into urea but is excreted as kreatinin, that is virtually as 

 kreatin, warns us to be careful in adopting the conclusion arrived 

 at above that the kreatin produced by muscular metabolism in the 

 living body is a conspicuous antecedent of the urea of the urine. 

 It is difficult to see why kreatin passing into the blood of the 

 capillaries of the muscle should be changed into urea while that 

 which passes into the capillaries of the portal system is not ; for 

 reasons which will be apparent presently we should rather expect 

 that the latter being more directly exposed to the influence of the 

 liver would be more readily and more completely converted than 

 the former. Indeed the question forces itself upon us, Is kreatin 

 after all the natural main product of the nitrogenous metabolism of 

 muscle ? Is it possible that in the normal metabolism of the living 

 muscle the nitrogen leaves the muscular substance and passes into 

 the blood in another form, as some substance not kreatin, and that 

 it is as the muscle dies that kreatin is formed, just as the solid 

 myosin is unknown to the living fibre but makes its appearance in 

 a dying one ? We have no positive evidence however that this 

 is so, and meanwhile may continue to suppose .that kreatin 

 is formed, and that in consequence kreatin is a conspicuous 

 antecedent of the urea of the urine ; but we must not regard 

 this as proved. 



486. Our knowledge of the metabolism of the nervous 

 tissues is, as we have seen, very imperfect ( 72), but the presence 

 of 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 cylinders 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 



