CHAP, iv.] METABOLIC PROCESSES OF THE BODY. 753 



of kreatinin in 24 hours. Moreover the kreatinin in urine vanishes 

 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 are now dwelling. The amount 



F. 48 



