592 FORMATION OF UREA. [BOOK n. 



any one moment. We can hardly suppose that the metabolism 

 of muscle is so slow as out of this stock only to provide the 1 

 grm. 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 nitrogenous metabolism of muscle) or by such conditions, 

 fever for instance, as notably increase the urea of urine by in- 

 creasing the nitrogenous metabolism of muscle. We infer there- 

 fore that the normal presence of kreatinin in urine is due to the 

 direct administration 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 mus- 

 cular metabolism in the living body is a conspicuous antecedent 

 of the urea of the urine. It is difficult to see why kreatin pass- 

 ing 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. In- 

 deed 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 mus- 

 cle 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 ap- 

 pearance in a dying one? We have no positive evidence how- 

 ever that this is so, and meanwhile may continue to suppose 

 that kreatin is formed, and that in consequence kreatin is a con- 

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

 regard this as proved. 



384. Our knowledge of the metabolism of the nervous 

 tissues is, as we have seen, very imperfect ( 67), but the pres- 

 ence 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 mus- 

 cle ; moreover, the energy set free by the metabolism of a mass 

 of nervous matter though 'higher' in quality is less in quantity 



