CHAP, v.] NUTRITION. 437 



considered as the normal form in which the kreatin of the muscles 

 passes out of the body. For the urinary kreatinin is exceedingly 

 variable in quantity, vanishing during starvation, and, though not at 

 all increased by exercise, is largely augmented by a flesh-diet ; and 

 kreatin injected into the blood, even in small quantities, reappears 

 as kreatinin in the urine. Without laying too much stress on the 

 last fact, we are led to conclude that the kreatinin or kreatin in 

 urine has an origin quite independent of that which is present in 

 the muscles, being probably derived directly from the food. 



Of the metabolism of the nervous tissues we know little ; but 

 kreatin is found in the brain, in some cases in not inconsiderable 

 quantity. Moreover the bodies of the nerve-cells are undoubtedly 

 composed of protoplasm ; the axis-cylinders of the nerve-fibres are 

 also protoplasmic in nature, and it is at least possible that much of 

 the peculiar matrix of the cerebral and cerebellar convolutions, and 

 of the grey matter generally, is also in reality protoplasmic. Hence 

 we may, with a certain amount of reason, suppose that the 

 nervous, like the muscular tissues, are continually, but to a 

 much less extent, supplying an antecedent to urea in the form of 

 kreatin. 



Lastly, the spleen contains a considerable quantity of kreatin, 

 as well as of xanthin, &a ; and these are present also in various 

 glandular organs. 



We thus have evidence of a continual formation of kreatin, 

 possibly in large quantities, in various parts of the body. On the 

 other hand, urea is certainly not present in muscle (save in certain 

 exceptional cases) and its presence in nervous tissue is extremely 

 doubtful. It is absent from the spleen (of the occurrence of urea 

 in the liver we shall speak presently), the thymus, and thyroid 

 bodies, and from the lymphatic glands, though uric acid, as we 

 have seen, appears to be a normal constituent of the spleen. It 

 seems very tempting to jump at once from these facts to the 

 conclusion that kreatin is the natural antecedent of .urea, and that 

 as far as nitrogenous excretion is concerned the labour of the 

 kidney is confined to the simple transformation of kreatin into 

 urea. We have only to suppose that the kreatin passes from these 

 several tissues into the blood, in which it may be found, and 

 while circulating in the blood is seized upon by the renal epithelium 

 and converted into urea. And in support of this view it has been 

 urged that while ligature of the ureters leads to an accumulation 

 of urea in the various tissues and fluids of the body, kreatin takes 

 the place of urea when the kidneys are wholly extirpated, the ex- 

 planation given of the different results being that in the former 

 case the kidneys continue to perform their functions, and to manu- 

 facture urea out of kreatin, and the urea thus formed is thrown 

 back upon the blood, whereas in the latter case the kreatin- 

 converting organs are absent. Further observations however have 

 clearly shewn that whether the kidneys be wholly extirpated or the 



