SEC. 2. THE HISTORY OF UREA AND ITS ALLIES. 



We may now return to the questions which we left unanswered 

 at p. 409. Where is urea formed ? what are its immediate ante- 

 cedents ? what are the various chemical links between it and the 

 proteid material of which it is the excretory representative ? 



We have seen, p. 69, that the muscular tissues contain kreatin, 

 together with smaller quantities of allied nitrogenous crystalline 

 bodies, such as xanthin, hypoxanthin, &c. ; and we cannot go far 

 wrong in supposing that these bodies are in some way or other the 

 products of muscular metabolism. We do not know in what 

 quantities they are formed; but since they are such bodies as 

 would readily be carried away from the muscle by the blood- 

 stream, and yet are always to be found in the muscle, we infer that 

 they are continually being formed, and as continually being con- 

 verted into some other bodies and carried away. And we- may 

 further say, that since kreatin exists in muscle to the extent 

 of '2 or '4 p.c., and since muscle forms so large a portion of the 

 whole body, and must be continually undergoing some nitrogenous 

 metabolism, even if the energy of the muscle (see p. 99) have a 

 non-nitrogenous source, it is at least possible, if not probable, that 

 a considerable amount of kreatin passes within twenty-four hours 

 into the blood, on its way to become transformed by other tissues 

 into urea, or into some stage nearer to urea than itself. The urine, 

 it is true, contains a certain amount ('9grm. in 24 hours) of 

 kreatinin, into which kreatin is easily converted ; but this can be 



