SEC. 5. ON UREA AND ON NITROGENOUS 

 METABOLISM IN GENERAL. 



482. We have seen that nitrogenous proteid material in 

 some form or other enters into the composition of all the tissues of 

 the body, and we have further seen that it is so conspicuously and 

 constantly present wherever living substances are manifesting 

 vital energies as to justify the conclusion that the changes which 

 it undergoes are in some way essential to the manifestation of 

 those energies. We have seen, it is true, reason to think that in 

 some tissues at least, in muscle for instance, a large part of the 

 energy set free during activity preexisted as potential energy in, and 

 had its immediate source in not proteid (nitrogenous) but some 

 other constituents of muscle ; and indeed, as we shall see later on, 

 the greater part of the whole energy of the body must be regarded 

 as the energy of carbon compounds and not of nitrogen compounds; 

 but this is quite consistent with the view that proteid material in 

 some way or other essentially intervenes in, we may perhaps go so 

 far as to say directs, the changes by which in the body energy is 

 set free in the peculiar way which we speak of as living. 



We have seen that at all events the greater part of the proteid 

 material of the food enters the blood as proteid material, and is 

 carried as proteid material to the tissues. 



We have seen that the nitrogen of proteid material leaves the 

 body so largely in the form of urea, that the other nitrogenous 

 excretions may for the time be left out of consideration. 



And lastly we have seen reason to think that this urea which 

 leaves the body in urine is brought to the kidneys as urea in the 

 blood, the kidneys themselves apparently having no special power 

 of forming urea out of something which is not urea, but only con- 

 tributing to the general stock of urea by virtue of their own proteid 

 metabolism. We have now to study the little we know concerning 

 the steps by which the proteid material of the food and of the 

 body is converted into this urea of the blood which is the source of 

 the urea of the urine. 



