SEC. 4. ON UREA AND ON NITROGENOUS METABO- 

 LISM IN GENERAL. 



380. 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 manifest- 

 ing vital energies as to justify the conclusion that the changes 

 which it undergoes are in some way essential to the manifesta- 

 tion 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 latent energy 

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

 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 pro- 

 teid material of the food enters the blood as proteid material 

 either as peptone or in some other form, 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 nitro- 

 genous 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 contributing 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. 



589 



