ON UREA AND ON NITROGENOUS METABOLISM. 457 



395. The question may be asked, Is the secretion of bile independent 

 of or in some way or other connected with the glycogenic activity of the 

 cells? To this we cannot at present give a definite answer. In some of the 

 in vertebra ta the cells in the organ, called a liver, which manufacture gly- 

 cogen, are distinct from those which secrete bile or other digestive juices; 

 and it might be inferred that in the vertebrate the two actions, though 

 taking place, as they certainly do, in the same cell, take place apart and 

 distinct. There are facts which seem to indicate that the two are intimately 

 connected ; but we have as yet no exact knowledge concerning the matter. 

 It has been urged that the portal blood is chiefly concerned with the forma- 

 tion of glycogen, and the blood of the hepatic artery witli the secretion of 

 bile ; but there is no adequate support of this view. It must be remembered, 

 moreover, that, in addition to the formation of glycogen and the secretion 

 of bile, other metabolic events, especially affecting proteid or at least nitro- 

 genous constituents of the body, are also taking place ; and to these we 

 must now turn. 



ON UREA AND ON NITROGENOUS METABOLISM IN GENERAL. 



396. 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 subtances are manifesting vital energies as to justify the conclusion 

 that the changes which it undergoes are in some way essential to the mani- 

 festation 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 proteid 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 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 kidney as urea in the blood, the kidneys 

 themselves apparently having no special power of forming urea out of some- 

 thing 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. 



397. In the first place we may take it for granted that the urea carried 

 to the kidney in the blood had an antecedent in something which was not 

 urea. We can hardly suppose that the proteid constituent of living sub- 

 stance, when in the course of its metabolism it ceases to be proteid, breaks 

 up at once into urea and into non-nitrogenous bodies. All we have learned 

 goes to show that what we call metabolism is not a single abrupt change, 

 but consists essentially in a series of changes ; and we may safely conclude 



