METABOLISM. 475 



the serum-albumin and serum-globulin is constantly of the same com- 

 position. It is inferred that the amino-acids and polypeptids of the 

 food-proteid are synthetically built up into proteid suited to that par- 

 ticular kind of animal. The serum-proteids are independent in com- 

 position of the kind of proteid-food for a horse fed with gliadin, five 

 times richer in glutaminic acid than serum-proteid, yet its serum- 

 proteid had the normal amount of glutaminic acid even when the 

 animal had been bled profusely and must generate a large amount of 

 serum-proteid. Hence the intestinal wall and the ferments have the 

 important function to always furnish the same proteids to the tissues. 

 It is evident that not all proteids can have the same food value to the 

 animal body and their value can not be determined by their content of 

 nitrogen. A food-proteid that yields very little serum-proteid when 

 its amido-acids are resynthesized in the intestinal wall can not replace 

 in equivalent amounts a food-proteid that yields amido-acids in more 

 nearly the same proportion in which they are found in serum-proteids. 

 Hence, if a certain food-proteid contains very much less of an amino- 

 acid than the serum-proteid does, then only a correspondingly small 

 amount of its other amino-acids can be used in the building up of 

 tissues. It is probable that the tissue cells have some selective power, 

 as the proteids in these cells of the tissues have a certain proportion of 

 the various amido-acids different from those of the serum-proteids. 

 The portal circulation carries to the liver the specific serum-proteids 

 with polypeptids, amido-acids and ammonia. The blood going to the 

 liver also conveys indol, skatol, phenol and cresol, products of 

 microbian digestion which go to the liver to form the conjugated sul- 

 phates. The liver also forms glycuronates, as indoxyl, skatol and 

 phenol glycuronates, which, with the ethereal sulphates, are carried to 

 the kidney and excreted. 



Leathes states that the two main products of proteid metabolism, 

 urea and carbonic acid, are to a great extent produced independently 

 of each other, and the reactions which result in the discharge of nitro- 

 gen are not those in which energy is set free, work done and carbonic 

 acid produced. This being so, it is plain that proteid metabolism, in 

 so far as it is concerned with evolution of energy in its exothermic 

 stages, may be almost entirely nonnitrogenous metabolism. 



We can not take the excretion of urea as a measure of proteid 

 metabolism, because a large part of it is formed from nitrogen that 

 has never penetrated the body beyond the liver. 



The body is able to interchange the proteids of the tissues which 

 are distinctly different; hence it can build specific proteid out of the 



