162 GENERAL PHYSIOLOGY 



that any product of proteid-decomposition originates by the simple 

 cleavage of preformed groups of atoms. 



It is important to become acquainted with the most essential 

 derivatives of the disintegrating proteid molecule. As has been 

 found by the investigation of the substances that are contained in 

 living substance,^ there can be distinguished among these products 

 of proteid transformation two groups — those containing nitrogen, 

 and those not containing nitrogen. Representatives of each group 

 appear in every cell, but their special composition differs in indi- 

 vidual cases according to the characteristic metabolism of the cell. 

 Among the substances that contain nitrogen the most wide-spread 

 are urea, uric acid, hippuric acid, creatin, and the nuclein bases — 

 xanthin, hypoxanthin or sarkin, guanin, and adenin. Regarding 

 the majority of these substances, thus far it is not known how they 

 originate from the decomjDosition of proteids, but for some at 

 least hypotheses concerning their immediate forerunners have been 

 formed. Thus, from the fact, which Schroder discovered, that 

 ammonium c'arbonate introduced into the fresh, excised, still-living 

 liver of a dog leaves the liver as urea, it has been supposed that 

 ammonium carbonate is the forerunner of urea, that from it the 

 liver- cells prepare urea by a transformation of the atoms and the 

 giving-off of two molecules of water : — 



(NHJ.,C03-2H,0 = (NH,),CO. 



But this is not conclusive, it is only a provisional hypothesis, for 

 the possibility is not to be excluded summarily that within the 

 organism itself still other substances are employed for the syn- 

 thesis of urea. With somewhat more certainty we know the 

 forerunner of uric acid, which is that substance in which, in rep- 

 tiles and birds, the greater part of the nitrogen that is derived 

 from the decomposition of proteids leaves the body. Its forerunner 

 is ammonium lactate. From experiments which Gaglio ('86) 

 carried out upon dogs it follows that the lactic acid of the blood 

 is derived from the decomposition of proteid, for the quantity of 

 lactic acid in the blood increases and decreases together with the 

 quantity of proteid food, and is wholly independent of the quantity 

 of ingested carbohydrate. While lactic acid is always found in the 

 blood, under normal conditions no trace of it occurs in the urine ; 

 it must, therefore, undergo transformation before it is excreted. 

 Minkowski('86) made these relations clear by an experiment, in which 

 he showed that geese after the extirpation of the liver excrete very 

 small quantities of uric acid but large quantities of lactic acid and 

 ammonia, both of the latter in the quantitative relations of ammo- 

 nium lactate. From this important fact Minkowski rightly concluded 

 that ammonium lactate is a preliminary stage in the formation of uric 

 acid, from which uric acid arises by rearrangement. We can also 



1 Of. p. 109. 



