ELEMENTARY PHYSIOLOGICAL CHEMISTRY 253 



bases are liberated. These are chemically closely related to urea, and 

 if they be given in the food they at once lead to an increase in the 

 urea excretion. It is almost certain, however, that no such process 

 occurs physiologically since these bodies cannot be detected in the 

 blood, so that, although large amounts of them can certainly be 

 obtained in an artificial tryptic digest, it is probable that, in the 

 intestine, the bulk of the proteid is at once absorbed when it reaches 

 the stage of peptone, and that only a trace is further hydrolysed into 

 these bodies. 



The absorbed serum albumin and globulin are carried to the 

 active tissues (i.e. muscles, glands, and nervous tissues) where they 

 undergo complicated changes leading to the production of disintegrative 

 products, among which is creatin. It will be remembered that this, 

 body can be very easily changed into urea in the laboratory (see p. 205), 

 and that it exists in the tissues in very large amount, whereas the 

 urine only contains a small quantity (as creatinine) (see p. 261). 

 Many experiments have accordingly been made to study how the 

 urea excretion behaves on administration of creatin as food, or after its 

 injection into the blood. It has invariably been found that the urea 

 is not increased in the slightest, but that all the administered creatin 

 reappears as urinary creatinin. It is possible, however, that this- 

 foreign or exogenous creatin is not quite the same thing as the creatine 

 naturally produced by the tissues, i.e. endogenous, and that, whereas 

 the latter becomes changed into urea, the former does not undergo- 

 this change. 



A third probable precursor of urea is ammonium salts. If 

 ammonium carbonate or citrate be given by the mouth, the urea- 

 excretion at once rises. These salts have also been found in small 

 quantities in the blood, more especially in that of the portal vein. 

 The chemical transformation is a very simple one (see p. 248), and 

 since they are found most abundantly in the portal blood, it is- 

 probable that before the amido acids or hexone bases, absorbed from 

 the intestine, are transformed into urea, they are split up into ammonium 

 carbonate and a residue containing most of their carbon, and which 

 is further made use of by the tissues to produce energy, or to 

 form fat. 



From one of the hexone bases, viz. arginine, urea can be very easily 

 obtained by boiling with baryta. Now arginine is chemically very 

 like creatin, one point of difference being that the former, when 

 administered as food or injected into the blood, causes an increase 

 in urea, the latter does not. 



There can be no doubt, then, that amido acids and hexone bases, and 



