EXCRETION BY THE KIDNEYS 



475 



The purin bases (sometimes called the nuclein bases, the alloxuric 

 bases, or the xanthin bases) are a group of substances allied to uric 

 acid, and including, besides xanthin itself, hypoxanthin, guanin, adenin, 

 and other bodies. They exist in very small amount in urine, but, like 

 uric acid, are increased in amount by the ingestion of nuclein-contain- 

 ing substances. The greater part of the purin bases produced in the 

 body is transformed into uric acid ; it is only the untransformed residue 

 which appears in the urine. An interesting fraction of the purin bases 

 in the urine which is not related to the nuclein metabolism is composed 

 of the so-called heteroxanthin, derived from caffeine in the .coffee and 

 tea, /-methylxanthin, derived from theobromine in the cocoa, and para- 

 xanthin, derived from theophyllin in the tea, consumed as beverages. 



Hippuric acid (C 9 H 9 NO 3 ) occurs in considerable quantity in the urine 

 of herbivora (Practical Exercises, p. 516); in the urine of carnivora and 

 of man only in traces; in that of birds not at all. Its amount is much 

 more dependent on the presence of particular substances in the food 

 than that of the other organic constituents of urine. Anything which 

 contains benzoic acid, or substances which can be readily changed into 

 it (such as cinnamic and quinic 

 acids), causes an increase of the 

 hippuric acid in urine. In fact, 

 one of the best ways of obtaining 

 the latter is from the urine of a 

 person to whom benzoic acid is 

 given by the mouth; the sweat 



Fig. 183. Kreatin. 



Fig. 184. Kreatinin-Zinc-Chloride. 



may also in this case contain a trace of hippuric acid. Chemically it 

 is a conjugated acid formed by the union of benzoic acid and glycin. 



Amino-Acids. The only amino-acid hitherto detected with certainty 

 in normal urine is glycin. 



Oxalic acid is always present, although in very small amount. Some 

 of it comes from the oxalates of the food, but a portion of it arises in 

 the metabolism of the tissues, probably from the decomposition of uric 

 acid. It is known that outside of the body uric acid may be made to yield 

 oxalic acid. Ca cium oxalate crystals are often seen in urinary sediments. 



Kreatinin (C 4 H 7 N 3 O) . Kreatinin is the anhydride of kreatin 

 (Fig. 183). Its formula differs from that of kreatin only in possessing 

 the elements of one molecule of water less; and kreatinin can be 

 obtained by boiling kreatin with dilute sulphuric acid. From its 

 alcoholic solution it crystallizes in colourless prisms. Kreatinin forms 

 crystalline compounds with various acids and salts. One of the best 

 known of these is kreatinin-zinc-chloride, formed on the addition of 

 zinc chloride to an alcoholic cr watery solution of kreatinin, often in 

 the shape of beautiful thick-set rosettes of needles (Fig. 184). A por- 



