CHEMICAL BASIS OF THE ANIMAL BODY. 1263 



usually employed consist in its precipitation with nitrate of mer- 

 cury or silver. From the urine of calves or from their allantoic 

 fluid, allantoin may usually be obtained in crystals by mere con- 

 centration and subsequent standing till crystallization occurs. 



Allantoin may be easily obtained by the careful oxidation 

 of uric acid with potassium permanganate. 



As prepared artificially it crystallizes readily in large pris- 

 matic hexagonal crystals. 



Fig. 216. Crystals of Allantoin prepared by the oxidation of Uric 



Acid. (After Ktikne.) 



In addition to the crystalline form and precipitability with 

 nitrates of mercury and silver, allantoin is further characterized 

 by yielding Schiff's reaction with furfuraldehyde (see above, 

 p. 1257, sub urea), but less readily and with less intense colora- 

 tion than does urea. It also reduces Fehling's fluid on pro- 

 longed boiling. 



THE XANTHINE GROUP. 



This group comprises a number of substances closely related 

 to uric acid and to each other. Some of them occur in small 

 amounts in the tissues (muscles) and excretions (urine) of the 

 body and are to be regarded as being, like urea and uric acid, 

 typical products of the downward destructive metabolism of 

 proteids. They are also obtained as products of the decompo- 

 sition of the true nucleins when boiled with acids. Some of 

 them are closely related to certain alkaloids which occur in 

 plants (theobromine and caffeine), and which probably play 

 some not unimportant part in the nutritional change of the 

 animal body, since they are constantly consumed, in some form 

 or other, by the larger part of the human race. This relation- 

 ship of the xanthine-bodies to certain vegetable alkaloids is 

 further interesting when it is remembered that the latter are 



