CHEMICAL BASIS OF THE ANIMAL BODY. 155 



THE UREA AND URIC ACID GROUP. 1 

 1. Urea. (NH 3 ),CO. (Carbamide). 



This is tin- chief nitrogenous constituent of normal urine in 

 mammalia and some other animals. The urine of birds also con- 

 tains a small amount, more particularly on a meat diet Average 

 normal human urine contains from 2'5 3 - 2 p.c., the average total 

 daily r.\rivti<>n varying from 22 35 grams or as a mean 30 grams. 

 It is also found in minute quantities in normal blood 2 (-OHf* p.r. ) 

 serous fluids, lymph, and aqueous humour: it is not usually met 

 with in the tissues except that of tin- liver. 3 It is never present 

 in normal mammalian muscles, but may make its appearance there 

 under certain pathological conditions. Under ordinary conditions 

 the amount of urea in sweat is almost inappreciable, but the older 

 statements of its occurrence in this excretion have recently received 

 continuation, and it appears that this source of nitrogenous loss to 

 tin- body may have to be taken into account 4 



FIG. 14. UREA CRYSTALS SEPARATED BY SLOW EVAPORATION FROM 

 AQUEora SOLUTION. (After Funkc.) 



When pure it crystallises from a concentrated solution in t In- 

 form of IOIIL:, thin glittering needles. If deposited slowly from 

 dilute solutions, the form is that of four-sided prisms with pyrami- 

 dal ends; these are always anhydrous. When tin- separation 

 rapidly, as for instance from a strong alcoholic solution on 

 --slide, the typical crystalline form is not readily obwned. 

 but rather that of irregular dendritic crystals. 



Urea is very soluble in mid water, distinctly less soluble in cold 

 alcohol, readily so in hot ; it is insoluble in anhydrous ether and 



1 For full details of tin- NMtfaM, pMOOtiM, Md methods of detcriniiiini: and 



dealing practically with tin- in<'inl>Ts of this rii]i, consult in all cawH NmhaiuT 11. 



.[ti>ili/xf il>ti Ifnrna. Salkow.ski U. Letlbe, Die I.thr- mm Hum. Hoppc 



r. /'/ii/xio/. -;;///. rfiiin. Antllt/Kt. 



' d II ' tw!g, 1 -"l 



Hut ne ii..|,|,,- s,.yi,T, /.i. t : ,,/i /././. r< nssi), S.348. 



ArpUinsky, 1'tlutrVr's Ar.-l,. H,| XLVI. (18i0). S. 594. 



