196 EXCRETION. 



that the weight of evidence is against the carbonate-of- 

 ammonia theory of uraemia. 



Except as regards the probable changes that take place 

 in the process of transformation of certain constituents of the 

 tissues into urea, the chemical history of this substance does 

 not present much physiological interest. Urea may be read- 

 ily extracted from the urine, by processes fully described in 

 all the modern works upon physiological chemistry ; and its 

 proportion may now be easily estimated by the new meth- 

 ods of volumetric analysis. It is not so easy, however, to 

 separate it from the blood or the substance of any of the 

 tissues, on account of the difficulty in getting rid of the other 

 organic matters and the great facility with which it under- 

 goes decomposition. 



When perfectly pure, urea crystallizes in the form of long, 

 four-sided, colorless, and transparent prisms, which are with- 

 out odor, neutral, and in taste something like saltpetre. 

 These crystals are very soluble in water and in alcohol, but 

 are entirely insoluble in ether. In its behavior to reagents, 

 urea acts as a base, combining readily with certain acids, 

 particularly the nitric and oxalic. It also forms combina- 

 tions with certain salts, such as the oxide of mercury, chlo- 

 ride of sodium, etc. It exists in the economy in a state of 

 watery solution, with perhaps a small portion of it modified 

 by the presence of chloride of sodium. 



Origin of Urea. There are two probable sources of 

 urea in the economy, assuming that it always preexists in 

 the blood and is not formed in the kidneys. One of these 

 is in the disassimilation of the nitrogenized constituents of 

 the tissues, and the other in a transformation in the blood 

 of an excess of the nitrogenized elements of food. Urea, as 

 we have already seen, exists in considerable quantity in the 

 lymph and chyle, and is found, also, in small proportion, in 

 the blood. It has lately been detected in still smaller quan- 

 tity in the muscular tissue ; 1 but chemists have thus far been 



1 ZALESKY, loc. dt. Meissner found urea in the muscles, liver, and brain, 



