194 EXCRETION. 



Urea, C 3 H 4 N 3 O 2 . 



As regards quantity, and probably as a measure of the 

 activity of the general process of disassimilation, urea is the 

 most important of the urinary constituents ; and this sub- 

 stance, with the changes which it undergoes in the urine 

 and the mode of its production in the system, has been most 

 carefully studied by physiologists. Regarding the daily ex- 

 cretion of urea as a measure of nutritive force and physio- 

 logical waste, its consideration would come properly under 

 the head of nutrition, in connection with all other substances 

 known to be the results of disassimilation ; but it is more 

 convenient N to treat of its general physiological properties, 

 and some of its variations in common with other excremen- 

 titious principles separated by the kidneys, in connection 

 with the composition of the urine. 



The formula for urea, showing the presence of a large 

 proportion of nitrogen, would lead us to suppose that it is 

 one of the products of the waste of the nitrogen ized princi- 

 ples of the body. It is found, under normal conditions, in 

 the urine, the lymph and chyle, the blood, the sweat, and 

 the vitreous humor. 1 Its presence has lately been demon- 

 strated also in the substance of the healthy liver in both 

 carnivorous and herbivorous animals; 2 and it has further 

 been shown by Zalesky that it exists in minute quantity in 

 the muscular juice. 8 Under pathological conditions, as has 

 been already intimated, urea finds its way into various 



1 MILLON, Presence de Puree dans Phumeur vitree de Fail. Annuaire de chimie, 

 Paris, 1848, p. 431. The discovery of urea in the vitreous humor was con- 

 firmed by Marchand and by Wohler (Ibid., 1849, p. 540). 



8 The presence of urea in the substance of the liver in diseased conditions 

 has frequently been observed, and lately its existence has been conclusively 

 demonstrated in the healthy liver by Meissner. (Eeitrdge zur Kenntniss des 

 Stoffwechsels im thierischen Organismus. Centralblatt fur die mediciniscJien Wis- 

 scnschaften, 1868, No. 18, S. 275.) 



3 ZALESKY, Untersuchungen iiber den Uraemischen Process, Tubingen, 1865, 

 Tabelle iii. 



