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EXCRETION BY THE SKIN AND KIDNEYS. 



Urea. As regards quantity, and probably as a measure of the activity 

 of the general process of disassimilation, urea CO(NH 8 ) g is the most im- 

 portant of the urinary constituents. Regarding the daily excretion of urea 

 as a measure of the physiological wear of certain tissues, its consideration 

 would come properly under the head of nutrition, in connection with other 

 substances known to be the results of disassimilation ; but it is convenient to 

 treat of its general physiological properties and some of its variations in 

 common with other excrementitious 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 to the supposition that this substance is one of the prod- 

 ucts of the wear of the nitrogenized constituents of the body. It is found, 

 under normal conditions, in the urine, the lymph and chyle, the blood, the 

 sweat, the vitreous humor, and a trace in the saliva. Its presence has been 

 demonstrated, also, in the substance of the healthy liver in both carnivorous 

 and herbivorous animals ; and it has been shown that it exists in minute 

 quantity in the muscular juice (Zalesky). Under pathological conditions, 

 urea finds its way into various other fluids, such as the secretion from the 

 stomach, the serous fluids etc. 



Urea is one of the few organic substances that have been produced artifi- 

 cially. In 1828, Wohler obtained urea by adding ammonium sulphate to a 

 solution of potassium cyanate. The products of this combination are potas- 

 sium sulphate, with cyanic acid and am- 

 monium in a form to constitute urea. 

 Ammonium cyanate is isomeric with 

 urea, and the change is effected by a 

 re-arrangement of its elements. It has 

 long been known that urea is readily 

 convertible into ammonium carbonate ; 

 and ammonium carbonate, when heated 

 in sealed tubes to the temperature at 

 which urea begins to decompose, is con- 

 verted into urea (Kolbe). 



Urea may readily be extracted from 

 the urine, by processes fully described in 

 works upon physiological chemistry; 

 and its proportion may now easily be 

 estimated by the various methods of 

 volumetric analysis. It is not so easy, however, to separate it from the blood 

 or from the substance of any of the tissues, on account of the difficulty in 

 getting rid of other organic matters and the readiness with which it under- 

 goes decomposition. 



When perfectly pure, urea crystallizes in the form of long, four-sided, 

 colorless and transparent prisms, which are without odor, neutral, and in 

 taste resemble saltpetre. These crystals are very soluble in water and in 

 alcohol, but they are entirely insoluble in ether. In its behavior with reagents, 



FIG. 119. Urea crystallized from an aqueous 

 solution (Funke). 



