MANUAL OF HISTOLOGY. 



of the fluid excreted, and is carried out of the body daily in con- 

 siderable quantity. It is found likewise in the blood in very minute 

 quantity (Strahl and Lieberkuhn, LeJimann, Verdeil, and Dollfuss), and 

 in the chyle and lymph of mammals (Wurtz). It is also stated, but 

 with uncertainty, by Millou, to be present in the aqueous humour of 

 the eye. Further, it has been met with in the brain of the dog (Staedeler), 

 and in normal sweat, according to Favre, Picard, and Funlte. Under 

 certain diseased conditions it may appear very widely distributed through- 

 out the system. 



Urea, like all allied substances, a decomposition product, and unfit, 

 owing to its solubility, to take part in the formation of tissues springs, 

 as we know by experience, from the protein compounds of the system ; 

 from those albuminous substances entering into the constitution of 

 tissues, as well as those received into the blood from the food, and super- 

 fluous there. Thus the amount of urea in the body is increased by 

 muscular exertion and abundant fleshy diet ; the introduction of many 

 alkaloids into the stomach has the same effect, as, for instance, thein, 

 glycin, alloxantin, and guanin. Finally, the injection of uric acid into 

 the circulation causes an augmentation in the amount of urea excreted 

 with the urine ( Wohler and Frerichs). 



In regard to detail, we are still in the dark as to the formation of urea 

 in the body. We do know that it is a decomposition product of the 

 protein compounds, and also that almost all the nitrogen which leaves 

 the system passes out in this way ; and yet, on the other hand, as to the 

 chemical mutation series whose end factor is urea, we are in possession of 

 but few facts. Two points, however, may be alluded to as throwing 

 some light upon the origin of the substance in question, namely, that 

 kreatin, a mutation product of the protein compounds, splits up into 

 sarkosin and urea under the action of alkalies ; and again, urea may be 

 obtained from guanin, among other substances, by treatment with oxidiz- 

 ing reagents (Strecker). But in this respect the presence of uric acid 

 is probably of greater importance as a source of urea in the system urea 

 being one of the usual decomposition products of the same, derived from 

 its oxidation. 



29. , 



We turn now to three substances closely allied one to the other, to 



be regarded as members of a mutation 

 series of histogenic matters, and which 

 possibly lead to the formation of uric 

 acid in their further physiological 

 transformation. 



They are compounds very insoluble 

 in water, but which dissolve readily 

 in alkalies and acids, forming with 

 the latter crystalline salts which are 

 partly decomposed by water. All 

 three evaporated with nitric acid, 

 form yellow substances which, on 

 the addition of potash without heat, 

 assume a red colour, turning to a 

 lively purple on the temperature 



Fig. 22. Crystals of chlorate of guanin. being raised. 



