532 MANUAL OF HISTOLOGY. 



first place, it possesses the nature of a decomposition product of the nitro- 

 genous constituents of the body, which is indicated by the production of ben- 

 zoic acid and oil of bitter almonds by the oxidation of albuminous matters. 

 This source, however, is not its greatest, for, after a purely fleshy diet, its 

 amount sinks to a minimum. In the next place, it takes its origin from 

 vegetable food, which yields the un nitrogenous constituent of the acid. 

 Consequently, its amount is much increased in man by a vegetable diet. 

 It is also very abundant in the urine of the herbivora, while again, that 

 of the calf is quite free from it so long as sucking (Wohler). It has been 

 already mentioned that benzoic acid, oil of bitter almonds, cinamic and 

 kinic acids, and oil of tolu, when taken into the stomach, are eliminated 

 by the kidneys as hippuric acid ( 26). 



The nitrogenous part of hippuric acid, which, on combination with 

 water, separates in the form of glycin ( 33), is originally a product of 

 the decomposition* of the glutin-yielding tissues in all probability. We 

 are not yet sure, however, in what way its construction takes place, or, 

 in other words, how hippuric acid is generated. It was supposed, some 

 years ago, that the process was carried on in the circulation of the liver 

 (Kuhne and Hallwachs). It has been shown, on the other hand, more 

 recently by Meissner and Shepard, that the acid is probably formed in 

 the kidney itself exclusively. 



Oxalate of calcium, as already stated, is possibly constantly present in 

 very small quantity in normal urine ; at all events, it appears very com- 

 monly there. The frequent appearance of oxalic acid, also, coincidently 

 with the decomposition of uric acid, is a point of some interest (p. 

 35). Kreatin may likewise play a part here. Of one point, however, 

 we are certain, that oxalic acid may take its origin from vegetable ali- 

 ment. 



Carbolic and taurylic acids (p. 36) are also possibly constant con- 

 stituents of human urine (Staedeler). 



We now come to two substances with all the characters of decomposi- 

 tion products of nitrogenous tissues, namely, of muscle and nervous 

 matter ; we allude to kreatin and kreatinin ( 30). The latter is always 

 to be found in human urine (Neubauer, Munk), in which kreatin may 

 also be present. Both bases are almost invariably to be met with in the 

 urine of dogs (Voit, Meissner). In considering this subject, sufficient 

 weight must be given to the fact that kreatin is converted into kreatinin 

 by the action of acids, whilst the latter may be transformed into the 

 former by contact with alkaline solutions. Their presence, then, in acid 

 or alkaline urine must be judged accordingly. The amount of these two 

 substances increases greatly under an abundantly fleshy diet. Injected 

 also into the blood they are eliminated with the urine (Meissner). In 

 starving animals, likewise, in which combustion of their own muscular 

 tissue is going on, the quantity of both alkaloids is found to rise (Voit, 

 Meissner). Muscular exertion, on the contrary, produces no effect on their 

 generation. It is an interesting fact, that the urine of dogs whose ureters 

 have been ligatured, and which has consequently been secreted under 

 high pressure, contains no urea, but an abundance of kreatin (M . 

 Hermann). 



Xantliin and Jiypoxanthin are likewise present in minute quantity in 

 human urine. The first is also to be found in the renal secretion of dogs 

 after moderate muscular exertion (Meissner}. 



As regards the existence of grape sugar as a normal constituent of urine 



