CARBOHYDRATES AND RELATED SUBSTANCES. 607 



detected, and Tcynurenic acid, a related substance, is an important consti- 

 tuent of dog's urine. 1 



Pathologically, the substances just described may become of considerable 

 importance. In carbolic acid poisoning many of them are excreted in greatly 

 increased amount; pyrocatechin and hydrochinon may be present in large 

 quantity, and then give rise, by their oxidation, to the peculiar coloration seen 

 in carboluria. In certain diseases other members of the group are increased, 

 and give rise to the phenomena of alcaptonuria. In this state the urine 

 develops, on standing, a dark colour, like that seen in carboluria ; and 

 Boedecker in 1861 isolated a substance which he termed alkapton, to the 

 oxidation of which he held the colour due. Alkapton was shown by Marshall 

 and Kirk to be impure uroleucic acid (vide supra). This latter substance, 

 however, is not wholly responsible for the coloration phenomenon. Wolkow 

 and Baumann 2 have recently shown that in a case investigated by them the 

 " alkaptonuria " was almost wholly due to the presence of homogeiitisic acid 

 (supra). Pyrocatechin is doubtless sometimes the cause. 3 Most hydroxy- 

 derivatives of benzene in alkaline solution develop a dark coloration on 

 exposure to the air 4 (cf . p. 630). 



The quantity of phenol and kresol in the urine is increased in extreme 

 constipation, in obstruction of the lower bowel, in peritonitis, and in pyaemia 

 (cf. indoxyl, infra, and p. 631). 



Indoxyl and skatoxyl. These, although nitrogenous compounds, 

 are closely related to the substances just treated, and may fitly be 

 considered here. 



Indoxyl (C ? H 4 .NH.CH.C.OH). The so-called urinary indican is 

 indoxylsnlphuric acid. In normal urine on a mixed diet, the quantity 

 present is only from 5 to 20 mgrins. In herbivora the quantity 

 is much larger. It is absent from the urine of new-born children 

 (Senator). Indoxyl is derived from, oxidation in the body of the indol 

 absorbed from the bowel, and its amount is increased, like that of the 

 urinary phenols, by all causes which lead to increased bacterial decom- 

 position of proteids, in the intestine or elsewhere ; and by circumstances 

 which favour the absorption of the indol when formed (intestinal 

 obstruction, etc.). Skatoxyl (C 9 H 8 NOH) is derived from skatol (methyl- 

 indol), and accompanies indoxyl into the urine by parallel paths and 

 from kindred causes. Like indoxyl, it is present as a conjugated 

 sulphate. 



By oxidation indoxyl forms indigo-blue and indigo-red, while skatoxyl 

 similarly yields red pigments. The consequent colour phenomena 

 which arise in the urine are discussed under the head of the pigments. 



CARBOHYDRATES AND BELATED SUBSTANCES. 



Normal urine contains small quantities of certain carbohydrates. 

 Under ordinary circumstances the physiological limit extends only to 

 a minute quantity of any one of these substances ; but, in the urine of 

 women during lactation, milk-sugar may occur in very considerable 

 amount, without departure from what must be considered physiological 

 conditions. 



1 For phenaceturic acid, see under hippuric acid. 2 Loc. cit. 



3 Cf. v. Jaksch, "Kliuische Diagnostik," 4th edition, 1896, p. 415. 



4 The behaviour of the alkaline solution of pyrogallic acid used in photography will 

 be an example familiar to many. 



