260 KETINAL PIGMENTS. 



is initially acid, it turns red on the addition of an excess of 

 alkali. 1 After the internal administration of copaiba, the urine 

 turns pink or rose-coloured on the addition of hydrochloric acid 

 and shows three absorption bands, one (narrow) in the orange to 

 the red side of D, one broad band in the green between D and E, 

 similar to that of fuchsin, and one in the blue. 2 Tannin leads to 

 the appearance in urine of gallic acid [C 6 H 2 . (OH) 8 . COOH], which 

 is hence sometimes found normally in the urine of herbivora 

 (horse). 3 In such cases the urine if made alkaline with caustic 

 potash turns brown, and bluish-black on the addition of ferric 

 chloride. It also yields a pink colouration with Millon's reagent, 

 similar to that given by proteids or tyrosin. After doses of anti- 

 pyrin [C 9 H 6 N 2 (CH 3 ) 2 ] the urine may be dark-coloured and gives 

 a brownish-red colour on the addition of ferric chloride. 4 Fuchsin 

 (hydrochloride of rosaniline C 20 H 19 N 3 . HC1) reappears partly un- 

 changed in the urine, to which it imparts a reddish tinge. It is 

 detected by making the urine alkaline with ammonia and shaking 

 with an equal volume of ether : the latter extracts the colouring 

 matter and into the solution thus obtained a thread of white wool 

 is dipped and allowed to dry spontaneously. If fuchsin is present 

 the wool is stained red. Salicylic acid (ortho-oxybenzoic acid, 

 OH . C 6 H 4 . COOH) is excreted partly in an unaltered form, partly 

 as salicyluric acid, OH . C 6 H 4 . CONH . CH 2 . COOH. These may 

 be detected by the intense violet colour they yield on the addition 

 of ferric chloride. Finally after the absorption of carbolic acid 

 (phenol) and many other aromatic compounds such as pyrocate- 

 chin, hydrochinon, &c., the urine turns greenish-brown and finally 

 dark-brown on exposure to air. 



KETINAL PIGMENTS. 5 



The pigments which have to be considered under this heading 

 are numerous. There is in the first place the extremely stable 

 dark-brown colouring-matter of the retinal epithelium, belonging 

 to that general class of pigments known as melanins (see p. 256) 

 and called in this case fuscin. In addition to this the retinal 

 epithelium of some animals contains a not inconsiderable amount 

 of fat globules whose yellow colour is due to lipochrin, a pigment 



1 For discrimination of these see Munk, Virchow's Arch. Bd. LXXII. (1878), 

 S. 136. 



2 Quincke, Arch. f. exp. Path. u. Pharm. Bd. xvn. (1883), S. 273. 



3 Baumann, Zt. f. physiol. Chem. Bd. vi. (1882), S. 193. 



* Umbach, Arch.f. exp. Path. u. Pharm. Bd. xxi. (1886), S. 161. 



6 The following account of these pigments is based upon Kiihne's article in 

 Hermann's Hdbch. d. Physiol. Bd. in. Thl. 1. 1879, and on the original papers in 

 Kiihne's Untersuch. a. d. physiol. Inst. zu Heidelberg, 1878 1882, in which the 

 literature is fully quoted. 



