1312 RETINAL PIGMENTS. 



If fuchsine is present the wool is stained red. Salicylic acid (ortho- 

 hydroxybenzoic acid, OH. C 6 H 4 . COOH) is excreted partly in an unal- 

 tered form, partly as salicyluric acid, OH . C 6 H 4 . COXH . 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 pyrocat- 

 echin, hydroquinone, etc., the urine turns greenish-brown and finally 

 dark brown on exposure to air. 



RETINAL PIGMENTS. 



1. Fuscin (Retinal melanin). 



This pigment is found as minute granules imbedded in the 

 cell-substance and processes of the retinal epithelium (see 

 576). These granules may be either irregular, as they 

 always are in the choroid, or may, especially as in birds, pos- 

 sess an elongated form with sharply pointed ends distinctly 

 suggestive of a crystalline structure. It is obtained by extract- 

 ing the tissues with boiling alcohol, ether and water, and then 

 digesting for some time with trypsin. The residue is freed 

 from nucleins by dissolving the latter in caustic alkalis and froin 

 neurokeratine (p. 1204) by decantation and straining through 

 fine gauze. The pigment when freshly prepared is practically 

 insoluble in all ordinary reagents, but is partially dissolved if 

 boiled for some time with strong caustic alkalis or sulphuric 

 acid. By prolonged treatment with dilute nitric acid it be- 

 comes soluble in alkalis, yielding yellow solutions. It becomes 

 similarly soluble by prolonged exposure to light with free access 

 of air (oxygen), and may be again precipitated from these solu- 

 tions by the addition of an acid. It is remarkable that not- 

 withstanding its extrema insolubility and resistance to the 

 action of most reagents fuscin is gradually bleached by exposure 

 to light, a result due to some oxidational change, since it only 

 occurs in presence of oxygen. The product to which the above 

 description refers contains much nitrogen, and leaves on in- 

 cineration a slight ash-residue containing traces of iron. 



Later investigations of the pigment (from the choroid and iris) 

 confirm the above statements of its insolubility in most reagents, 

 and further shew that it contains neither sulphur nor iron. The 

 black pigment from hairs is stated to contain less nitrogen and a not 

 inconsiderable amount of sulphur but no iron, and to be readily sol- 

 uble in alkalis. 



2. Lipochrin. 



The fat globules in the retinal epithelium from which this 

 1 The pigments of the retinal epithelium and choroid are apparently identical. 



