CHEMICAL BASIS OF THE ANIMAL BODY. 263 



later destroyed (bleached) by sufficiently prolonged exposure to light. 

 The possibilities hereby suggested of a photochemical explanation of 

 retinal excitation have however as yet thrown no real light on the 

 nature of the process. It may be that the impulses result from the 

 changes which these pigments undergo, and it is possible that the 

 coloured globules of the cones play a part in the whole process not 

 merely by the instability of their colours but also by acting as coloured 

 though transparent screens, and thus at the same time determining the 

 advent to the photochemical apparatus of rays of certain wave-length 

 only. Such speculations are interesting but for the present devoid of 

 any decisive experimental support ( 773). 



1. Fuscin (Retinal melanin) 1 . 



This pigment is found as minute granules imbedded in the cell- 

 substance and processes of the retinal epithelium (see 746). These 

 granules may be either irregular, as they always are in the choroid, or 

 may, especially as in birds, possess an elongated form with sharply 

 pointed ends distinctly suggestive of a crystalline structure. It is 

 obtained by extracting 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 from neuro- 

 keratin (p. 86) 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 becomes 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 solutions by the addition of an acid. It is remarkable that 

 notwithstanding its extreme 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 con- 

 tains much nitrogen, and leaves on incineration 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 show 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 soluble in alkalis 2 . When the several substances described 

 under the general term melanins are compared each with the other it is found that 



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



2 Sieber, Arch. f. exp. Path. u. Pharm. Bd. xx. (1886), S. 362. 



