262 LIPOCHRIN. 



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 contains much nitrogen, and leaves on incinera- 

 tion a slight ash-residue containing traces of iron. 



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

 firm 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 incon- 

 siderable amount of sulphur but no iron, and to be readily soluble in 

 alkalis. 1 When the several substances described under the general 

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

 are by no means identical, but in the absence of any guarantee of the 

 purity of each product or of the absence of change during its prepara- 

 tion, all specific statements of differences must be received with caution. 

 Possibly they are all closely allied and probably in some cases, as in 

 the melamemia of the malarial fever 2 or the melanuria (and melanotic 

 pigmentation) accompanying certain kinds of tumours (p. 256), they 

 are derived from the colouring-matter of the blood. The divergence 

 in views as to their derivation from haemoglobin has apparently turned 

 in many cases on the presence or absence of iron in the pigments un- 

 der examination. Some of the melanins may contain iron, some none, 

 but whether they do or do not is not a decisive test of their derivation. 

 If they do it makes the connection more probable, if they do not they 

 may still take their origin from blood-pigments, as in the case of the 

 highly coloured but iron-free haematoporphyrin. 



2. Lipochrin. 



The fat globules in the retinal epithelium from which this pig- 

 ment is obtained are more especially abundant in the frog. It is 

 soluble in chloroform, ether, benzol, carbon bisulphide, &c. When 

 dissolved in ether it gives two absorption bands between F and G ; 

 in carbon bisulphide two bands, one each side of F? The pigment 

 of the body-fat of frogs gives similar absorption spectra when 

 dissolved in the same solvents. Solutions of lipochrin are slowly 

 bleached by exposure to a strong light. The pigment is probably 

 closely allied to the yellow colouring-matter of many other animal 

 fats. (See below sublutein.) 



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



2 For references see Gamgee, Physiol. Chem. Vol. i. (1880), p. 162. 



3 See Kiihne and Ayres, Jl. of Physiol. Vol. i. (1878), p. 109. 



