264 RETINAL PIGMENTS. 



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 preparation, all specific 

 statements of differences must be received with caution. Possibly they are all 

 closely allied and probably in some cases, as in the melanaemia of the malarial 

 fever 1 or the melanuria (and melanotic pigmentation) accompanying certain 

 kinds of tumours (p. 258), 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 under 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 hsematoporphyrin. 



2. Lipochrin. 



The fat globules in the retinal epithelium from which this 

 pigment 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.) 



3. Chromophanes 3 . 



These are, as stated above, the colouring substances of the 

 fat-globules which occur between the outer and inner limbs of 

 the retinal cones. They are prepared, as yet chiefly from the 

 eyes of birds, as follows. The retinas are dehydrated with alcohol 

 and extracted with ether. The ethereal solution of the fats is 

 then evaporated to dryness, the residue dissolved in hot alcohol and 

 saponified with caustic soda. The hard coloured soaps thus obtained 

 are then extracted in succession with petroleum ether (see note p. 156), 

 ether and benzol : of these solvents the first dissolves out the yellowish- 

 green chlorophane, the second the yellow xanthophane and the third 

 the red-coloured rhodophane. 



(i) Chlorophane. Soluble in petroleum ether, ether, carbon bisul- 

 phide and in alcohol. When dissolved in the first two of these solvents 

 it shows two absorption-bands between F and G ; in solution in the 

 latter, the two bands lie one each side of F. 



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



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



3 Kiihne and Ayres, loc. cit. and ibid. p. 189. 



