262 RETINAL PIGMENTS. 



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 pyrocatechin, liydro- 

 chinon &c., the urine turns greenish-brown and finally dark brown on 

 exposure to air. 



RETINAL PIGMENTS. 1 



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. 258) and called in this 

 casefuscin. 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 closely allied to that of other fats 

 of the body and known under the generic name of lipochronies or 

 luteins. Passing from the epithelium to the retina proper we find in the 

 outer end of the inner limb of the cones highly coloured fat globules 

 from which three distinct pigments known as chromophanes, also 

 belonging to the general class of lipochromes, may be obtained ; to 

 these the names rhodophane, chlorophane and xanthophane have been 

 given in correspondence with their respective red, green and yellow 

 colours. In addition to the above the outer limbs of the rods (not 

 the inner limbs or either the inner or outer limbs of the cones) after 

 the retina has been shielded for some time from the action of light, are 

 found to present a distinct reddish-purple colour which is very marked 

 when the retina is examined as a whole. This colour 2 is due to 

 an exceedingly unstable 3 pigment called by Kiihne 'visual-purple' 

 or rhodopsin. The stability of the above pigments other than visual 

 purple is merely relative not absolute, since they are all sooner or 



1 The following account of these pigments is based upon Kuhne's article in 

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

 Kuhne's Untersuch. a. d. physiol. Inst. zu Heidelberg, 18781882, in which the 

 literature is fully quoted. 



2 First observed in the retina of vertebrates by H. Miiller (1851) and extended 

 by Ley dig in 1857. 



3 The instability on exposure to light was first described by Boll, 1876. 



