CHEMICAL BASIS OF THK ANIMAL I:<>1>\ 



closely allied to that of other fata of the body ami ku<>\vn under 

 tin- ^eiienc name of lipochromes or luteins. Pas- the 



epithelium tn tin- ivtina proper we lind in tin- outer end of the 

 inner liiul> iif the cones highly coloured fat globules from \vlii-h 

 three distinct pigments known as chroniophanes. also belon^in^' 

 to the u'eneral class of lipochromes, may be obtained 

 names rli'H/ojifniiif, chlorophanf,;(\\<\ ' nthujihn," 

 in correspondence with their ivsp-ctive red, ^n-i-n. ami yellow 

 colours. In addition to the above the outer limits ot tin 

 the inner limbs or either the inner or outer limits of the . 

 after the retina has lieen shielded for some time from the ;i< tion 

 of liidit, are found to present a distinct reddish-purple colour 

 which is very marked when the retina is examined as a \\li.-le. 

 This colour 1 is due to an exceedingly unstable- pigment <-alled by 

 Kuhne ' visual-jturple' or rhodopsin. The staltility of the. 

 piiMuents other than visual-purple is merely relative not al. solute, 

 since they are all sooner or later destroyed 1. leached l.y suffi- 

 ciently prolonged exposure to li^'ht. The powibilitii 



sted of a photochemical exjtlanation of retinal excil 

 have however as yet thrown no real lij:ht on the nature of the 

 process. It may be that the impulses result from the changes 

 which these pigments undergo, and it is psilile that the col 

 ^loltules of the cones play a part in the whole proce-. not merely 

 I iy the instability of their colours but also l.y acting as eoloun-d 

 tlioui.'li traiisj.arent screens, and thus at the same time determin- 

 ing the advent to the photochemical apparatu- 



leii-th only. Such speculations are interesting luit fir the 

 devoid of any decisive exjK-rimental Mipport >; 7 



1. Fuscin (Retinal melanin). 3 



This- ].iL r iiient is found as minute granules iml.edded in the cell- 

 sulistance and jirocesses of the retinal epithelium (see 

 Th'--e -ninnies niav lie either irr.-i.Mi la r. as they always are in the 

 ehoroid, or may, especially as in turds, possess an elongated form 

 with sharply pointed ends distinctly su^'u'->-tiv illine 



structure. It is obtained by -\t ra.-t in-j l! vsith boiling 



alcohol, eth.-r, and water, and then di- MIC time with 



tryp-in. The residue is fr ..... 1 from nin-I.-iiis by di--o Ivin^ the 

 latter in caugtk alkalis. and from neiiiokerat in (p. S7i bv 

 tion an<l straining through fine The pigment v\h' 



pivpar.'d is practically insoluble in all ordinal 

 partially dissolved if'boiled for some time with stioi; 

 alkalis 'or sulphuric acid. I'.v prolonged tieatment with dilut- 



i First ..hsri-voil id tin- n-tina ( * : ' II \\<>'.' 1 - : < t*od 



l>v I.i-\ <\\ in I - 



iiiM.-it.ili! . IN t.. liiilif w:v t: 



8 The- j.i.i:iii.-:it-; ..f :!,. i- :n.:il r|iitli-limn :nnl <-tioroid r mppamitly ulontlcal. 



