122 LESLIE B. AIIEY 



pass, those obtMiued from tho two other branches of retinal physiol- 

 ogy, namely, the study of the visual purple and of the action 

 currents in the optic nerve. 



In order that the reader may understand the present status 

 of this subject, it is desirable to summarize briefly the general 

 conclusions that have been established relative to the efficiency 

 of light in producing movements in the retinal elements. For 

 a more extensive review of this literature reference may be made 

 to a separate paper by the writer (Arey, '15) or to the excellent 

 compilation of the German author, Garten ('07). 



Although a variability in the position of the retinal pigment 

 had been noted by early workers (H. Miiller, '56; Morano, '72) 

 the cause remained unsuspected until Boll ('78) and Kiihne 

 ('77) independently discovered that in the light the retinal pig- 

 ment of the frog extends nearly to the external limiting mem- 

 brane whereas in darkness it retreats, thereby forming a com- 

 pact layer next to the choroid^ (figs. 1 and 4). Later obser- 

 vations have corroborated Kiihne 's original view that pigment 

 migration is not due to the extension and retraction of cell proc- 

 esses but to the movement of pigment granules in the proto- 

 plasm of relatively fixed cells. 



The most extensive pigment migration occurs in fishes (Stort 

 '86) and in anuran amphibians (Boll, '78, and Kiihne '77, on 

 the frog; Arcoleo, '90, on the toad), whereas the positional changes 

 in the pigment of urodeles are relatively limited (Angelucci, '78). 

 Well defined movements of the retinal pigment are also found 

 among birds, including not only those that are diurnal, but also 

 some that are nocturnal in habit. Among reptiles (Angelucci, 

 '90) and mammals (Angelucci, '78) limited pigment changes 

 have probably been detected, notwithstanding the contradictory 

 evidence presented by various workers. 



' Czerny ('67) found that when sunlight was concentrated by a lens on the 

 retinas of various animals, the pigment became more highly expanded in the 

 affected regions than in other portions which had been exposed to light but 

 not so treated. In his experiments no comparisons were made with dark-adapted 

 retinas. These somewhat pathological tests were not well substantiated by the 

 later work of Deutschmann ('82). 



