CHAP. XL] PERIPHERAL NERVOUS END-ORGANS. 463 



Having given this brief account of the progress of discoveries on 

 the visual purple, a short abstract of all the more important facts 

 which have been brought to light may be given. 



Distribution If the retina of a rabbit or a frog preferably of 



of the Visual one which has been kept for some time in the dark 

 Purple in the k e quickly removed from the perfectly recent eye, in a 

 fcetina. room lighted with the help of a monochromatic yellow 



light, and be taken into the daylight, it will be observed to be of a 

 purple-red colour, which quickly bleaches on exposure. On a closer 

 inspection it will be found that in a horizontal plane cutting the 

 retina the purple colour is more intense, forming a distinct purple 

 band, whilst the macula lutea and a rim 3 4 millimetres broad, at 

 the ora serrata, are devoid of colour. If the retina be examined 

 under the microscope the purple colour will be found to be limited to 

 the rods, and to the outer segments of these, all other parts of the retina 

 looking greenish by contrast. Thus the purple colour varies in fulness 

 directly with the richness of the retina in rods. The more cones, the 

 less visual purple : and vice versa. Hence the absence of purple from 

 the fovea centralis which contains cones only, and its entire deficiency 

 in the rod-less retinae of reptiles. But, although the colour is 

 confined to the outer limbs of the rods, it must not be supposed that 

 every rod is purple. The rods in the neighbourhood of the fovea 

 centralis (viz. in the macula lutea} lack colour, as also do the rods in 

 the colourless margin near the ora serrata. The cause of the greater 

 depth of purple in the horizontal zone previously referred to has not 

 been discovered, as, for instance, whether it is due to a more intense 

 colouration of each rod segment, or to a greater length of the rod 

 segments. 



(11) Ewald u. Kiihne, " Untersuchungen iiber den Sehpurpur. " (Scliluss.) Ibid. 

 Vol. i. part iv. p. 370. 



(12) Kiihne, " Beobachtungen iiber Druckblindheit." Ibid. Vol. n. Part i. p. 46. 



(13) C. Fr. W. Krukenberg, "Ueber die Stabchenfarbe der Cephalopoden. " 

 Ibid. p. 58. 



(14) Kiihne, "Beobachtungen an der frischen Netzhaut des Menschen. " Ibid. 

 p. 59. 



(15) Kiihne, " Fortgesetzte Untersuchungen iiber die Retina und die Pigmente des 

 Auges." Ibid. p. 89. 



(16) Ayres u. Ktihne, "Ueber Regeneration des Sehpurpurs beim Saugethiere." 

 Ibid. p. 215. 



(17) Ewald, "Ueber die entoptische Wahrnehmung der Macula Lutea und des 

 Sehpurpurs." Ibid. p. 241. 



(18) Kiihne, "Zur Abwehr einiger Irrthu'mer iiber das Verhalten des Sehpurpurs." 

 Ibid, p. 254. 



(19) Kiihne, "Notiz iiber die Netzhaut der Eule." Ibid. p. 257. 



(20) Mays, "Ueber das braune Pigment des Auges." Ibid. Heft in. p. 324. 



(21) Kiihne, "Notizen zur Anatomic und Physiologie der Netzhaut." Ibid. p. 

 378. 



The first two papers in the above list were translated from the German by Mrs 

 Foster, edited with notes by Dr Michael Foster and published under the title "On 

 the Photochemistry of the Retina and on the Visual Purple." London, Macmillan and 

 Co.* 1878. 



Kiihne has recently given a systematic- account of his researches under the title of 

 "Chemische Vorgange in der Netzhaut" in Hermann's Handbuch der Physiologie, Vol. i. 

 Part i. (1879) p. 235337. 



