776 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



from those which perform the same function for the cones. Whatever be the 

 precise mode of connection between the rods and cones and the fibres of the 

 optic nerve, it is evident that each retinal element cannot be connected with 

 the nerve-centres by a separate independent nerve-channel, since the retina 

 contains many millions of rods and cones, while the optic nerve has only 

 about 438,000 nerve-fibres, 1 though of course such a connection may exist in 

 the fovea centralis, as Cajal has shown is probably the case in reptiles and birds. 

 -Changes Produced in the Retina by Light. We must now inquire 

 what changes can be supposed to occur in the rods and cones under the influ- 

 ence of light by means of which they are able to transform the energy of the 

 ether vibrations into a stimulus for the fibres of the optic nerve. Though in 

 the present state of our knowledge no satisfactory answer can be given to this 

 question, yet certain direct effects of light upon the retina have been observed 

 which are doubtless associated in some way with the transformation in 

 question. 



The retina of an eye which has been protected from light for a considerable 

 length of time has a purplish-red color, which upon exposure to light changes 

 to yellow and then fades away. This bleaching occurs also in monochromatic 

 light, the most powerful rays being those of the greenish-yellow portion of 

 the spectrum i. e. those rays which are most completely absorbed by the pur- 

 plish-red coloring matter. A microscopic examination of the retina shows 

 that this coloring matter, which has been termed visual purple, is entirely con- 

 fined to the outer portion of the retinal rods and does not occur at all in the 

 cones. After being bleached by light it is, during life, restored through the 

 agency of the pigment epithelium, the cells of which, under the influence of 

 light, send their prolongations inward to envelop the outer limbs of the rods 

 and cones with pigment. If an eye, either excised or in its natural position, 

 is protected from light for a time, and then placed in such a position that the 

 image of a lamp or a window is thrown upon the retina for a time which may 

 vary with the amount of light from seven seconds to ten minutes, it will be 

 found that the retina, if removed and examined under red light, will show the 

 image of the luminous object impressed upon it by the 

 bleaching of the visual purple. 



If the retina be treated with a 4 per cent, solution of 

 alum, the restoration of the visual purple will be pre- 

 vented, and the so-called " optogram " will be, as pho- 

 tographers say, " fixed." 2 



K ? ure 236 shows the W^ance of a rabbit's retina 

 on which the optogram of a window has been impressed. 

 Although the chemical changes in the visual purple under the influence of 

 light seem, at first sight, to afford an explanation of the transformation of the 

 vibrations of the luminiferous ether into a stimulation for the optic nerve, yet 

 the fact that vision is most distinct in the fovea centralis of the retina, which, 



1 Salzer : Wiener Sitzungsberichte, 1880, Bd. Ixxxi. S. 3. 



2 Kiihne : Untersuchungen a. d. phys. Inst. d. Universitdt Heidelberg, i. 1. 



