250 OBSERVATIONS ON VISION. 



A^> learn from the analoniy of the retina that in the fovea centralis 

 and a portion of the iiiaciila lutea cones only are fonnd, rods being 

 entirely wantin^i', while on the remaining portion of the retina there 

 are both rods and cone«, arranged, indc'ed, in such a manner that 

 toward the periphery the rods outumber the cones. We know, too, 

 that the fovea centralis is the special spot used when the eyes are 

 fixed upon an (jbject and focused for direct vision. In fixed or 

 direct vision (foveal or central vision) the rods therefore take no 

 part; in indirect (peripheral or oblicjue) vision the rods as well as 

 the cones are employed. Thus in feeble illumination the two visual 

 organs enter into sharp competition, and when the light becomes suf- 

 ficiently dim the advantage lies with the color-blind rods, so that 

 everj^thing then appears gray on a gray background — that is, in 

 colorless light. 



B. Recent Anatomical Discoveries Relating to Vision. 



Recent investigations of the retina " show that very frequentl}' 

 several rods are attached to a connnon nerve lil)er, while the cones — 

 at least those of the fovea centralis — have each a special central con- 

 nection. This enables us to partially understand why the stimula- 

 tion of the rods is felt befoi'e that of the cones, especially as they are 

 affected by all light waves wliile the cones difi'erentiate the waves 

 according to length. It is estimated that we have in all 11:1,000,000 

 rods and 7,000,000 cones (of which only 4,000 are in the fovea cen- 

 tralis and 8,000 to 18,000 in the macula lutea), whereas connected to 

 them all there are only 1,000.000 nerve fil)ers, Avhich, though inter- 

 laced in a bundle like a cable, separately convey to the brain the 

 excitations of light. 



Besides this there is, corresponding to the mosaic of the rods and 

 cones of the retina, an equally regular mosaic of ganglion cells in the 

 cortex of the occipital lol)e of tlu' brain, in the particular lobule 

 known as the " cuneus,"' where jNliink has located the " Sehsphiire," 

 or center for cortical vision. The ivtinal elements nuiy therefore be 

 likened to the keys of a piano, by whose means, through the agency 

 of the conducting nerve fibei-s, the strings in the visual center are set 

 in vibration. Yet the [)ath fi'om the retinal elements to the cuneus 

 is not a direct one, but is sevei-al times interrui)ted. For examj^le, in 

 the anterior i)air of corpoi'a (|uadrigemina the incoming nerve fibers 

 break up into nuuiy ramifications, while the path leading thence on- 

 ward to the cortex l)egins there with similar ramifications. The 

 finest branchlets are not, however, so connected as to allow direct 



« See R. Greeff, The microscopic anatomy of the visual nerve and the retina. 

 From the Ilandhuch tier Angenlieillunulo of Graefe iuid Saniiseli 2, Autl. 1 {'>), 

 Berlin, I'JUl. 



