250 OBSERVATIONS ON VISION. 



We learn from tlie anatomy oi" the retina tliat in the fovea centralis 

 and a portion of the macnhi hitea cones only are found, rods being 

 entirely wanting, wliile on the ren)ainin_i>- portion of 1lie retina there 

 are both rods and cones, arranged, indeed, in sucli a manner that 

 toward the peripliery the rods outuniber the cones. We know, too, 

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

 fixed upon an object and focused for direct vision. In fixed or 

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

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

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

 organs enter into sharj) comj^etition, and when the light becomes suf- 

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

 everything 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 frequently 

 several rods are attached to a connnon nerve fiber, while the cones — 

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

 nection. This enables us to partiall}^ understand why the stinuda- 

 tion of the rods is felt before that of the cones, especially as they are 

 afi'ected by all light waves while the cones differentiate the waves 

 according to length. It is estimated that we have in all 113,000,000 

 rods and 7,000,000 (;ones (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 mily 1,000,000 nerve fibers, which, 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 e(|ually regular mosaic of ganglion cells in the 

 cortex of the occi})ital lobe of the brain, in the [)articular lobule 

 known as the " ctuieus,'"' where Munk has located the " Sehsphiire,"" 

 or center for cortical vision. The i-etinal elements may therefore be 

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

 of the conducting nerve hbers, the sti'ings in the visual center are set 

 in vibration. Yet the path from the retinal elements to the cuneus 

 is not a direct one, but is sevei'al times interrupted. For exami)le, in 

 (lie aidei'ior i)air of corpora (|uadrigemina the iucoiuiug nerve Hbers 

 bi'cak u}) into numy ranufications, while fhe ])ath leading thence on- 

 ward to the cortex begins (here with similar ramiHcations. The 

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



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

 From tlie Ilandbuch dor Angenlieillcunde of Grai'fe and Siimisch 2, AuH. 1 (5), 

 Berlin, 19U1. 



