448 SPECIAL PHYSIOLOGY. 



the left eye is closed, it will be found, that when the paper is held 

 about four times as far from the eye as the dots are from each other, 

 the right-hand dot will be no longer visible, for its image falls upon 

 the optic eminence. In the same manner, when the image of an ob- 

 ject is made, by artificial means, to fall simultaneously upon both optic 

 eminences, no visual impressions are excited by it. Since this portion 

 of the retina, which is thus deficient in sensibility to light, consists 

 only of the diverging fibres of the optic nerve, all the other elements 

 of the retina being absent, whilst that portion of the retina most sen- 

 sitive to light, viz., the fovea centralis, is destitute of all the retinal 

 elements, except the cones and gray ganglionic nerve vesicles, it would 

 seem that the optic nerve fibres are excited only by the changes in- 

 duced by luminous rays in some other retinal structures, and are them- 

 selves only indirectly excitable by light. The rods and cones alone 

 appear to be the proper recipient organs. The yellow spot, the part 

 most sensitive to light, contains closely-packed cones and ganglionic 

 cells, but is destitute of nerve fibres ; whilst at the sides and anterior 

 parts of the retina, the rods become less numerous, and the sensibility 

 to light is diminished. Moreover, exposure of the optic nerve itself 

 to the strongest light, gives rise to no luminous sensations, and excites 

 no reflex contractions of the iris ; besides this, in the eyes of insects, 

 as we shall hereafter describe, the rods and cones only are exposed to 

 the action of light, the optic nerve fibres being covered by pigment. 

 The rods and cones, or perhaps even their outer and more highly re- 

 fractive segments only, may, indeed, be regarded as the parts which 

 receive the undulatory movements of the luminiferous ether, and 

 modify, or translate them, into nervous energy or force, which then 

 manifests itself, by propagation along certain of the radial nuclear 

 fibres to the ganglionic nerve cells, thence to the plexiform fibres of 

 the retina, through these to the optic nerves and tracts, and finally to 

 the optic sensorial centres. 



It may here be remarked that, whilst the parallel coMear elements, 

 the rods of Corti, receive the sonorous impulses at right angles to 

 their own direction, the parallel retinal elements, the rods and cones, 

 receive the luminous rays in lines corresponding with their own direc- 

 tion. This difference may be connected with the condition, already 

 adverted to, viz., that the supposed movements of the luminiferous 

 ether are transverse to the direction of the imaginary lines called 

 rays, whilst, as is well known, the movements which produce sound, 

 take place in the direction of the sonorous rays. In both the ear and 

 the eye, therefore, the microscopic recipient organs, connected with 

 the extremities of the nerves, are so arranged, that their proper 

 exciting motions do not pass inoperatively between them, but agitate 

 them transversely. 



In the retina of Man and of the Vertebrata, all the light must 

 pass through the nerve-fibres, ganglionic cells, and bloodvessels, and 

 also through the granular layer, before it reaches the rods and cones, 

 or true excitable elements. The existence of these last-named struc- 

 tures appears indispensable to distinct vision ; their outer free ends 

 form a mosaic surface, on which local points of light fall evenly, and 



