THE SENSE OF VISION. 



327 



The fundus of the eye thus observed presents a reddish background on 

 which the retinal vessels are distinctly visible. 



Retina. — Having considered the mechanism by which optical images of 

 objects at various distances from the eye are formed upon the retina, we must 

 next inquire what part of the retina is aifected by the rays of light, and in 

 what this affection consists. To the former of these questions it will be found 

 possible to give a fairly satisfactory answer. With regard to the latter nothing 

 positive is known. 



The structure of the retina is exceedingly complicated, but, as very little 

 is known of the functions of the ganglion cells and of the molecular and 

 nuclear layers, it will suffice for the present purpose of physiological descrip- 

 tion to regard the retina as consisting of fibres of the optic nerve which are 

 connected through various intermediate structures with the layer of rods and 

 cones. 



Fig. 144.— Diagrammatic representation of the retina. 



Figure 144 is intended to show, diagrammatically, the mutual relation of 

 these various portions of the retina in different parts of the eye, and is not 

 drawn to scale. It will be observed that the optic nerve 0, where it enters the 

 eye, interrupts the continuity of the layer of rods and cones R and of the 

 intermediate structures I. Its fibres spread themselves out in all directions, 

 forming the internal layer of the retina N. The central artery of the retina 

 A accompanying the optic nerve ramifies in the layer of nerve-fibres and in 

 the immediately adjacent layers of the retina, forming a vascular layer V. In 

 the fovea centralis F of the macula lutea (the centre of distinct vision) the 

 layer of rods and cones becomes more highly developed, while the other layers 

 of the retina are much reduced in thickness and the blood-vessels entirely dis- 

 appear. This histological observation points strongly to the conclusion that 

 the rods and cones are the structures which are essential to vision, and that in 

 them are found the conditions for the conversion of the vibrations of the 

 luminiforous ether into a stimulus for a nerve-fibre. This view derives con- 

 firmation from the observations on the retinal blood-vessels, for it is found 

 that the distance between the vascular layer of the retina and the layer 

 of rods and cones determined by histological methods corresponds with that 

 which must exist between the vessels and the light-perceiving elements of the 

 retina, as calculated from the apparent displacement of the shadow caused by 

 given movements of the source of light used in studying intraocular images 1 as 

 1 "Dimmer: Verb. d. phys. Clubs zu Wien, 24 April, 1894," Centralbl fiir Physiologic, 1894, 159. 



