SENSE OF SIGHT, 619 



The con es differ from the rods mainly in their tapering form and the 

 greater diameter of their internal portion, which, as a general rule, is 

 from two to three times as thick as that of the rods. They have the 

 same transparent, highly refractive appearance, and are intercalated 

 among the rods in the same position, that is, perpendicularly to the 

 surface of the retina. Their outer extremities, in some regions, stop 

 short of the external surface of the retina, while in others, particularly 

 in that of most perfect vision, they reach the same level with the ends 

 of the rods. Each cone is connected at its inner extremity with a nu- 

 cleated fibre belonging to the preceding layer, the only difference in this 

 respect being that both the fibres and the nuclei connected with the 

 cones are larger than those connected with the rods. 



Over the greater part of the retina the rods are more abundant than 

 the cones. When viewed from the external surface (Fig. 195, J), their 

 closely packed extremities present the appearance of a fine mosaic pat- 

 tern, while the cones are interspersed among them in smaller numbers. 

 At the borders of the macula lutea (p. 623), on the other hand, the 

 cones are more abundant, being only separated from each other by single 

 ranges of rods (-S); and at its central portion (G) there are only cones, 

 the rods being entirely absent. The cones at this point are also longer 

 and more slender than elsewhere. The following figure indicates the 

 appearance of the rods and cones, as shown in an external view of 

 different parts of the retina. The smaller circles represent the rods, 

 the larger circles the cones. In the interior of each cone is seen the 

 section of its conical extremity. 



OUTER SURFACE OF THE RETINA, showing the ends of the rods and cones. A. From 

 the lateral portion of the eyeball. B. From the posterior portion, at the edge of the macula 

 lutea. C. From the macula lutea. (Helmholtz.) 



Beside the distinctly marked layers above described, there are vari- 

 ous others of less certain signification and less uniformity of extent, 

 which are found in different parts of the retina. Throughout the mem- 

 brane there also exists a certain proportion of delicate connective tissue, 

 which serves for the support and attachment of its remaining anatomi- 

 cal elements. 



Perception of Luminous Impressions by the Retina. It appears, 

 from the description given above, that the retina is not simply an ex- 

 pansion of the fibres of the optic nerve. It is a membrane of special 

 structure, connected with the extremities of the optic nerve fibres, but 

 containing also many additional anatomical elements. It is accordingly 

 a peculiar nervous apparatus, adapted to receive the impression of lumi- 

 nous rays, and connected, by means of the optic nerve, witli the central 



