618 



THE SENSES. 



Fig. 194. 



3. Layer of Nuclei. The layer of nuclei is so called because its most 

 characteristic elements have, in the main, the aspect of nuclei ; although 

 by some observers (Kolliker, Schultze), they are regarded as having 

 rather the signification of nucleated cells, in which the enveloping cell- 

 substance is in small quantity as compared with the size of the nucleus. 

 The nuclei themselves, sometimes called "grains" or "granules," are 

 oval bodies, placed with their long axes perpendicular to the surface 

 of the retina. There are two varieties of them mingled together, which 

 differ mainly in size ; the larger being from 9 to 13 mmm. in length, the 

 smaller one-half or two-thirds as long. They are all contained in the 

 interior of varicose enlargements of slender fibres, which are also di- 

 rected perpendicularly to the surface of Jthe retina, and extend uninter- 

 ruptedly through the whole thickness of the layer. These fibres are 

 presumed to be of the nature of modified nerve fibres, and to represent, 

 either directly or indirectly, the continuations of those derived from 

 the expansion of the optic nerve. At their outer extremities they are 

 immediately continuous with the elements of the following layer. 



4. Layer of Rod* and Cones This is undoubtedly the most remark- 

 able of the retinal layers, since it consists of elements which are more 

 peculiarly constituted than those found elsewhere, and which are most 



immediately connected with the physiology 

 of luminous impressions. As the name 

 indicates, these elements are of two kinds; 

 distinguished, according to their shape, by 

 the name of "rods" and "cones." There 

 is reason to believe that their offices are 

 essentially similar, and that they are to be 

 regarded as modifications of each other. 



The rods (Fig. 194) are straight, elon- 

 gated, cylindrical bodies, composed of a 

 transparent, homogeneous substance, re-' 

 markable for its highly refractive power. 

 They are about 50 mmm. in length by a lit- 

 tle less than 2 mmm. in diameter. They are- 

 all placed parallel with each other, closely 

 packed side by side, standing perpendicu- 

 larly to the surface of the retina, and ex- 

 tending through the whole thickness of 

 the layer. At its outer extremity each 

 rod terminates by a plane perpendicular 

 to its axis ; at its inner extremity it tapers 

 / ^A A I \ h ' ) suddenly to a point and is continuous 



wifch a fibre of the P recedin g Kver, and 

 thus with one of its nucleated enlarge- 

 ments or grains. According to Schultze, 

 the internal half of each rod is slightly 

 thicker, and exhibits rather less refractive 

 power than its external half. 



DIAGRAMMATIC SECTION, 

 from the posterior portion of the 

 human retina. 1. Layer of rods 

 and cones. 2. Layer of nuclei. 

 (Schultze.} 



