THE SENSES. 



527 



FIG. 135. 



parts of the membrane. The longest fibres continue their course to 

 the ora serrata, beyond which none are visible. 



2. Ganglionic Layer of Nerve Cells. This layer, which is situated 

 immediately outside the former, contains multipolar nerve cells, similar 

 to those of the gray substance of the brain. According to Kolliker, 

 they vary in size from 9 to 36 mmm. in diameter, and are provided 

 with pale, ramified prolongations. Some of these prolongations are 

 directed toward the more external portions of the retina ; others pass 

 in a horizontal direction, and, according to some observers (Kolliker, 

 Miiller, Corti), become continuous with optic nerve fibres. 



3. Layer of Nuclei. The most characteristic elements of this layer 

 have, in the main, the aspect of nuclei ; although by some observers (Kol- 

 liker, Schultze), they are regarded as nucleated cells, in which the envel- 

 oping cell-substance is scanty, as compared with the size of the nucleus. 

 The nuclei, sometimes called "grains " or "granules," are oval bodies, 

 with their long axes perpendicular to the surface of the retina. They 

 are of two varieties, differing 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 varicose enlargements of slender fibres, also 

 directed perpendicularly to the surface of 



the retina, and extending through the whole 

 thickness of the layer. These are presumed 

 to be of the nature of nerve fibres, and to 

 represent, directly or indirectly, the continu- 

 ations of those from the optic nerve. At 

 their outer extremities they are continuous 

 with the elements of the following layer. 



4. Layer of Rods and Cones. This is 

 the most remarkable of the retinal layers, 

 consisting of elements more peculiar in form 

 than those found elsewhere, and most directly 

 connected with the physiology of luminous 

 impressions. As their name indicates, these 

 elements are of two kinds, namely, the 

 " rods " and the "cones." There is reason 

 to believe that they are modifications of 

 each other, and that their offices in vision 

 are essentially similar. 



The rods (Fig. 135) 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- DIAGRAMMATIC SECTION, from the 

 tie less than 2 mmm. in diameter. They S^-i^LayeVof^r^ 

 are placed parallel with each other, closely cones. 2. Layer of 

 packed side by side, perpendicularly to the (Schultze.) 

 surface of the retina. At its outer extremity each rod terminates by 



nuclei. 



