STRUCTURE OF RETINA. 



665 



The rods (a) are elongated solid and grooved particles, which are pointed 

 at the inner end, and are more numerous than the other elements ; from 

 their inner ends fine threads (rod-fibres) are prolonged through the outer 

 set of granules (c), and are there connected with elliptical transversely 

 striated bodies. The cones (6) are shaped like a flask with a long neck, 

 and have the larger end turned inwards : they do not project so far out as 

 the rods. When viewed on the outer surface, they form large isolated swell- 

 ings (m) amongst the ends of the rods, and at a deeper level. By their 

 inner ends they are united with a pear-shaped cell (fig. 234) in the outer 

 part of the granular layer, and are said to reach as far as the molecular 

 material (d) between the two sets of granules. 



Fig. 234. 



1. Columnar layer with rods a, and cones, b. 



2. Granular layer with outer, c, and inner granules, e, and 



intermediate non-granular part, d. 



3. Nervous layer with g, nerve cells, and ft, nerve fibres ; 



outside tho cells is a finely granular part,/, and in- 

 side the fibres a limiting layer, i, formed by fibres of 

 Mailer. 



On the left of the vertical section is an ideal plan of the 

 connection of the several parts. Thus the nerve cell, g, 

 unites internally with nerve fibres, and externally with 

 the inner granular layer ; and the inner granular layer 

 is further joined by a thread (fibre of Miiller) to the 

 outer granular layer. 

 k. Microscopic appearance of the outer surface of the retina 



over the yellow spot, where there are only cones. 

 1. Appearance of the retina near the yellow spot a single 



circle of rods surrounding each cone. 



m. Appearance of the middle of the retina, a large number 

 of the rods surrounding each cone. In all three 

 figures the larger rings represent the cones, and the 

 smaller ones the rods seen endwise. 



MAGNIFIED VERTICAL SECTION OF THE RETINA (altered from Kolliker). 



In the fresh state both are soft, clear, and homogeneous, with a glisten- 

 ing appearance, but these characters are soon destroyed by water and other 

 fluids : their structure is very uncertain. 



The granular layer ( 2 ) consists of innumerable rounded and ovalish 

 bodies with nuclei, which are collected into two sets (c and e), with an 

 intervening fine molecular material (d). 



In the inner set (e) there are several kinds of cells, whose nature is 

 unknown : two sets, which are oval and nucleated, are said to have this 

 arrangement : One is connected with the fibres of Miiller ; and the other 

 has a fibre at each end, like a bipolar ganglion-cell, which is continued 

 outwards and inwards into the molecular material (d and /'). 



In the outer set (c) two kinds of nucleated cells are to be recognized, 

 which are connected with the fibres from the rods and cones in Jacob's 

 layer. The cells of the rod-fibres, the most numerous, are elliptical, and 

 marked by cross stria? (Henle) (fig. 234). The cells of the cone-fibres are 



