VISUAL ORGAN. 349 



The cones are shorter than the rods, measuring only about 

 30 fi. Like the rods, they show an outer and an inner seg- 

 ment. The outer segment is much shorter than that of the 

 rod, and is slightly conical in form. It sometimes presents 

 cross-striations. The inner segment is somewhat shorter and 

 much thicker (6 //) than that of the rod, and is rounded. The 

 ellipsoid of the cone is larger than that of the rod, and lies in 

 the peripheral part of the inner segment, occupying about 

 two-thirds of this. 



Each cone is continuous with a cone fibre. At the junction 

 of these two parts of the cone cell, immediately inside the 

 external limiting layer, lies the cone nucleus. The cone 

 fibres end in the outer reticular layer by means of a conical 

 expansion, from which fine fibres spread out. 



The number of rods is far greater than that of the cones. 

 They are distributed less uniformly, so that in a section taken 

 at right angles to the surface two or three rods are found 

 between each two cones. 



The rods and cones lie in a row, the lower boundary of 

 which is the membrana li mi tans externa (Fig. 263, B}. This 

 membrane is a product of the Miiller's fibres. Outside these lie 

 the rod and cone fibres, together with their nuclei, forming the 

 outer granular layer (C). This consists usually of granules 

 crowded closely together. In the region of the macula lutea the 

 inner segments of the rod and cone fibres are elongated, and form 

 the so-called Henles fibre layer (D), which contains no granules. 



The outer reticular layer (E] is made up of the thickened 

 ends of the visual cell fibres and the end arborizations of cells 

 whose bodies lie in the outer ganglionic layer. 



The main constituents of the outer ganglionic layer (inner 

 granular layer) (F) are the bipolar ganglion cells, whose 

 processes end in the outer and inner reticular layers. Some 

 cells (Fig. 263, d) establish a communication between the rod 

 cells and the optic nerve fibres in such a way that the outer 

 arborizations come in contact with the ends of the rod cells, 

 and the inner processes reach to the inner border of the inner 

 reticular layer to surround the ganglion cells there. Other 



