232 THE NERVOUS SYSTEM. 



from the neuroglia-elements and the processes of neighboring 

 nerve-cells. 



The inner nuclear (or granular) layer is a narrower zone 

 crowded with nerve-cells having prominent nuclei ; the nuclei 

 of Miiller's fibres are also in this layer. 



The outer reticular (or molecular) layer is a narrow reticu- 

 lum of neuroglia- fibres and nerve-cell processes. 



The outer nuclear (or granular) layer is a broad zone crowded 

 with the nuclei and the lower portions of the rod and cone cells. 



The external limiting membrane is a thin perforated mem- 

 brane, through the openings of which pass the rod and cone 

 cells. 



The layer of rods and cones is composed of the portions of 

 the rod and cone cells external to the limiting membrane. 

 The rod and cone cells are elongated neuro-epithelium cells, 

 arranged perpendicularly, divided into two zones by the limit- 

 ing membrane, their nucleated portions occupying the outer 

 nuclear layer and their terminal portions forming the layer of 

 rods and cones. 



The portion of the rod-visual cells in the nuclear layer is a 

 slender fibre, the " rod-fibre," with a bulbous expansion con- 

 taining the transversely banded nucleus. The portion external 

 to the limiting membrane, or " rod," consists of two segments, 

 the inner one continuous with the rod-fibre being fusiform, 

 and granular or fibrillated (especially in its outer part) ; while 

 the outer segment is a slender cylinder containing rhodopsin 

 and possessing special staining properties. 



The portion of the cone-visual cells outside of the limiting 

 membrane, or the "cone," also consists of two segments, an 

 outer part similar to the corresponding portion of the rods ex- 

 cept that it is shorter, conical, and contains no rhodopsin; and 

 an inner broad fusiform portion, fibrillated or granular exter- 

 nally. The portion of the cone-cell within the outer nuclear 

 zone consists of a slender " cone-fibre" with an expansion, 

 containing the nucleus, next to the limiting membrane and 

 uniting with the fusiform segment. The rod-cells consider- 

 ably outnumber the cone-cells. 



The pigment-layer of the retina, lying next to the vitreous 

 membrane of the choroid, consists of a single layer of hex- 

 agonal columnar epithelium-cells, with their nuclei next to the 



