:J48 MK^ROSCOPIC ANATOMY OF THE ORGANS. 



it is distributed equally throughout the cell. In consequence 

 of this the rods and cones become surrounded by pigment 

 granules in the region of the external limiting membrane. 

 After exposure to stronger light the pigment moves to the 

 outer part of the cell and collects in a thin layer there, so 

 that the visual cells are quite free from it. 



The neuro-epithelial layer is formed of the visual cells. Of 

 these, there are three layers : the layer of rods and cones, the 

 outer granular layer, and the sheath of Henle. The external 

 limiting membrane is derived from the supporting cells of 

 Miiller (see below). 



We distinguish two kinds of visual cells, rod cells and cone 

 cells (Fig. 263, b, c). Each rod cell consists of a rod and a rod 

 fibre. The latter contains the nucleus. The rods are elongated 

 cylindrical structures, about 50 u long and 2 ^ thick. They 

 may be divided into two parts, the outer segment and the inner 

 segment. The outer segment is cylindrical and doubly refrac- 

 tive. It contains the visual purple, and when acted upon by cer- 

 tain reagents breaks up into many discs. The inner segment is 

 slightly spindle-shaped, finely granular, and singly refractive. 

 In the outer part of the inner segment there is in most verte- 

 brates an ellipsoidal body which shows a fibrous structure. 

 This is the so-called ellipsoid of Krause. 



Each rod is continuous at its inner end with a fine fibre, the 

 rod fibre. This ends in the outer reticular layer in a small 

 globular thickening. Each rod fibre shows somewhere in its 

 course a nucleated enlargement, the rod nucleus. This may 

 occur at various levels, so that the outer granular layer contains 

 many rows of nuclei. In some animals (cat, rabbit, guinea-pig, 

 horse, etc.) the nucleus shows a distinct transverse striation, 

 which is due to the arrangement of the chromatin substance in 

 two to four plate-like segments. In man the nucleus shows a 

 reticular structure, and only seldom do we see an indistinct 

 cross-stria ti on, which is due to annular thickenings of the 

 chromntin network on the surface of the nucleus. 



The cone cells consist also of two parts, the cone and the 

 cone fibre. 



