458 



THE EYE. 



supporting tissue of the retina and Miiller's fibers, together with the 

 internal and external limiting membranes. 



The visual cells are either rod-visual cells or cone-visual cells. 

 The rod-visual cells consist of a rod and a rod-fiber with its 

 nucleus. The rod (40 // to 50 // in length) consists of two seg- 

 ments, an outer and an inner, the former of which is doubly refrac- 

 tive and may be separated into numerous transverse discs by the 

 action of certain reagents. The inner is less transparent than the 

 outer segment, and its inner end shows a fine superficial longitu- 

 dinal striation due to impressions from the fiber-baskets formed by 

 Miiller's fibers. In the lower classes of vertebrates a rod-ellipsoid 



Layer of nerve- ' 



fibers. 



Ganglion-cell layer. 



Inner molecular ., 



layer. 



Inner nuclear layer. i 



Outer molecular 

 layer. 



Outer nuclear 

 layer. 



Ext. limiting mem- 

 brane. 

 Inner segment of 



rod. 



Inner segment of 

 cone. 



Outer segment of 



cone. 

 Outer segment of 



rod. 



Fig. 358. Section of the human retina ; X 7- 



(a fibrillar structure) may easily be demonstrated in the outer region 

 of each inner portion ; in many mammalia and in man the demon- 

 stration of this is more difficult. This structure is a planoconvex, 

 longitudinally striated body, the plane surface of which is coincident 

 with the external surface of the inner segment, its inner convex sur- 

 face lying at the junction of the outer and middle thirds of the inner 

 segment. The rod-fibers extend as far as the outer molecular layer 

 of the retina, where they end in small spheric swellings. The nuclei 

 of the rod-visual cells are found at varying points within the rod- 

 fibers, but rarely close to the inner segment. When treated with 

 certain fixing agents and stains, the rod-nuclei of certain animals (cat 

 and rabbit) are seen to show several zones, which stain alternately 



