752 



THE EYE AND VISION. 



[CH. LV. 



9. Layer of rods and cones. This layer is the nerve-epithelium 

 of the retina. It consists of two kinds of cells, rods and cones, 

 which are arranged at right angles to the external limiting mem- 

 brane, and supported by hairlike processes (basket) proceeding from 

 the latter for a short distance (fig. 570). 



The rods. Each rod (fig. 571) is made up of two parts, very 

 different in structure, called the outer and inner limbs. The outer 

 limb of the rods is about $op. long and 2/>t broad, is transparent, and 

 doubly refracting. It is said to be made up of fine superimposed 

 discs. It stains brown with osmic acid but not with hsernatoxylin, 

 and resembles in some ways the myelin sheath of a medullated 

 nerve. It is the part of the rod in which the pigment called visual 

 purple is found. In some animals, a few rods have a greenish 

 pigment instead. The inner limb is about as long but slightly 

 broader than the outer, is longitudinally striated at its outer and 



granular at its inner part. 

 It stains with hsematoxylin 

 but not with osmic acid. 

 Each rod so constructed is 

 connected internally with a 

 rod fibre, very fine, but here 

 and there varicose ; in the 

 middle of the fibre is a rod 

 granule, really the nucleus of 

 the rod, striped broadly trans- 

 versely, and situated about 

 the middle of the external 

 nuclear layer ; the internal 

 end of the rod fibre termi- 

 nates in branchings in the outer molecular layer. 



The cones. Each cone (fig. 571), like the rods, is made up of 

 two limbs, outer and inner. The outer limb is tapering and not 

 cylindrical like the corresponding part of the rod, and about one- 

 third only of its length. There is, moreover, no visual purple 

 found in the cone. The inner limb of the cone is broader in the 

 centre. It is protoplasmic, and under the influence of light has 

 been seen to execute movements. In birds there is often a 

 coloured oil globule present here. Each cone is in connection by 

 its internal end with a cone fibre, which has much the same 

 structure as the rod fibre, but is much stouter and has its nucleus 

 quite near to the extemal limiting membrane. Its inner end 

 terminates by branchings in the external molecular layer. 



In the rod and cone layer of birds, the cones usually pre- 

 dominate largely in number, whereas in man the rods are by 



Fig. 574. Pigment-cells from the retina. A, 

 cells still cohering, seen on their surface ; 

 a, nucleus indistinctly seen. In the other 

 cells the nucleus is concealed by the pig- 

 ment granules. B, two cells seen in profile ; 

 a, the outer or posterior part containing 

 scarcely any pigment, x 370. (Henle.) 



