CH. LVIII.] THE RODS AND CONES 817 



The large oval nuclei (fig. 514) belonging to the Miillerian fibres 

 occur in this layer. 



6. Outer molecular layer. This layer closely resembles the inner 

 molecular layer, but is much thinner. It contains the branchings of 

 the rod and cone fibres on the one hand and of the bipolar cells on 

 the other. 



7. External nuclear layer. This layer consists of small cells 

 resembling at first sight those of the internal nuclear layer ; they 

 are classed as rod and cone granules, according as they are connected 

 with the rods and cones respectively, and will be described with 

 them. They are lodged in the meshes of a frame-work, which is 

 formed by the breaking up of the Miillerian fibres. 



8. Membrana limitans externa. This is a well-defined membrane, 

 marking the internal limit of the rod and cone layer, and made up 

 of the junction of the sustentacular or Miillerian fibres externally. 



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. 514). 



Each rod (fig. 515) is made up of two parts, very different in 

 structure, called the outer and inner limbs. The outer limb of the 

 rods is about 30 /x, long and 2 /UL 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 haematoxylin, 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 haematoxylin, but not with osmic acid. Each rod 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 transversely, and situated about 

 the middle of the external nuclear layer; the internal end of the 

 rod fibre terminates in branchings in the outer molecular layer. 



Each cone (fig. 515), 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 cones. 

 The inner limb of the cone is broader in the centre. It is proto- 

 plasmic, and under the influence of light has been seen to execute 

 movements. In birds, reptiles and amphibia, 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 



