THE EYE 1523 



outer molecular layer, where it comes into relation with the arboriza- 

 tions of the external process of a rod-bipolar. 



The cone-granules are larger than the rod-granules, but not so 

 numerous, and each contains an oval nucleus. They are situated 

 in the outermost part of the outer nuclear layer, close to the mem- 

 brana limitans externa. The outer end of each granule is con- 

 tinuous with one of the cones of the bacillary layer. The inner 

 end is prolonged into a cone-fibre, which passes into the outermost 

 part of the outer molecular layer, where it expands into a foot- 

 plate, from which filaments are given off. These filaments come 

 into relation with the arborizations of the external process of a cone- 

 bipolar cell. 



7. Layer of Rods and Cones, or Bacillary Layer. — ^This layer 

 consists of two elements, namely, rods and cones, the former being 

 cylindrical, and the latter flask-shaped. The rods are much more 

 numerous, longer, and narrower than the cones, and both are placed 

 perpendicularly . 



Each rod and cone consists of two segments — outer and inner. 

 In the case of the rods the two segments are of almost equal length, 

 the inner segment being rather larger than the outer. The outer 

 segment is the only seat of the colouring matter known as visual 

 purple or rhodopsin. In the case of the cones, which, as stated, 

 are flask-shaped, the inner segment of each forms two-thirds of the 

 cone, and is of large size ; whGst the outer segment forms one-third, 

 is narrow, and represents the tapering part of the flcisk. The outer 

 segments of both rods and cones present faint transverse striations. 

 The inner segments of both are subdivided into tvvo parts — outer 

 and inner. The outer part is composed of delicate fibrils, longi- 

 tudinally arranged, and therefore presents a longitudinally striated 

 appearance. The inner part is faintly granular. The rods and 

 cones are continued at their inner ends through the membrana 

 limitans externa into the rod-fibres and cone-fibres, which belong 

 to the outer nuclear layer. The outer ends of the rods project into 

 the pigmentary layer. 



8. Pigmentary Layer. — ^This is the most external layer of the 

 retina, and it is in close contact with the choroid coat. It consists 

 of a single layer of hexagonal epithelial cells, which contain pigment. 

 The deep surfaces of the cells give off processes which extend into 

 the interv^als between the outer ends of the rods and cones. 



Sustentacular Fibres, or Fibres of Miiller. — These fibres constitute 

 the supporting tissue of the retina, and extend from its internal 

 surface to the boundary-line between the outer nuclear layer and 

 the layer of rods and cones. The inner ends of the fibres are ex- 

 panded, and cohere by their edges so as to present the appearance 

 of a distinct retinal layer, which is called the membrana limitans 

 interna. Their outer ends, which are very numerous owing to the 

 breaking up of the fibres, also expand, and form what is known as 

 the membrana limitans externa, which lies between the outer nuclear 

 layer and the layer of rods and cones. (The membrana limitans 



