THE EETINA. 



817 



(6) The horizontal cells are of two varieties : (1) small, flattened, star-like cells, lying 

 immediately internal to the outer molecular layer, and sending a tuft of dendrites outwards, 

 towards the bases of the cone fibres, while their axons are directed horizontally, for a 

 variable distance ; (2) large, irregular cells, lying internal to the above and ending in 

 finger-like ramifications in the outer molecular layer. Their axons run horizontally for 

 some distance, and end, in extensive varicose arborisations, under the spherules of the 

 rod fibres. 



(c) The spongioblasts are situated in the innermost part of the inner nuclear layer ; 

 their processes ramify in the inner molecular layer, it may be in one stratum (stratified 

 spongioblasts) or in several strata (diffuse spongioblasts). 



5. Outer molecular or outer plexiform layer. This is constituted by the 

 interlacement of the dendrites of the bipolar and horizontal cells, just described, with 

 the spherules of the rod fibres and the ramifications of the foot-plates of the cone fibres. 

 It is divided into two strata : (a) external, indicating the contact of the rod bipolars with 

 the spherules of the rod fibres ; (b) internal, the line of contact between the cone bipolars 

 and the branches of the cone fibres. 



6. Outer nuclear layer or layer of outer granules. This is made up of clear 

 granules which somewhat resemble those of the inner nuclear layer, and are divisible into 

 two kinds : (a) cone granules, (b) rod granules. The cone granules are the larger, and each 

 contains an oval nucleus ; they lie immediately inside the outer limiting membrane, through 

 which they are continuous with the cones of the next layer. Each is prolonged internally 

 as a straight fibre, which, on reaching the outer molecular layer, expands to form a foot- 

 plate, from which several horizontal fibrils are given off. The rod granules are far more 

 numerous than the cone granules, and each contains a small oval nucleus, which is 

 transversely striated. Their outer processes are continuous, through the outer limiting 

 membrane, with the rods of the next layer, while their inner processes pass into the outer 

 molecular layer and end in free, unbranched spherules amongst the arborisations of the 

 rod bipolars. 



7. Layer of rods and cones. This consists of two sets of structures, viz., rods 

 and cones. Except at the macula lutea the rods are far more numerous than the cones, 

 and assume the form of elongated cylinders, while the cones are shorter than the rods, 

 and taper externally to fine points. Each rod and cone consists of two segments inner 

 and outer. The inner segment of the rod only slightly exceeds in diameter its outer 

 segment, whereas the inner segment of the cone greatly exceeds its outer part. The inner 

 segments of both rods and cones have an affinity for staining reagents, and consist of a 

 basal homogeneous portion and an outer longitudinally striated part, the proportion of the 

 latter to the former being greater in the cones than in the rods. The outer segments 



ive not the same affinity for reagents, but tend to break transversely into numerous discs 

 689, B). The colouring matter, rhodopsin, already referred to, is found only in the 

 iter segments of the rods, the terminal parts of which extend into the layer of pigmented 

 )ithelium. 



8. Stratum pigmenti. This consists of a single stratum of cells which, on surface 

 )\v, are hexagonal (Fig. 690), their outer flattened surfaces being firmly attached to 



the chorioid. When seen in profile the outer part of each cell contains 



a large oval nucleus and is devoid of 



pigment, while the inner portion is 



filled with pigment and extends as a 



series of thread-like processes amongst 



the outer segments of the rods and 



cones. ' When the eye is kept in the 



dark the pigment accumulates near 



the outer part of the cell, but when 



exposed to light it streams in between 



the rods and cones (Fig. 691). 



Sustentacular fibres of the 



retina. These form a framework for 

 RETINA (viewed from the support of the nervous structures 



(Fig. 688 M). They begin at the FlG 69L _ SECTION THROUGH OUTBR 



inner surface of the nerve-fibre layer 

 in single or forked expanded bases, by the apposition of 

 which a delicate membrane, the membrana limitans in- 

 terna, is formed. In the ganglionic layer they give off a few side branches, and, on 

 passing through the inner nuclear layer, supply ramifications amongst the inner granules 



53 



LAYERS OF RETINA (semi-diagram- 

 matic). 



