RODS AND CONES OF THE RETINA. 



47 



tinguished by their shape, which is somewhat pyriform, by the absence of transverse 

 striatiou, and by their position for they occupy the part of the outer nuclear layer 

 nearest the membrana limitans externa, 1 and the larger end of each is thus in close 

 proximity to the base of the corresponding cone (fig. 48), with which it is directly 

 connected, or there is at most a short, comparatively thick stalk uniting the two 

 (see fig. 52). At the macula lutea, however, where only cone-granules are met with, 

 many of them are further removed from the limiting membrane, and the stalk is 

 then longer (fig. GO). The nucleus of each cone-granule, which, as in the case of 

 the rod-granules, occupies almost all the enlargement, is spheroidal, and contains a 

 distinct nucleolus. The cone-fibre is very much thicker than the rod-fibre above 

 described, and is itself finely striated or fibrillated. It passes from the smaller end 

 of the pear-shaped enlargement straight through the outer nuclear layer to reach 

 the outer molecular layer, upon which it rests by a somewhat pyramidal base (cone- 

 foot), from which ramifications may be traced into the substance of the molecular 

 layer where they interlace with the ramifications of the peripheral processes of the 

 cone-bipolars (see above) (fig. 52). 



7. The layer of rods and cones. The elements which compose this layer 

 are, as their name implies, of two kinds, those of the one kind the rods having an 



Fig. 53. OUTER SURFACE OF THE COLUMNAR LAYER OF 

 THE RETINA (Kblliker). 350 DIAMETERS. 



a, part within the macula lutea, where only cones 

 are present : ?>, part near the macula, where a single 

 row of rods intervenes between the cones ; c, from a 

 part of the retina midway between the macula and 

 the ora serrata, showing the preponderance of the 

 rods. 



elongated cylindrical form (about 0'060 mm. long and 0*002 mm. 

 diameter) ; the cones, on the other hand, being shorter (0'035 mm.), 

 much thicker (O'OOG mm.), bulged at the inner end or base, and 

 terminated externally by a finer tapering portion. Both rods and 

 cones are closely set in a palisade-like manner over the whole extent 

 of the retina between the membrana limitans externa and the pig- 

 mentary layer (fig. 48, 7). Except at the macula lutea, where only 

 cones are met with, the rods far exceed the cones in number. Their 

 relative number and arrangement is well exhibited when the layer is 

 viewed from the outer surface, as in fig. 53, where a represents a portion 

 of the layer from the macula lutea ; Z>, from the immediate neighbour- 

 hood of the latter ; and c, from the peripheral part of the retina. 



Fig. 54. A ROD AND A CONE FROM THE HUMAN RETINA (Max Schultze). 



(Highly magnified. ) 



In the rod the longitudinal striation of both the outer and inner segments is 

 shown ; in the cone the transverse striation of the outer segment and the longitudinal 

 of the inner segment ; Z, limitans externa. 



The total number of cones in the human retina has been calculated to exceed three 

 millions ; and of rods many times this number. It may be of interest here to note that the 

 number of fibres in each optic nerve is about half a million (Salzer). 



The rods and the cones, although differing thus in shape and size, agree in many 

 points of structure. Thus, each consists of two distinct segments an inner and an 

 outer ; the division between the two occurring, in the case of the rods, about the 

 middle of their length (in man) ; in the cones at the junction of the finer tapering 



In Teleostei the cone-nuclei lie outside the limitans externa. 



