256 



TWENTY-FOURTH LECTURE. 



Our present knowledge concerning the cones (Fig. 203, B, 



Fig. 205 2), is uncertain. 



In man, they have the form of a slen- 

 der bottle. Their base rests on the 

 membrana limitans externa. Upwards, 

 the cone passes into a shorter, conical, 

 infinitely changeable structure, the so- 

 called cone rod (Fig. 203, B, above c, 

 Fig. 205, 2 a). It is the equivalent of 

 the external member of the rod, charac- 

 terized by its great tendency to break 

 up into transverse discs. The inner 

 member, or the cone body <Iig. 205, 2 

 b), also shows the longitudinal striation, 

 similar to the equivalent portion of the 

 rod. 



At the base of the rod, immediately 

 beneath the limitans externa, we meet 



with a cell-like 

 body, the so- 

 call ed cone 

 granule (Fig. 

 203, B, C, Fig. 

 205, 2, below d). 

 A broad cone 

 filament (up to 

 0.0029 mm - m 

 thickness) finally 

 runs downward, 

 passing through 

 the outer granular layer (Fig. 203, below c'). It is a fascicu- 

 lus of primitive fibrillar. 



Interesting local variations (Fig. 206) in regard to the num- 

 ber of cones and rods occur in the human eye. 



In the macula lutea, the seat of the most distinct vision, 

 we meet with cones alone, which have become extremely fine 

 (1). In the vicinity the latter are still quite crowded, and are 



Fig. 205. — Fibrillated cover- 

 ing of the rods and cones ; i, 

 rods ; 2, cones of man ; a, 

 outer ; 6, inner member ; c, 

 rod-filament ; d, limitans ex- 

 terna ; 3, rod of the sheep. 

 The fibrillie project beyond the 

 inner member ; the outer mem- 

 ber is wanting. 



Fig. 206. — The rod layer 

 seen from without ; a, cones ; 

 b, cone rods ; c, ordinary 

 rods ; 1, from the macula lu- 

 tea : 2, at the margin of the 

 same ; 3, from the centre of 

 the retina. 



