SENSE OF VISION. 



755 



uective tissue, in which extraordinarily fine fibrils are imbedded, running 

 obliquely or parallel to the surface of the retina, and attached to the peri- 

 pheric processes of the internal granules, and to the fibres of the rods and 

 cones; and finally the layer of rods and cones (9), often also called the ba- 

 cillnry layer, or after its discoverer, Jacob's membrane. The rods are cylin- 

 drical bodies, with a length at the back of the Eye in Man of fifty to sixty 

 micromillimetres, and a diameter of two micromillimetres, but becoming 

 shorter anteriorly. Distributed at regular intervals between the rods, except 

 at the macula lutea and ora serrata, are the flask-shaped cones. The dis- 

 tance between two cones amounts on an average to eight or ten millimetres, 

 the intervening space being occupied by three or four rods in a straight line. 

 In both structures two essentially different segments are distinguishable. (See 

 Fig. 268.) The distinction is most marked in the cones, in which the outer 



FIG. 267. 



FIG. 268. 



FIG 269. 



; r-" "1 



w^ 



r - o ,- 



r ^llr 3? a 



FIG. 267. Diagrammatic representation of the connections of the nerve-fibres in the Retina. 1. 

 Meiubrana limitans interna. 2. Optic nerve-fibre layer. 3. Layer of ganglion-cells. 4. Internal 

 granulated or molecular layer. 5. Internal granule-layer. 6. External granulated or molecular layer. 

 7. External granule-layer. 8. Membrana limitans exterior. 9. Bacillary layer, or layer of rods and 

 cones. 



FIG. 268. Rod and cone from the Retina of Man preserved in a two per cent, solution of perosmic 

 acid, to show the fine fibres of the surface, and the different lengths of the internal segment. The 

 outer segment of the cone is broken up into disks, which, however, are still adherent to one another; 

 at the base of the cone are seen a few fine hairs (X 1000 diam.). 



FIG. 269. Diagrammatic representation of the connective tissue of the retina as seen near the ora 

 serrata. The numbers correspond to those of the several layers of the retina shown in Fig. 267. 



segment is conical and highly refractile, whilst in the rods the segment is 

 usually regularly cylindrical. The plane of junction of the two segments 

 is more anterior in the cones than in the rods. After removal from the body 

 the outer segments of both the rods and cones undergo a kind of coagula- 



