56 SPECIAL SENSES. 



1. An external layer, situated next the choroid, called Ja- 

 cob's membrane, the bacillar membrane, or the layer of rods 

 and cones. 



2. The external granule-layer. 



3. The inter-granule layer (cone-fibre plexus, of Hulke). 



4. The internal granule-layer. 



5. The granular layer. 



6. The layer of nerve-cells (ganglion-layer). 



7. The expansion of the fibres of the optic nerve. 



8. The limitary membrane. 



The layer of rods and cones is composed of rods, or cylin- 

 ders, extending through its entire thickness, closely packed, 

 and giving to the external surface a regular, mosaic appear- 

 ance ; and, between these, are a greater or less number of flask- 

 shaped bodies, the cones. This layer is about -^^ of an inch 

 in thickness at the middle of the retina; T ^ of an inch, 

 about midway between the centre and the periphery; and, 

 near the periphery, about ^-g of an inch. At the macula 

 lutea, the rods are wanting, and the layer is composed en- 

 tirely of cones, which are here very much elongated. Over 

 the rest of the membrane, the rods predominate, and the 

 cones become less and less numerous toward the periphery. 



The rods are regular cylinders, their length corresponding 

 to the thickness of the layer, terminating above in truncated 

 extremities, and below in points, which are probably continu- 

 ous with the filaments of connection with the nerve-cells, 

 though they have been actually traced only into the exter- 

 nal granule-layer. Their diameter is about 13 ^ 00 of an inch. 

 They are clear, of rather a fatty lustre, soft and pliable, but 

 somewhat brittle, and so alterable that they are with difficulty 

 seen in a natural state. They should be examined in perfect- 

 ly fresh preparations, moistened with liquid from the vitreous 

 humor or with serum. Their intimate structure, as well 

 as that of the cones, has recently been very closely studied, 

 especially by German anatomists. 1 When perfectly fresh, it 



1 An excellent review of the recent investigations into the anatomy of the 



