SPECIAL SENSES. 



Over the rest of the membrane the rods predominate, and the cones be- 

 come less and less frequent toward the periphery. 



The rods are regular cylinders, their length corresponding to the thick- 

 ness of the layer, terminating above in truncated extremities, and below in 

 points which are probably continuous with the filaments of connection with 

 the nerve-cells. Their diameter is about ls } 00 of an inch (2 /*). They 



are clear, of rather a fatty lustre, soft and plia- 

 ble, but somewhat brittle, and so alterable that 

 they are with difficulty seen in a natural state. 

 They should be examined in perfectly fresh prep- 

 arations, moistened with liquid from the vitreous 

 humor or with serum. When perfectly fresh it 

 is difficult to make out any thing but an entirely 

 homogeneous structure ; but shortly after death 

 each rod seems to be divided by a delicate line 

 into an outer and an inner segment, the outer 

 being a little the longer. At the upper extrem- 

 ity of the inner segment, is a hemispherical body, 

 with its convexity presenting inward, called the 

 lentiform body (linsenformiger Korper). The 

 entire inner segment is somewhat granular, and 

 it often presents a granular nucleus at its inner 

 extremity. The outer segment apparently differs 

 B. Rods from the frog : i. Fresh, in its constitution from the inner segment and 



magnified 500 diameters ; a, in- . ., , . , , , 



ner segment ; b, outer segment ; is not similarly affected by reagents. Treated 



c, lentiform body; d, nucleus. ,-, 1M , ,. . n ,, . , 



2. Treated with dilute acetic acid witn dilute acetic acid, the outer segment be- 

 comes broken up transversely into thin disks. 



The cones are probably of the same constitution as the rods, but that 

 portion called the inner segment is pyriform. The straight portion above 

 (the outer segment) is sometimes called the cone-rod. The entire cones are 

 about half the length of the rods and occupy the inner portion of the layer. 

 The outer segment is in its constitution precisely like the outer segment of 

 the rods. The inner segment is slightly granular and contains a nucleus. 

 The cones are connected below with filaments passing into the deeper layers 

 of the retina. The arrangement of the rods and cones is seen in Fig. 247, 

 which shows the different layers of the retina. 



At the fovea centralis, Jacob's membrane is composed entirely of elon- 

 gated cones, with no rods. These are slightly increased in thickness at the 

 macula lutea, but are diminished again in thickness^ by about one-half, at the 

 fovea centralis. At the fovea the optic nerve-fibres are wanting ; and the 

 ganglion-cells, which exist in a single layer over other portions of the retina, 

 here present six to eight layers, except at the very centre, where there are 

 but three layers. Of the layers between the cones and the ganglion cells, 

 the external granule-layer and the inter-granule layer (cone-fibre plexus) re- 

 main, in the fovea, while the internal granule-layer and the granular layer are 

 wanting. At the fovea, indeed, those elements of the retina which may be 



FIG. 246. Rods of the retina 



(Schultze). 



From the monkey. A. Rods, after 

 maceration in iodized serum, the 

 outer segment (6) truncated, the 

 inner segment (a) coagulated, 

 granular, and somewhat swol- 

 len ; c, filament of the rods ; d, 

 nucleus. 



