58 SPECIAL SENSES. 



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. 3, which shows the different layers of the retina. 



At the f ovea centralis, the external layer is composed en- 

 tirely of immensely elongated 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 fo- 

 vea centralist At the fovea, the optic nerve-fibres are want- 

 ing ; the ganglion-cells, which are elsewhere over the retina 

 in a single layer, here present from six to eight layers, ex- 

 cept 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) remain, in the fovea, while the internal granule-layer 

 and the granular (molecular) layer are wanting. At the fo- 

 vea, indeed, those elements of the retina which may be re- 

 garded as purely accessory seem to disappear, leaving only 

 the structures that are concerned directly in the reception of 

 visual impressions. 



The external granule-layer is composed of large granules, 

 looking like cells, which are each nearly filled with a single 

 nucleus. These are connected with the filaments from the 

 rods and cones. They are rounded or ovoid, and measure 

 from xaooo to -ginro ^ an mcn ^ diameter. The inter- 

 granule layer (cone-fibre plexus) is composed apparently of 

 minute fibrillse and a few nuclei. The internal granule-layer 

 is composed of cells nearly like those of the external granule- 

 layer, but a little larger, and probably connected with the 

 filaments of the rods and cones. The granular (molecular) 

 layer is situated next the layer of ganglion-cells. 



The layer of ganglion-cells is composed of multipolar 



1 SCHULTZE, Zur Anatomic und Physiologic der Retina. Bonn, 1866, S. 52. 



