THE SENSE OF VISION 607 



1. Adjacent to the choroid is a single layer of polygonal pigment 

 cells, from which elongated processes containing fine pigment (fuscin) 

 granules pass into the next layer, and end between the rods. 



2. The receptor mechanism of the eye, the layer of rods and cones 

 A rod consists of an outer elongated part, about 30 /w long and 2 (J. 

 broad. It is transparent, transversely striated, and in it are to be 

 found the minute pigment granules which form the " visual purple." 

 The inner part of the rod spreads out somewhat in the shape of a 

 carrot, the upper part being longitudinally and the lower part trans- 

 versely striated. From the tip arises a rod-fibre, which pierces the 

 external limiting membrane, and swells out as the rod granule 

 or nucleus, finally passing on to form a varicose synapse (see 

 Fig. 340) 



A cone also consists of an inner and outer portion. The outer 

 portion is tapering and pointing, about one-third the length of the 

 corresponding portion of the rod. It is transversely striated, and 

 contains no pigment. The inner part of the cone is broad, and ends 

 in a cone-fibre. This is somewhat thicker than that of the rod, but, 

 like it, pierces the external limiting membrane, contains a nucleus or 

 granule, and ends as an arborizing synapse. 



3. The external limiting membrane is a well-defined membrane 

 formed by the outer processes of the sustentacular fibres. The susten- 

 tacular fibres of Miiller reach from this layer to the internal limit- 

 ing membrane. This membrane serves to support the rods and 

 cones. 



4. The external nuclear layer is made up of the rod and cone granules 

 (the nuclei of the rod and cone fibres), and also of fine fibres 

 belonging to the supporting cells of the internal limiting membrane 

 (Fig. 340). * 



5. The outer molecular layer is really a synapse layer, consisting of 

 the ramifying interdigitations of the rod and cone fibres and the 

 synapses of the cells in the next layer the bipolar cells (Fig. 340). 

 It also contains a few supporting cells, probably of a neuroglial 

 nature. 



6. The inner nuclear layer consists chiefly of the bipolar cells which 

 form the first neXiron (Fig. 340). One process, as we have seen, arborizes 

 in the preceding layer ; the other passes into the inner molecular layer 

 of synapses, to end around the terminations of the dendrites of the 

 ganglion cells of the optic nerve. In this layer are also oval cells, 

 termed npongioblasts, which send a single process into the inner 

 molecular layer, and other small cells, termed amacrine cells, which 

 send a short process into the outer molecular layer. There are also 

 a few bipolar cells. The large oval nuclei of the supporting fibres of 

 Muller are found in this layer. 



7. The inner molecular layer is a synaptic layer containing the 

 synapses of the bipolar cells, and of the ganglion cells. It also 

 contains neuroglial cells. 



8. The layer of ganglion cells consists of the large flask-shaped 

 ganglion cells which constitute the second set of neurons (Fig. 340). 



