834 



THE ORGANS OF SPECIAL SENSE. 



retina, they radiate from their point of entrance over the surface of the retina, 

 grouped in bundles, and in many places, according to Michel, arranged in plexuses! 

 Most of the fibres in this layer are centripetal, and are the direct continuations of 

 the axis-cylinder processes of the cells of the next layer, but a few of them 

 (centrifugal fibres) pass through it and the next succeeding layer to ramify in the 

 inner molecular and inner nuclear layers, where they terminate in enlarged 

 extremities (Fig. 448, 1, m). The layer is thickest at the optic nerve entrance, 

 and gradually diminishes in thickness toward the ora serrata. 



3. The ganglionic layer consists of a single layer of large ganglion-cells; 

 except in the macula lutea, where there are several strata. The cells are some- 

 what flask-shaped, their rounded internal margin resting on the preceding layer 



FIG. 446 and 447. Vertical sections of the human retina. Fig. 446, half an inch from the entrance of the 

 optic nerve. Fig. 447, close to the latter. 1. Layer of rods and cones, Jacob's membrane, bounded underneath 

 by the membrana limitans externa. 2. Outer nuclear layer. 3. Outer molecular layer. 4. Inner nuclear layer. 

 5. Inner molecular layer. 6. Ganglionic layer. 7. Layer of nerve-fibres. 8. Sustentacular fibres of Miiller. 

 9. Their attachment to the membrana limitans interna. 



and sending off an axon which is prolonged as a nerve-fibre into the fibrous layer. 

 From the opposite extremity numerous thicker processes (dendrites) extend 

 into the inner molecular layer, where they branch out into flattened arborizations 

 at different levels (Fig. 448, vn). The ganglion-cells vary much in size, and the 

 dendrites of the smaller ones as a rule arborize in the inner molecular layer as 

 soon as they enter it ; while the processes of the larger cells ramify close to the 

 inner nuclear layer. 



4. The inner molecular layer is made up of a dense reticulum of minute fibrils, 

 formed by the interlacement of the dendrites of the ganglion-cells with those of 

 the cells contained in the next layer, immediately to be described. Within the 

 reticulum formed by these fibrils a few branched spongioblasts are sometimes 

 imbedded. 



5. The inner nuclear layer is made up of a number of closely packed cells, of 

 which there are three different kinds. (1) A large number of oval cells, which are 

 commonly regarded as bipolar nerve-cells, and are much more numerous than either 

 of the other kind. They each consist of a large oval body placed vertically to the 

 surface, and containing a distinct nucleus : they are surrounded by a small amount 

 of protoplasm, which is prolonged into two processes : one of these passes inward 

 into the inner molecular layer, is varicose in appearance, and ends in a terminal 

 ramification, which is often in close proximity to the ganglion-cells (Fig. 448 1, c). 



