204 



Basic Structure of Vertebrates 



a set of fibrils belonging to a cell in the adjacent layer. The correspond- 

 ing process of a cone ends in a cluster of short branches which, simi- 

 larly, are engaged by fibrils of a cell in the next layer. The middle 

 layer consists mainly of these bipolar neurons, each having peripheral 

 fibrils (dendrites) connecting with rods or cones and, at its opposite 

 pole, fibrils related to those of a neuron in the next (toward the center 

 of the eyeball) ganglionic layer. It is from the large cells of this inner- 

 most ganglionic layer that most of the fibers of the optic nerve 

 arise. Each cell produces a liber which, emerging from the inner (cen- 

 tral) surface of the layer, bends sharply (Fig. 178) and passes into a 

 layer of such fibers which converge toward a point at the back of the 

 eyeball, where they are collected into a bundle which is the optic nerve 

 (Figs. 174, 175). But an occasional large "stellate cell" (Fig. 178) in the 

 middle layer of neurons produces a nerve-fiber which penetrates the 

 inner ganglionic layer without making a synapse with any of its cells 

 and continues directly into the optic nerve. Also, a few fibers of the 

 optic nerve come from cells in the brain and terminate in the middle 

 layer of the retina. A consequence of the manner of origin of the optic 

 nerve is that the retinal tissue adjacent to the vitreous humor is a 

 fairly thick layer of nerve-fibers. At the place of exit of the optic nerve, 

 there are no rods and cones. It is therefore a "blind spot." 



Pigmented 

 epithelium. 



Lr.yer of rods and J 

 cones. | 



Membrana limitans 

 externa. 



Outer nuclear J 

 layer. 1 



Her.le's fiber layer. 



Outer reticular 



layer. 



Inner nuclear-) 



layer. [ 



Inner reticular f 



layer. { 



Ganglion cell layer 



Nerve fiber layer.- 



Membrana limitans 

 interna. 



-—Vessels of the 

 / choriocapillaris. 



--Lamina basalis. 



-Rods \ Outer- 

 . Cones / segment. 



Cones') 



I Inner 

 f segment. 



^ Base of a cone fiber. 



Nucleus of a radial 

 fiber. 



Nucleus of an 

 amakrine cell. 



'yramidal base of 

 a radial fiber. 



Blood vessels. 



Fig. 177. Section perpendicular to the surface of a human retina. (X 36.) (Cour- 

 tesy, Bremer: "Text-Book of Histology," Philadelphia, The Blakiston Company.) 



