ENCEPHALIC NEKVES. 



451 



Fig. 296. 



Nerve-cells with processes, from the re- 

 tina of the ox. Magnified 350 diameters. 

 (KoUOcer.) 



1. The limitary membrane, A, is merely a simple membrane, 



of an inch thick. Its inner surface presents towards the hyaloid 

 membrane of the vitreous body. On 

 its outer surface Todd and Bowman 

 found a layer of epithelial (?) cells (c), 

 but which Kolliker has not seen. 



2. The next layer the expansion 

 of the optic nerve (1) has just been 

 described (p. 450). 



3. The layer of gray cerebral sub- 

 stance (2) penetrates between the fibres 

 of the optic nerve, but is quite well 

 defined on its outer surface. It is com- 

 posed of a finely granular matrix, 

 exactly resembling that in the gray 

 substance on the surface of the cere- 

 brum and cerebellum; containing nu- 

 merous scattered nerve-cells. Some 

 of the latter, in the outer half of this 

 layer, are small (^Vo to ^Vo of an 

 inch); others, in the inner portion, 



2iiW to ?io f an inch. They have from two to six, or more, pale 

 branched processes (Fig. 296), like those of the central nerve-cells; 

 these processes being at first g^^ of an inch in diameter, but di- 

 minishing in repeated divisions to 

 scarcely ^oio^ f an inch. They ap- 

 pear always to be given off towards 

 the exterior, but curve so as to ramify 

 in the gray layer itself. Its entire 

 thickness varies at points from g J c ^ 

 to 2^ of an inch. The distribution 

 of its capillaries is shown by Fig. 297. 



4. The granular layer (Fig. 295, 

 g). In man, the granules are dis- 

 posed throughout most of the retina 

 in two layers an internal, thinner 

 (s*W to T5 1 OIi of an inch), and an ex- 

 ternal, thicker (9 2 3 to ^is f an inch). 



Between these is a clear, finely granular, and somewhat vertically 

 striated layer (3), ^^^ to jJw of an inch thick. Towards the ora 

 serrata, however, the two form a single structure of not more than 



Fig. 297. 



Distribution of capillaries in the vascular 

 (gray) layer of the retina. 



