452 



THE TISSUES. 



f an i nc h thick. The granular bodies themselves are round 

 or oval, and g^^ to -3^3 of an inch in diameter, with very fine 

 filaments extending from both sides, of ^J^ to ^J^ f an i n h. 



5. The external layer of the retina is the bacillar layer, or Jacob- 



^SOttrs membrane (m). Its structure is very remarkable, presenting two 



elements the rods (bacilli) and the cones (coni) blended together 



in a single layer, ^-^ of an inch thick at the bottom of the eye, -g fa 



more anteriorly, and quite in front not more than -gfa of an inch. 



The rods are cylindrical, slender, elongated corpuscles, consisting 

 of two portions, the larger external end, or proper rod, and the 

 internal, or filament. The former is a cylinder ygVo to g&o of an 

 inch long, and T^^ of an inch broad, and truncated at the outer 

 end; while the inner is produced into a short point, ? oW to 4oV<? 

 of an inch long from the inner extremity of the rod ; is of uniform 

 width, and extends through the inner half of the bacillar layer. 

 The substance of the rods is clear and homogeneous, with a faint, 

 glistening, fatty aspect, and is very soft, flexible, and fragile. They 



Fig. 298. 



Retinal elements of man. 1. "Kods" and radiating fibres: fc, proper "rod;" r, prolongation of 

 Its pointed inner extremity ; h, "granule" (cell) of the outer granular layer ; I, enlarged extremity 

 of the radiating fibres, proceeding from them to the surface of the optic layer ; fc', "rod" seated on 

 a "cone" (i) ; r', fibre proceeding from the latter, connected with the "granule" (/) of the inner 

 granular layer, and the terminal enlargement (Z) on the inner surface of the retina ; n, one of the 

 fibrous bundles in which the radiating fibres frequently terminate at their innermost extremity. 2. 

 Rods torn from their fibres. 3. Fibres of optic nerve ; c, fine a and b, varicose. 4. Two cones torn 

 from their processes, d; with attached rod, a, at outer end ; c, nucleus. Magnified 350 diameters. 

 (Kolliker.) 



