328 THE BIOLOGY OF THE FROG chap. 



The optical portion of the retina which hnes the whole pos- 

 terior region of the eye and extends as far forward as the 

 ciliary body, is a complex structure formed mainly of nervous 

 elements. It is the part of the eye which is especially sen- 

 sitive to light and upon which images of external objects are 

 thrown. In addition to the outer pigmented epithelium it is 

 composed,as in the higher vertebrates, of nine layers, the latter 

 forming the inner or thicker stratum of this structure. These 

 kiyers taken in order from within outward are as follows : — 



1. The inner Umiting membrane, a thin supporting mem- 

 brane lying next to the vitreous humor. It is connected with 

 elongated cells ( Midler's fibers) which extend to the outer 

 limiting membrane and give off branches which form a sort 

 of supporting network for the nervous elements of the retina. 



2. A layer of nerve fibers, formed by the ramifications of 

 the optic nerve. 



3. The inner ganglionic layer, formed by comparatively 

 large ganglion cells whose processes extend into the preced- 

 ing and following layers. 



4. The i7i7ier reticular layer, formed by a network of 

 nerve fibers. 



5. The itiner nuclear layer, composed of several layers of 

 ganglion cells whose processes extend into both of the 

 adjacent layers. 



6. The outer reticular layer, a narrow layer formed by a 

 network of nerve fibers derived from the inner nuclear layer 

 and the layer of rods and cones. 



7. The outer nuclear layer contains the nucleated por- 

 tions of the cells forming the rods and cones and a few 

 isolated ganglia. 



8. The outer limiting membrane, a very thin supporting 

 membrane, perforated in numerous places by the cells of the 

 outer layer. 



