PHYSIOLOGY. 



an inch across and is one-tenth of an inch to the outer side of the 

 point of entrance of the optic nerve. Its center is the fovea cen- 

 tralis. In the fovea there are no rods; cones only are present, and 

 these are longer and narrower than those of the other parts of the 

 retina. 



When the optic nerve penetrates the eye it projects somewhat 

 beyond the inner surface of the eyeball as a papilla. In this papilla 

 there are none of the essential nerve-elements of the retina, so that 

 rays of light cannot be perceived by this particular area; hence the 

 name of blind spot. 



The nervous layer of the retina is composed principally of the 

 terminal nerve-elements of the optic nerve. Externally, it is coated 



Fig. 337. Right Eye, Normal Fundus Oculi. (BALL.) 



with a pigment-layer; internally, it is lined with a homogeneous, 

 transparent structure, the hyaloid membrane. 



Histological Structure. The histological structure of the retina 

 is very complicated. The retina is really an outward expansion of 

 the original forebrain. The retina is usually divided into eight 

 layers : 



1. The layer of nerve-fibers. 



2. The layer of ganglionic cells. 



3. The inner molecular layer. 



4. The inner nuclear layer. 



5. The outer molecular layer. 

 G. The outer nuclear layer. 



7. The layer of rods and cones. 



8. The hexagonal pigment-layer. 



