THE EYE 



»5«9 



eyeball, and about ^V ^^ch below its level, is the porus opticus, 

 or optic disc. This is circular in outline, and its circumference is 

 slightly elevated. It represents the point of entrance of the fibres 

 of the optic nerve, and the centre of the disc is pierced by the 

 arteria centralis retinae, which immediately divides into two 

 branches — upper and lower. The optic disc consists entirely of 

 nerve-fibres, and is known as the * blind spot,' vision being absent 

 in this situation. 



Structure of the Retina. — ^The retina consists of eight super- 

 imposed layers, seven of which are nervous and one pigmentary. 



Macula Lutea, with 

 Fovea Centralis 



Poms Opticas 



Fig 633. 



-The Posterior Portion of the Right Retina 

 (Anterior View). 



In addition to these, there are sustentacular fibres. The eight 

 layers arfe as follows, from within outwards : 



1. Stratum opticum, or layer of nerve-fibres. 



2. Ganglionic layer, or layer of nerve-cells. 



3. Inner molecular (inner plexiform) layer. 



4. Inner nuclear or granular layer. 



5. Outer molecular (outer plexiform) layer. 



6. Outer nuclear or granular layer. 



7. Layer of rods and cones, or bacillary layer. 



8. Pigmentary laj'er. 



In addition to the foregoing layers, there are two very delicate 

 membranes, which really belong to the sustentacular fibres of the 

 retina, but are known as the membrana limitans interna and mem- 

 brana limitans externa. The membrana limitans interna covers 

 the retina on its internal surface, and the membrana limitans 



