HISTOLOGY OF THE RETINA 



541 



In order to understand the structure of these layer?,' it is necessary to keep 

 the fact in mind that the optic nerve and cup arc outgrowths of part of the 



EXTERNAL 



OUTER. MOLECULAR 

 LAYER. 



GANGLIONIC LAYER 



STRATUS* QPTICUH 



FIG. 271. Diagram of transverse section of retina. 



brain. We must therefore be prepared to find in the retina the presence of all those 

 structures which are found in the case of every sensory neive to intervene 

 between the sense cell and the brain nucleus. Be it taste-cell or touch-cell, or 

 sensory cell of any other kind, the stimulus is conveyed in every case through three 

 sets of neurons or relays before it reaches the brain. We must therefore expect to 

 find in the retina all these three sets of neurons represented. 



THE NERVE FIBRE LAYER (stratum opticum) consists of the non-myelinated 

 (non-medullated) axons of the large ganglion cells found in the second layer. These 

 axons are the third order neurons which become myelinated after they have passed out 

 of the eyeball and travel by paths to the occipital cortex described on page 52. 

 Beside the fibres conveying visual impressions there are others which belong to the 

 pupillo-motor reflex. Others again bring impulses from the brain to the retina ; their 

 functions will be considered below. 



THE GANGLION NERVE CELL LAYER consists of a single layer of large oval 

 cells. These are nucleated and give off the axons which we have already described 



