71 



and its outer surface rests against the skull and the lateral ligaments of the skull, 

 and gives off the large optic tract which enters the optic ganglion at once. 



The optic gangUa lie in the concavities of the lateral processes of the skull, 

 and behind and inside the eyes. Each ganglion is bean-shaped and is attached to 

 the pleural ganglion by the optic tract which enters the concave side or hilum 

 of the ganglion. The opposite (ventro -external) side rests upon the dorso-internal 

 surface of the eye. The longest axis of the ganglion is directed obliquely upward 

 and forward , and the gaps above and below between the ganglion and the eye are 

 filled by the white bodies. The artery which supplies the ganglion enters thru 

 the optic tract. The veins of the ganglion open into the optic sinus. 



The numerous fibres of the optic nerve arise from the retinal cells and, 

 forming small bundles, pass thru the foramina in the sclera and, converging, 

 pass thru, the narrow slit between the white bodies. They then diverge and, 

 passing over the surface of the ganglion for some distance, finally enter it. 

 Almost the entire surface of the ganglion , except the hilum , is covered by the 

 fibres of the optic nerve. 



The optic ganglion is formed of a stratified cortex and a coarsely reticular 

 medulla. The cortex covers the convex surface of the ganglion and the concave 

 hilum is covered largely by the entrance of the optic tract. The structure of 

 the optic ganghon of Cephalopods has been thoroughly studied by Lbnhossek, 

 whose work we summarize here. 



The cortex of the gangUon is composed of an outer granular layer , a plexi- 

 form layer and the inner granular layer. The plexiform layer contains two 

 zones, the inner and the outer plexus, having a preponderance of horizontal 

 fibres, which separate three zones having a larger proportion of vertical fibres. 

 In a small adult of Loligo pealii the outer granular layer is 72 micra, the 

 plexiform layer 170 to 180, and the inner granular layer, 180 micra thick. The 

 outer granular layer contains unipolar cells of three types : - numerous small 

 cells which lie near the inner surface of the layer, intermediate cells located 

 near its middle , and large cells lying at or near its outer surface. The largest 

 cells are about 40 micra long, their nuclei are about 13 micra in diameter and 

 the nuclei of the small cells are about 5 micra in diameter. The cell process of 

 each of the small inner cells passes thru the outer clear zone and ends in a brush 

 in the outer horizontal plexus. The processes of the large cells are sparingly bran- 

 ched and terminate in the horizontal plexuses and the middle zone. The processes 

 of the intermediate cells are like those of the large cells except that they bear 

 irregular brushes in the outer horizontal plexus. The plexiform layer is formed 

 by the interlocking processes of the ganglion and of the retinal cells together 



