THE EVIDENCE OF THE ORGANS OF VISION 



89 



1. The optic ganglion of the Arthropoda consists of two parts, of 

 which the one stands in direct inseparable connection with the 

 facetted eye, and together with the layer of retinal rods forms the 

 retina of the facetted eye, while the other part is connected rather 

 with the brain, and is to be considered as an integral part of the 

 brain in the narrower sense of the word. 



2. In all arthropods examined by him, the retina consists of five 

 layers, as follows :— 



(1) The layer of rods and their nuclei. 



(2) The layer of nerve-bundles. 



(3) The nuclear layer. 



(4) The molecular layer. 



(5) The ganglion cell layer. 



Berger passes under review the structure and arrangement of 

 the optic ganglion in a large number of different groups of arthropods, 

 and concludes that in 

 all cases one part of 

 the optic ganglion is 

 always closely attached 

 to the visual end-cells, 

 and this combination 

 he calls the retina. 

 On the other hand, the 

 nerve-fibres which con- 

 nect the peripheral part 

 of the optic ganglion 

 with the brain, the so- 

 called optic nerve, are 

 by no means homolo- 

 gous in the different 

 groups ; for in some 

 cases, as in many of 

 the stalk-eyed crusta- 

 ceans, the whole optic 

 ganglion is at the pe- 

 riphery, while in others, as in the Diptera, only the retinal ganglion 

 is at the periphery, and the nerve-stalk connects this with the rest 

 of the optic ganglion, the latter being fused with the main brain- 

 mass. In the Diptera, in fact, according to Berger, the optic nerve 



Fig. 38.— The Retina of Musca. (After Berger.) 



Br., brain; O.n., optic nerve; n.l.o.g., nuclear layer of 

 ganglion of optic nerve; m.L, molecular layer 

 (Punktsubstanz) ; n.l.r.g.i. and n.l.r.g.o., inner and 

 outer nuclear layers of tbe ganglion of tbe retina ; 

 f.lr.r., terminal fibre-layer of retina; r., layer of 

 retinal end-cells (indicated only). 



