634 SUMMARY OF CURRENT RESEARCHES RELATING TO 



fibrils, which are similar to those described by Gerlach in the 

 mammalian brain and spinal cord. 



The comparative anatomy of these parts is thus summed up : " In 

 the young Periplaneta the optic nerve-fibrils vv^hich leave the peri- 

 opticon pass without decussating, to the ommateum (eye proper) ; in 

 the adult Periplaneta there is a partial decussation, in Nepa there is 

 no decussation, but the anastomosis is complicated by the presence 

 of looped and transverse anastomoses. In Musca, the fibrils are split 

 up into little cylindrical blocks of neurospongium, which I have 

 called the elements of the peri-opticon ; in bees, wasps, and many 

 Lepidoptera, the elements of the peri-opticon are long, slender, and 

 close-set ; in ^schna they have partially fused with one another ; 

 and in Bomhyx, Eristalis, and the Crustacea they have completely 

 fused to form a complete and continuous ganglion, similar in every 

 way to the opticon and epi-oj)ticon. 



Three series of pigment- cells are very constant throughout the 

 Hexapoda ; there are (1) a series of pigment-cells which insheath 

 the cone and prevent extraneous rays of light from escaping ; they 

 may be called the cone pigment-cells. (2) In the outer region of the 

 rhabdom there is a series of external pigment-cells, which have long 

 processes passing between the retinulae and elsewhere. (3) The 

 name of internal pigment-cells is given to the series which usually 

 rests upon the basilar membrane. This last varies considerably in 

 thickness. 



In the historical and critical portion of his paper, Dr. Hickson 

 deals only with what has been published since 1879, the date of 

 Grenacher's great work. With regard to the view of Mr, Lowne 

 that the retinulae are not the nerve-end-cells at all, and that the true 

 retina is situated behind the basilar membrane, the author remarks 

 that not only does anatomy teach us that the optic nerve-fibrils end 

 in the retinulae, but morphology teaches us that they are homologous 

 with the nerve-end-cells of other animals, while the few physiological 

 experiments yet made show that they are eminently adapted for light- 

 perceiving purposes. These considerations are clenched by Leydig's 

 discovery of a true retina-purple in the retinula. 



The view of Ciaccio, Berger, and others, that the layer of retinulae 

 and rhabdoms cannot be considered as the equivalent of the retina 

 of other animals is accepted ; it is only part of the retina, or that 

 which bears the nerve-end-cells, and corresponds functionally to the 

 layer of rods and cones in the eyes of Vertebrates. We cannot 

 compare layer for layer the different strata of eyes in different 

 animals ; all we can say is that in all animals with highly organized 

 eyes, there are certain complicated nervous structures, between the 

 nerve end-cells and the brain, which have probably the function of 

 elaborating and combining the sensations received by the end-cells. 

 The author thinks that all the nerve-structure lying between the 

 crystalline cone-layer and the optic nerve is analogous with the retina 

 of other animals ; in other words, the retina of insects consists of the 

 retinulsB, peri-opticon, epi-opticon, opticon, and all the intermediate 

 nerve-tracts. 



