224 Di$coveries of Milller and others in the 

 _* 37 



38 



This zone {^fig» 35. g), of a deeper black colour than the pre- 

 ceding, and of greater thickness at the front than at the back 

 part of the eye, contains the nervous filaments, which, arising 

 from the bulb or ganglion of the optic nerve, terminate in the 

 transparent cylinders already described. Like these latter, 

 the filaments converge from the circumference towards the 

 centre, are linear, straight, and as nearly parallel as their 

 radiated disposition will permit ; but they are much smaller in 

 diameter than the cylinders, and, notwithstanding their slen- 

 derness, appear, under the microscope, somewhat opaque and 

 of a fibrous texture. Surrounded by a dark choroid secretion, 

 (Jig, 36. 2*), these filaments, on account of their great tenuity, 

 cause the pigment to appear much thicker and darker, when 

 regarded en masse, than that portion of it represented as 

 passing between the cylinders. These latter are almost in 

 immediate contact with each other : the nervous filaments, on 

 the contrary, are separated by spaces much exceeding in size 

 their own diameter. 



In the centre of the eye is the optic ganglion {Jg, 33. J), 

 which, however pulpy and homogeneous it may appear at first 

 sight, exhibits nevertheless a fibrous and radiated structure 

 when submitted to moderate compression. Indeed, it may in 

 some degree be regarded as the optic nerve passing into the 

 filamentary arrangement observed a little farther fi-om the 

 centre. 



Such are the anatomical details exhibited in the eye of the 

 grey Libellula and of other insects, with some modifications 

 to be hereafter noticed. In examining each of these parts, we 

 may, to a certain extent, refer them hypothetically to the 



