278 INSECTA. 



From the nervous expansion so formed arise the proper optic 

 nerves (e), appropriated to the individual eyes or ocelli, as we shall 

 term them. These nervous filaments are as numerous as the 

 facets of the cornea, and traverse the common choroid to radiate to- 

 wards the individual eyes whereunto they are respectively destined, 

 and the structure of which we must now proceed to examine. 

 In fig. 123, B, a portion of the circumference of the compound 

 eye is represented upon a very large scale, in order to show the 

 construction of the hexagonal ocelli that enter into its composi- 

 tion. Each cornea (i) is a double convex lens, adapted by its 

 shape to bring to a focus the rays passing through it. Behind 

 every lens so constituted is placed an hexaedral transparent prism 

 (A), which from its office may be compared to the vitreous hu- 

 mour of the human eye ; and it is upon the posterior extremity 

 of these prisms that the proper optic nerves (jig- 1 23, A, e) spread 

 themselves out, so as to form so many distinct retinae. When we 

 reflect upon the extreme minuteness of the parts above alluded to, 

 we may well expect slight discrepancies to occur between the ac- 

 counts given of them by different anatomists. Strauss Durckheim 

 represents every optic nerve as terminating in a minute pyriform 

 bulb (fig. 123, B,/), and points out a dark layer of pigment (g), 

 which forms a choroid tunic proper to each ocellus ; while, accord- 

 ing to Muller and Duges, the vitreous humours (h) are conical, and 

 terminate posteriorly in a sharp point, upon which the terminal 

 expansion of the optic nerve spreads out without any pyriform 

 enlargement : they likewise deny the existence of the proper 

 choroid (g) in the situation indicated by Strauss, but find a black 

 pigment situated immediately behind the cornea, that at first sight 

 would appear to be continuous over the whole surface of the eye. 

 Even Cuvier seems at one time to have adopted this opinion ; 

 Muller, however, found that, upon carefully removing the internal 

 structures of the organ, leaving the pigment untouched, the dark 

 varnish in question, although very thick at the lines of union of 

 the different facets, where it is continuous with a choroid that 

 separates the individual ocelli, yet towards the centre of each facet 

 it becomes exceedingly thin, and at the very centre is quite want- 

 ing, so that a minute perforation or pupil is thus left, through 

 which the rays of light enter. The existence of the secondary 

 optic nerves (b) and common retina (c) is likewise disputed by 

 Muller and Duges, who consider the proper optic nerves to arise 

 immediately from the surface of the brain. 



