VISION. 345 



black pigment (x,) probably connected with a choroid 

 coat, which, from the delicacy of its texture, has hitherto 

 escaped observation. There exists opposite to the centre, 

 or axis of each corneule, a circular perforation (p,) which 

 performs the functions of a pupil.* Duges states, in- 

 deed, that he has witnessed in this part movements of con- 

 traction and dilatation, like those of the iris in vertebrated 

 animals. He has likewise found that there is a small space 

 (A) intervening between the extremity of each corneule and 

 the iris, and filled with an aqueous humour. The compart- 

 ments formed by the substance of the choroid (x) are conti- 

 nued inwards towards the centre of the general hemisphere, 

 the cylindrical spaces which they enclose being occupied 

 each by a transparent cylinder (v,) consisting of an outer 

 membrane, filled with a viscid substance analogous to the vi- 

 treous humour. Their general form and situation, as they lie 

 embedded in the pigment, may be seen from the magnified 

 sections; each cylinder commencing by a rounded convex 

 base, immediately behind its respective pupil, and slightly 

 tapering to its extremities, where it is met by a filament 

 (N) of the optic nerve; and all these filaments, after passing 

 for a certain distance through a thick mass of pigment, are 

 united to the large central nervous bulb (G, Fig. 427,) which 

 is termed the optic ganglion.^ 



* This pupillary aperture was discovered by Muller, and had eluded all 

 the efforts of former observers to detect it; and it was accordingly the pre- 

 vailing notion that the black pigment lined the whole surface of the cornea, 

 and interposed an insuperable barrier to the passage of light beyond the 

 cornea. It was evidently impossible, while such an opinion was entertained, 

 that any intelligible theory of vision, with eyes so constructed, could be 

 formed. 



t Numberless modifications of the forms of each of these constituent parts 

 occur in different species of insects. Very frequently the vitreous humour 

 (v, ) instead of forming an elongated cylinder, has the shape of a short cone, 

 terminating in a fine point, as shown in Fig. 426. Straus Durckheim ap- 

 pears to have mistaken this part for an enlarged termination of the optic 

 nerve, believing it to be opaque, and to form a retina applied to the back of 

 the corneule, which latter part he considered as properly the crystalline lens. 

 In his elaborate work on the anatomy of the Melolontha, he describes the 



VOL. II. ^44 



