VISION. 489 



movements of contraction 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 cor- 

 neule and the iris, and filled with an aqueous 

 humour. The compartments formed by the sub- 

 stance of the choroid (x) are continued inwards 

 towards the centre of the general hemisphere, 

 the cylindrical spaces which they inclose being 

 occu2:)ied each by a transparent cylinder (v), 

 consisting of an outer membrane, filled with a 

 viscid substance analogous to the vitreous hu- 

 mour. 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, imme- 

 diately 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* 



the whole surface of the cornea, and interposed an insuperable 

 barrier to the passage of light beyond the cornea. It was evi- 

 dently impossible, while such an opinion was entertained, that 

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

 be formed. 



* Numberless modifications of the forms of each of these con- 

 stituent parts occur in different species of insects. Very fre- 



