VISION. 



437 



with a choroid coat, which, from the delicacy of its 

 texture, has hitherto escaped observation. There 



425 



426 



exists opposite to the centre, or axis of each cor- 

 neule, a circular perforation (p), which perforins 

 the functions of a pupil.* Duges states, indeed, 

 that he has witnessed in this part 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 corneule and the iris, and filled 

 with an aqueous humour. The compartments 

 formed by the substance of the choroid (x) are con- 

 tinued inwards towards the centre of the general 

 hemisphere ; the cylindrical spaces which they 



* This pupillary aperture was discovered by MuUer, after it had 

 eluded all the efforts of former observers to detect it ; and it was 

 accordingly the prevailing 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 impos- 

 sible, while such an opinion was entertained, that any intelligible 

 theory of vision, with eyes so constructed, could be formed. 



