48B 



THE SENSORIAL FUNCTIONS. 



smaller than the convexity of the former, its 

 optical effect is that of a meniscus, or concavo- 

 convex lens, with power of converging to a dis- 

 tant focus the rays of light which traverse iti 



425 



428 



EG^ 



Within these corneules there is extended a layer 

 of an opaque black pigment (x), probably con- 

 nected with a choroid coat, which, from the deli- 

 cacy of its texture, has hitherto escaped obser- 

 vation. There exists opposite to the centre, or 

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

 which performs the functions of a pupil.* Dug^s 

 states, indeed, that he has witnessed in this part 



* 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 



