30 



EVOLUTION OF THE HUMAN EYE 



sections show the mass to be made up of flattened epithelial 

 cells, which line and form columns between numerous, 

 variously-sized circular spaces. 



The function of the corpus nigrum has so far not been 

 definitely determined. Lindsay Johnson 3 suggested that it 

 served as a shade for the large oval pupil. Such a suggestion 

 does not explain the presence of a projection from the lower 

 margin of the pupil as well as the upper. In some animals, 



FIG. 5. Front of horse's eye from which cornea has been removed to expose 



iris with the "corpus nigrum." 



e. </., gazelles, camels and llamas, the black bodies from the 

 upper and lower margins of the pupil have indentations 

 capable of fitting into one another. On examination of 

 horses ' eyes, when exposed to bright sunlight, I have observed 

 that contraction of the pupil causes the pigmented bodies 

 from the two borders to meet and fill up its center, leaving 

 the two extremities still open. So that the horse then 

 has, instead of one oval pupil, two circular ones, looking 



