536 



THE CELL AND MAMMALIAN HISTOLOGY 



with appreciation of low light intensities (twilight vision) the 

 cones alone can detect colour. How they do this is still a matter of 

 dispute The action of the rods is connected with the breakdown 

 of a pigment called visual purple, which is closely related to 

 vitamin A ; deficiency of this vitamin is well known to cause 



nQr^e fibres 

 ganglionic layer 



inner synapse layer 



inner nuclear layer 



outer synapse layer 



outer nuclear layer 



Fig. 419. 



rods and cones 



epithelium 



-A transverse section of the human retina. 

 Elements of Anatomy. 



-From Quain, 



difficulty in seeing in dim light (' night blindness '). The sensitivity 

 of the fovea is very nearly at the theoretical limit both in the 

 intensities which will cause a response and in the size of particles 

 which can be discriminated. 



The sclerotic consists mainly of fibrous tissue, with pigmented 

 connective tissue inside it. The cornea has fibrous tissue continuous 

 with that of the sclerotic, and it is covered externally by a 



