SENSE OF SIGHT. 443 



though in very small quantities, in the other parts of the 

 retina. 



Finally, the retina contains vessels, and terminal branches 

 of the central artery of the optic nerve, which emerges in 

 the centre of the papilla, and surrounds the yellow spot 

 with its ramifications (Fig. 121). 



121. Appearance 01 tne posterior half of the retina of the left eye, 

 examined with the ophthalmoscope (Liebreich). 



The retina forms essentially the sensitive membrane of the 

 eve j by whatever cause its sensibility is excited, it always 

 gives rise, as a subjective phenomenon, to what is known by 

 the name of luminous sensation. If the retina be pricked 

 (Magendie), compressed (phosphenes,phosphainoe, studied by 

 Serre of Uzes), twitched by any sudden movement of the 

 eye, or, in short, excited in any way, an impression of light 

 will be produced ; the same effect follows the use of electric- 

 ity. The special method, by means of which the luminous 

 sensation is distinguished from all others, does not, therefore, 

 reside in the qualities which are peculiar to external light ; 

 there is no exclusive connection between light and luminous 



