V] 



FORM AND FUNCTION 



125 



the picture of the world without. It is made up of 

 very delicate rods and cones — -30,000,000 of the 

 former and 3,000,000 of the latter in the human eye, 

 at the lowest estimate. On these millions of sensitive 

 points the image is formed. They extend over 

 the back of the eyeball, but there is a central mark, 

 called the Macula Lutea (ML) or yellow spot, which 



The entrance of 



is the region of clearest vision 

 the nerve is rather towards 

 the nose (ON), and at this 

 point the eye is blind, as 

 a simple experiment will 

 show (BL). 



Hold the book at arm's 

 length, close the left eye, and 

 fix the right upon the cross 

 mark, the image of which 

 will fall upon the Macula 

 Lutea. The dot will be also 

 visible. Now move the book 

 slowly towards you, and the 

 image of the dot must at 

 length fall upon the point 

 where the nerve enters the 

 eye. At this moment the 



+ 

 dot will disappear, then again at a nearer distance 

 come again into view, as the image of it gets once 

 more clear of the blind spot. The rods and cones 

 cover the whole of the back of the eye, except this one 

 point. Another experiment shows that it is the hind- 

 most layer of the retina that is sensitive. Let a candle 



Fig. 32. — Section of Retina of duck 

 (after Cajal) ; C, Cone; R, Rod; 

 S, cells of supporting tissues. 



