VI 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 
is the region of clearest vision. The ‘entrance of 
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 | = : 
Vic. 32.—Section of Retina of duck 
where the nerve enters the GG ee Bere to aa 
eye. At this moment the ~ tik de 
+ 
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 acandle 
Oe 
ON 
