578 ANIMAL PHYSIOLOGY. 
Affections of the Retina.—There is no doubt that the fibers of 
the optic nerves can not of themselves be directly affected by 
light. This may be experimentally demonstrated to one’s self 
by a variety of methods, of which the following are readily car- 
ried out: 1. Look at the circle (Fig. 421) on the left hand with the 
Fig. 421 (after Bernstein). 
right eye, the left being closed, and, with the page about twelve 
to fifteen inches distant, gradually approximate it to the eye, 
when suddenly the cross will disappear, its image at that dis- 
tance having fallen on the blind-spot, or the point of entrance 
of the optic nerve. 2. Fixing the eye as before on a mark on 
a sheet of white paper made by a pen, draw the latter outward 
till its point disappears from view. Mark the location of the 
pen-point when this occurs, and continue the movement till it 
again appears. Mark this point also. This process may be 
continued in other directions besides the horizontal, and, by 
joining these points, an irregular outline is formed, marking 
off a portion of the “visual field,” within which there is really 
no vision. 3, A small image from a flame projected on the 
blind-spot by a mirror is not visible, though readily perceived 
when it falls on the retina proper. 
Fig. 422.—Diagrammatic section of macula lutea (after Huxley). a, a, pigment of choroid ; 
b,c, rods and cones; d,d, outer granular or nuclear layer; f, f, inner granular layer ; 
g, g, molecular layer ; h, h, layer of nerve-cells ; ¢, 2, fibers of optic nerve. ; 
It remains, then, to determine what part of the retina is 
affected by light. The evidence that it is the layers of rods 
