NULLIFICATION OF THE BLIND SPOT 179 



a physiological (normal) scotoma or gap in the visual field within which 

 nothing can be seen. We humans are not aware of our blind spots, for 

 since the two retinal topographies are mirror-images of each other and 

 both are aimed forward, any object whose image falls within the disc of 

 one retina is simultaneously imaged upon functional retina in the other 

 eye. We are not even aware of the blind spot when one eye is kept 

 closed, and can demonstrate it to ourselves only in an experiment such as 

 is shown in Figure 73. An animal whose eyes are on the sides of his 

 head, however, might as well have one eye closed so far as concerns what 

 the other fundus is seeing; and hence he cannot fill in, with each eye, 

 the blind spot of the other. 



The blind spot becomes a serious matter only where the disc is rela- 

 tively large; but this happens to be inevitable when the eye is especially 

 well adapted for diurnality. For, it will then have a preponderance of 

 cones, and the consequent great numbers of ganglion-cell axons make 

 for a relatively heavy optic nerve and a large disc. On the other hand, 



Fig. 73 — Demonstration of the blind spot. 



Cover the right eye; fixate the star steadily and move the book slowly toward and away 

 from the face. The words at the left will disappear and reappear as their image swings 

 on and off of the head of the left optic nerve. 



the disc of a mouse is a mere dot, for each of the few optic nerve fibers 

 is connected with hosts of rods. 



In three diurnal assemblages the disc has become a narrow, greatly 

 elongated oblong : in the squirrels, the birds, and the predaceous pikes, 

 salmonoids, and percoids among the teleost fishes. Elsewhere it is usually 

 circular but it may be oval, reniform, triangular — always, however, com- 

 pact. A fatally large, compact disc has been avoided in the fishes by 

 permitting the developing optic nerve fibers to fill in the whole length of 

 the embryonic fissure of the optic cup, instead of massing them at the 

 apex of the fissure (see p. 108) as other vertebrates do. The optic nerve 

 thus often departs from the fish eyeball as a ribbon rather than a cord, 

 and becomes crumpled edgewise to gain a circular cross-section between 



