MULTIPLE OPTIC PAPILLA 367 



ventrally along the course of the embryonic fissure (see p. 107) ever 

 casts a shadow on the fundus at all unless the bird looks up at the sky. 

 The shadow may not often be very useful in terms of Menner's hypoth- 

 esis; and, assuming that there usually is a shadow, it may not operate 

 entirely as Menner believes it to. Any moving object — the object itself, 

 not its motion — should be seen more clearly if seen intermittently. One 

 can often count the blades of a rotating electric fan by blinking the eyes 

 rapidly so that each glimpse is only momentary. This, in fact, is the 

 very essence of stroboscopy — everyone knows that it takes a fast camera 

 shutter to 'stop' fast motion. 



If a moving object is seen only intermittently, its nature can be better 

 made out since each image of it on the retina is less blurred by dragged 

 after-images. But it does not seem as if the fingers of a pecten shadow 

 are numerous enough and sufficiently close together to afford a series of 

 snap-shots of a moving object, analogous to those obtained by Eadweard 

 Muybridge's row of cameras. And if they were, they would hinder vision 

 in general for the bird, which would hardly benefit from being made to 

 look at the world as if through a picket fence. But if there is anything 

 to Menner's theory, it may help to explain another peculiarity of ocular 

 structure : multiple optic papillae. 



Multiple Optic Papillce — In a number of fishes (for example Amei- 

 urus, Misgurnus, Polyipnus, and Polypterus) , in various salamanders, 

 and in members of the deer family, the optic nerve on approaching the 

 eyeball divides into as many as a dozen or more separate rootlets, form- 

 ing an equal number of separate little blind spots with functional retinal 

 tissue around and between them. In the squirrels (pp. 179-80) we saw 

 a deformation of the blind spot which is intended to minimize the 

 scotoma effect, thus promoting overall visual acuity. The situation in 

 the Cervidas may have this same meaning. But the multiple blind spots, 

 which look as though they might have the same purpose of avoiding a 

 single huge scotoma, mostly occur in forms with abysmally low visual 

 acuity. Of the fishes listed above, Polyipnus is a deep-sea form, and none 

 of the others has much, visually, beyond brightness- and movement-per- 

 ception. Such animals have not needed the break-up of their blind spots 

 to enable them to see more sharply, for they are beyond any such help. 

 But if a turning on and off of the reception of a moving retinal image 

 helps the movement to break into their dull minds, the multiple papillae 

 may do for them what Menner thinks the pecten does for the birds. 



