3 
here and there so quickly that the human eye can not follow it, and 
yet comes suddenly to rest on an almost invisible twig; in the 
woodcock that flics through the thick woods, avoiding every tree, 
shrub and branch as if they were non-existent; in the owl that 
combines good diurnal with good nocturnal vision, and in the 
kingfisher that secs as well in the air as he does in water. 
Bird vision, then, is not only the highest expression of eyesight 
both as to acuity and variety, but it repeats the oft-told tale of the 
correlation of sight in the animal to its life history as witnessed 
chiefly in the pursuit of food and escape from death or injury. If, 
then, as ophthalmologists we engage in the study of the physiology 
and mechanics of vision, surely a consideration of the ocular appa¬ 
ratus of birds—even in the superficial fashion in which I shall be 
obliged to present it to you here—will not be profitless. 
The size and shape of the eyeball of birds must be noted. It is 
unusually large in comparison with that of other animals and in 
proportion to the size of the avian body. Of course, size alone is 
not the only consideration in determining effectiveness of function 
in an organ, but when one pursues the subject still further it will 
be seen that the cerebral and spinal parts of the ocular system are 
proportionately better developed and larger than in the great 
majority of other vertebrates. Almost all birds whose habits re¬ 
quire the widest range of vision—eagles, hawks and vultures, for 
example—are noticeable for very prominent eyeballs, laterally 
placed. In the owl, whose eyes are adapted to searching only space 
in front of the globes and orbits, they are disposed as in man and 
the higher apes. Like them, the owl has the power and possesses 
the apparatus common to all animals that enjoy the privileges of 
binocular vision. 
THE EYELIDS OF BIRDS. 
First of all, let us consider their eyelids. There is something 
very human about the true lids of most birds. They have, as a 
rule, the same dermal folds, the same cilia, and much the same 
minute structure as we have—all employed in the same way for 
the same purpose. 
The Nictitating Membrane .—The third eyelid found in reptiles, 
some mammals, and commonly seen as a vestigial remains in man 
(plica semilunaris) and the higher apes, finds its highest develop- 
