264 GENERAL ORNITHOLOGY part ii 



mals. The true sensitive membrane, in which the nervous filaments 

 end, is that investing ethmoidal (septal and turl)inal), not maxillary 

 parts. An associate structure of the olfactory organ is the nasal 

 gland, sometimes called the superorbital gland, from its position in 

 many birds. Thus it is of great size in a loon, and lodged in a 

 large deep crescentic depression on top of the skull over the orbits 

 (Fig. 63, tv) ; these crescents nearly meeting each other in the middle 

 line. In other birds it is smaller, and within the cavity of the 

 orbit, but never in that of the nose itself, its secretion being 

 poured into the nasal chamber by a special duct. 



Sense of Sight : Vision. — The eye is an exquisitely perfect 

 optical instrument, like an automatic camera obscura which adjusts 

 its own focus, photographs a picture upon its sensitised retinal 

 plate, and telegraphs the molecular movements of the nervous sheet 

 to the optic " twins " of the brain, where the result is " biogenised " ; 

 that is, translated from the physical terms of motion in matter to 

 the mental terms of consciousness. But no part of the nervous 

 tract, from the surface of the retina to the optic centre, sees or 

 knows anything about it, being simply the apparatus through which 

 the Bird looks, sees, and knows. In this class of Vertebrates, the 

 optic organs, both cerebral and ocular, are of great size, power, and 

 effect ; their vision far transcends that of man, unaided by artificial 

 instruments, in scope and delicacy. The faculty of accommodation, 

 that is, of adjusting the focus of vision, is developed to a marvellous 

 degree : rapid, almost instantaneous, changes of the visual angle 

 being required for distinct perception of objects that must rush into 

 the focal field with the velocity at least of the bird's flight. Birds 

 are therefore far-sighted or near-sighted (presbyopic or myopic) 

 according to the degree of tension the nerve-tide excites in the eye 

 by the mechanism described farther on ; and the transition from 

 one to the other state is effected with great quickness and correct- 

 ness. Observe an eagle soaring aloft until he seems to us but a 

 speck in the blue expanse. He is far-sighted ; and scanning the 

 earth below, descries an object much smaller than himself, which 

 would be invisible to us at that distance. He pi'epares to pounce 

 upon his quarry ; in the moment required for the deadly plunge he 

 becomes near-sighted, seizes his victim with unerring aim, and sees 

 well how to complete the bloody work begun. A humming-bird 

 darts so quickly that our eyes cannot follow him, yet instantaneously 

 settles as light as a feather upon a tiny twig. How far off it was 

 when first perceived we do not know ; but in the intervening frac- 

 tion of a second the twig has rushed into the focus of distinct 

 vision, from many yards away. A woodcock tears through the 

 thickest cover as if it were clear space, avoiding every obstacle. 

 The only things to the accurate perception of which birds' eyes 



