Cassell's book of birds; vol. 1

by Alfred Edmund Brehm

The upper part of the skull has been removed to show the brain and eyes in situ. a, a, the Cerebral Hemispheres; b, the Cerebellum; c, the Spinal Cord. All the above parts are represented covered by their investing membranes. e, Sinuses of the Dura Mater; g, g, Walls of the Skull; h, h, Base of the Beak; m, Optic Nerve of the Left Eye; p, p, Large sentient Nerves supplying the Bill; v, Bony Ring in front of the Eye-ball; x, x, Transparent Cornea; 1, 3, 5, 7, Muscles moving the Eye-ball.

In strict correspondence with this exalted condition of their cerebral organisation are the senses whereby they hold intercourse with surrounding nature. Their power of vision is beyond our comprehension, and the elaborate contrivances whereby the eye of a bird is adapted to its peculiar mode of life, might furnish materials for a lengthy treatise, imperfect as is our knowledge of the numerous delicate arrangements demonstrable by anatomical skill in every part of its structure. At present we can but briefly allude to a few of the more conspicuous peculiarities wherein the visual apparatus of a bird differs from that of other creatures.

The distances from which the vulture and the hawk can see their prey are almost incredible. To have the "eye of a hawk," to see with "eagle glance," are expressions which, though common enough, give but a very feeble idea of the extent to which those birds are gifted in this respect, or of the vast expanse bounded by their horizon. The falcon sees its diminutive prey from an altitude at which it is itself invisible, and from the very sky swoops down upon its quarry with the velocity of a shot, rarely missing its victim, and thus proving at once the perfection of its sight and the steadiness of its aim. The eye of these birds must therefore be constructed after the plan of a telescope, and its focus adapted to long-sightedness. Its axis must be lengthened to an extent greater than is compatible with a spherical form of the eye-ball. To meet this requirement a circlet of bony plates, constituting a firm but at the same time somewhat flexible ring or hoop, is introduced into the composition of the outer coat of the eye, whereby the requisite elongation is effected, and the organ is thus adapted for perfect vision at a great distance. (See Fig. 4.)

The above beautiful arrangement, however, constitutes but a part of the mechanism required. A telescope adjusted for distant vision is quite useless when brought to bear upon an object close at hand, and its focus must necessarily be altered in accordance with the changed conditions. In the case of the telescope, the needful adjustment would be effected by shortening or lengthening the sliding tube; but in the bird some other plan is evidently indispensable, and few contrivances in animal mechanics are more admirable than that which is adopted. Embedded in the transparent vitreous humour of the eye is a peculiar apparatus called the "marsupium," the texture of which resembles that of the human iris. Now the iris, as we all know, being eminently sensitive to the intensity of light, by its spontaneous contractions and dilatations is enabled to alter the diameter of the pupil of the eye, and thus exactly control the quantity of light admitted. The marsupium, equally sensitive, and equally spontaneous in its action, swells or contracts its dimensions, filling or emptying itself like a sponge, and thus adjusting the lenses of the eye so as to secure perfect vision at whatever distance the object to be seen may be placed. The quickness of sight with which birds are gifted is equally remarkable. The swallow is proverbially one of the swiftest flyers in the feathered creation, and yet in the full career of its flight it is looking on the right hand and on the left, upwards and downwards, for its food. The insects upon which it preys are often exceedingly minute, sometimes flying above and sometimes below the level of the swallow's course, and yet they are seen and captured without any diminution of the prodigious rate at which the bird is flying. Nay, more, any one who attentively watches one of these birds skimming over a meadow, may perceive that it will capture two or even three insects in such quick succession as to convince him that the swallow must have "had an eye upon them" all at once, and yet they are caught, as it were, in a moment.

Fig. 5.—SECTION OF THE HEAD OF AN EAGLE, SHOWING THE STRUCTURE OF THE EYE.

Another admirable contrivance peculiar to the feathered race, is the existence of a thin, semi-transparent veil, which, when requisite, can be instantaneously drawn over the front of the eye. This apparatus, generally known as the "nictitating membrane," is useful for a variety of purposes; it sweeps over the eye to cleanse it from dust, it diffuses the tears which keep it bright and polished, it will act as a screen to shut out the too great intensity of light, so that with its assistance the eagle can confront the sun even at noon-day; it will likewise defend the eye from sudden injuries, and yet, even when drawn like a curtain over the pupil, not shut out the light. The commodious manner in which this membrane lies folded up in the inner corner of the eye, and the quickness with which it executes its purpose, are known to every observer; but what is equally admirable, though not quite so obvious, is the employment of two kinds of material, and the combination of two kinds of force, by which the movements of this membrane are effected. It is not, as in ordinary cases, by the action of two antagonist muscles, the one pulling it forward and the other backward, but the membrane itself, being elastic, is capable of being drawn out like a thin sheet of india-rubber, and of returning to its former position when the force acting upon it is removed. Such being its nature, in order to adapt it for its office it is connected by a tendon with a muscle situated at the back part of the eye. This tendon, though strong, is so fine as not to obstruct the sight, even when it passes across the pupil, and the muscle which moves it being situated deeply within the orbit, derives from its situation the advantage of not only being secure from injury, but of being out of the way, which it hardly would have been in any position that could be assigned to it in the front of the eye, where its function really lies. When this muscle contracts, the membrane, by means of the communicating tendon, is instantly drawn, as it were, by a thread, over the transparent cornea, and when the muscle ceases to act, the elasticity of the membrane is sufficient to bring it back into its former position. (See Fig. 6.)

Fig. 6.—EYE OF AN OWL, SHOWING THE ARRANGEMENT OF THE NICTITATING MEMBRANE.

Both eyelids are divided through their middle, and everted, so as to display the Nictitating Membrane, a, and the passage for the tears (puncta lacrymalia), b.