Cassell's book of birds; vol. 1

by Alfred Edmund Brehm

But this is not all. In the arrangement of the muscle which, though placed behind the eye, draws the nictitating membrane in front of it, there is what justly deserves to be called a marvellous piece of mechanical contrivance. The extent of contraction necessary to draw the membrane over the whole front of the visual organ would require a much longer muscle than could have been placed in such a situation; in order to meet this difficulty, the tendon which draws forth the nictitating membrane is made to pass through a loop in another muscle, as represented in the next wood-cut (Fig. 7), where it is evident that, by the simultaneous contraction of both these muscles, the extent of their action when drawing the nictitating membrane over the eye is considerably increased. Neither is this the only advantage derived from so ingenious a contrivance; were it not for the plan adopted, the tendon of the muscle u x would press upon the optic nerve, and thus materially interfere with vision—an inconvenience that by the existing arrangement is totally prevented. Devices like these, whereby special machinery is introduced for special purposes, speak for themselves; we acknowledge their beauty, and in them we recognise at once the wisdom and the goodness of the Creator.

Fig. 7.—MUSCLES OF THE EYE-BALL AND OF THE NICTITATING MEMBRANE.

The Muscles of the Eye-ball, p, q, r, s, are separated from their origins, and turned aside, to show the "trochlearis" or "quadratus," t, and the "pyramidalis," u, x; the latter of which passes through a loop in the former, so as to gain a double extent of effect with a given length of fibre.

The SENSE OF SMELL in birds has afforded subject-matter for much discussion, and great obscurity still exists with reference to the extent to which they make use of their olfactory organs. It has been generally asserted that birds of prey are gifted with an acute perception of odours, and are thus enabled to discover their food at a distance; but the rapidity with which vultures are known to assemble round the carcase of an animal too recently killed to attract them by putrefactive exhalations, has induced many observers to consider them as being directed entirely by sight. That this latter is the preferable theory appears to be sufficiently established by the experiments of Audubon, which go to show that these birds possess a sense of smell very far inferior to that conferred upon carnivorous quadrupeds, and that, so far from guiding them to their prey from a distance, it affords them no indication of its presence even when close at hand.

Having procured the skin of a deer, M. Audubon stuffed it full of hay, and after the whole had become perfectly dry and hard, he placed it in the middle of an open field, laying it down on its back in the attitude of a dead animal. In the course of a few minutes he perceived a vulture flying towards and alighting near it. Quite unsuspicious of the deception, the bird immediately proceeded to attack the carcase, as usual, in the most vulnerable points. Failing in this, he next, with much exertion, tore open the seams with which the skin had been stitched, and appeared earnestly intent upon getting at the flesh which he expected to find within, and of the absence of which not one of his senses was able to inform him. Finding that his efforts, which were long reiterated, led to no other result than the pulling out of sundry quantities of hay, he at length, though with evident reluctance, gave up the attempt, and took flight in pursuit of other game.

Another experiment, the converse of the preceding, was then tried:—A large dead hog was concealed in a narrow and winding ravine, about twenty feet deeper than the level of the ground around it, and filled with briars and high cane. This was done in the month of July, in a tropical climate, where putrefaction takes place with great rapidity; yet, although many vultures were seen sailing in all directions over the spot where the putrid carcase was lying covered only with twigs of cane and light underwood, none of them appeared at all to suspect its presence.

Nevertheless, notwithstanding the apparently decisive result of the above experiments, anatomy teaches us that the olfactory apparatus in this class of animals is largely developed, and indicates by its extent that it is well adapted to investigate the odorous properties of the air taken in for respiration.

The SENSE OF HEARING in birds is remarkably acute, as might be readily inferred from the vocal capabilities conferred upon many of these gifted songsters. Their music is certainly not less appreciated by the performers than it is by their auditors. "Nobody can doubt," observes Bishop Stanley, "who sees a bird singing, clapping its little wings, turning from side to side, and glancing its bright eyes in all directions, as if courting attention and admiration, that it feels delight and satisfaction. Did we require further proof, we have but to recollect that the song-bird is most alert with the music of its voice when its affections and interests are awakened by its mate during the time of rearing its young." It is, indeed, principally during the breeding-season that the singing power of birds is in full activity; and seeing that in general it is only the male that possesses the musical faculty, we may naturally suppose that its exercise is intended for the solace and amusement of his mate during her confinement to her nest. The nightingale himself becomes voiceless so soon as the appearance of his nestlings calls him to more profitable employment.