VESPERTILIO (see Encycl.) has been subjected to some cruel, but curious experiments, by the Abbé Spallanzani and M. de Jurine. The former of these philosophers having let loose several bats in a chamber perfectly dark, found that they flew about in it without any impediment, neither rushing against any thing in the apartment, nor touching the walls with their wings. This surprised him; but imagining that they were conducted by some glimpse of light which he did not perceive, he blindfolded them with a small and very close hood. They then ceased to fly; but he observed, at the same time, that this did not proceed from any deprivation of light, but rather from the constraint thence occasioned, especially when a hood of a very light texture was attended with the same effect.
He then conceived the idea of pasting up the eyes of the bats with a few drops of size or gum; but they still flew about in the same manner as if their eyes had been open. As this, however, was not sufficient, he pasted up the eyes of these animals with round bits of leather; and this even did not impede them in their flight.
That he might at length be certain of his object, he blinded them entirely, either by burning the cornea with a red hot wire, or by pulling out the pupil with a pair of small pincers, and scooping out the eye entirely. Not contented even with this precaution, he covered the wounds with pieces of leather, that the light might have no influence whatever on the remains of the organs which had been destroyed. The animals seemed to suffer very much by this cruel operation; but when they were compelled to use their wings, either by day or by night, and even in an apartment totally dark, they flew perfectly well, and with great caution, towards the walls, in order to suspend themselves when they wished to rest. They avoided every impediment, great or small, and flew from one apartment to another, backwards and forwards, through the door by which they were connected, without touching the frame with their wings. In a word, they showed themselves as bold and lively in their flight as any other animals of the same species which enjoy the use of their eye-sight.
These experiments were repeated by M. Jurine, and with the same results. Spallanzani had supposed that the bat possessed some organ or sense which is wanting in the human species, and which supplies to these animals the place of vision; and Jurine determined to ascertain the truth or falsehood of this hypothesis by anatomical researches. During the course of these, he found the organ of hearing very great in proportion to that of other animals, and a considerable nervous apparatus assigned to that part. The upper jaw also is furnished with very large nerves, which are expanded in a tissue on the muzzle.
M. Jurine then extended his experiments to the organ of hearing and that of smell. Having put a small hood on a long-eared bat, it immediately pulled it off, and flew. He stopped up its ears with cotton; but it freed itself in the like manner from that inconvenience. He then put into its ears a mass of turpentine and wax. During the operation the animal showed a great deal of impatience, and flew afterwards very imperfectly.
A long-eared bat, the ears of which had been bound up, flew very badly; but this did not arise from any
pain occasioned by the ligature; for when its ears were sewed up, it flew exceedingly well. In all probability the animal would have preferred having its ears bound up to having them sewed. Sometimes it flew towards the ceiling, extending its muzzle before it settled.
M. Jurine poured liquid pomatum into the ears of a bat which enjoyed the use of its sight. It appeared to be much affected by this operation; but when the substance was removed it took flight. Its ears were again filled, and its eyes were taken out; but it flew then only in an irregular manner, without any certain or fixed direction.
The ears of a horse-shoe bat, which had the use of its sight, were filled with tinder mixed with water. It was uneasy under the operation, and appeared afterwards restless and stunned; but it conducted itself tolerably well. On being blinded, it rushed with its head against the ceiling, and made the air resound with strokes which it gave itself on the muzzle. This experiment was repeated on other bats with the like effect.
The tympanum of a large horse-shoe bat was pierced with a pin (trois quart). The animal appeared to suffer much from the operation, and fell down in a perpendicular direction when thrown into the air. It died next morning. The same effect was produced on piercing the tympanum of a long-eared bat with a needle.
The author then made very accurate researches on the difference between the organisation of the brain of these two kinds of bats; and, after a careful dissection, found that the eye of the long-eared bat is much larger than that of the horse-shoe bat, but that the optic nerve is proportioned to it. The outer part of the ear of the former is much larger than that of the latter, but the interior part is smaller.
The horse-shoe bat is indemnified for this difference by a greater extension of the organ of smell, as evidently appears when the external elevations and irregularities of its muzzle are examined. When it is about to take flight, it agitates its nose much more than the long-eared bat.
From these experiments, the author concludes: First, That the eyes of the bat are not indispensably necessary to it for finding its way; secondly, That the organ of hearing appears to supply that of sight in the discovery of bodies, and to furnish these animals with different sensations to direct their flight, and enable them to avoid those obstacles which may present themselves.