Page:Proceedings of the Royal Society of London Vol 1.djvu/209

Rh not governed, Mr. Carlisle says, by the form, habits, or voice of the animal, except in'those mammalia. which inhabit the waters, such as the seal. the walrus, and the whale tribe: in these the stapes is more massive; but in the otter, which only dives occasionally, the stapes does not differ from that of the fox. In the tiger, the dog, and other ferae, the crura are straight, and meet in an acute angle; but the same ﬁgure occurs in the horse, in the beaver, in the goat, and in many other herbivorous quadrupeds. In the cete, the muscle of the stapes pulls the capitulum at such an angle, as very much to depress its subjacent end into the fenestra vestibuli; and the joint appears capable of considerable motion. In the walrus, this ossicle is entirely solid : in the seal, and in the cete, the bone has only a small per- foration instead of the crural arch.

Mr. Carlisle has discovered a very remarkable singularity in the stapes of the marmot, and in that of the guinea-pig. In those ani- mals, the bone is formed of slender crura, constituting arounded arch: through this arch an osseous bolt passes, so as to rivet it to its situation. This bolt, to which Mr. Carlisle has given the name of Pessulus, is placed near the top of the arch, so that, by the action of the stapedeus muscle, the upper part of the straight crus is brought into contact with the pessulus. The use of this mechanism is not obvious, there being nothing in these animals, excepting their shrill whistle, peculiarly diEel‘ent from others which are destitute of such mechanism. In the kangaroo, the stapes is like the corresponding ossicle in birds, called Columella. In the two species of Ornitho- rhynchus (paradoxus and hystrix), this resemblance to the columella is still more striking, and forms an additional point of similarity be— tween these singular quadrupeds and birds. These columellze are articulated to a small bone, which performs the ofﬁce of the manu- Jn'ium of the malleus; whereas, in birds, the capitulum of the colu- mella is slightly expanded, and is joined to a triangular plate of car. tilage attached to the membrana tympani. In some birds, a small foramen occurs in the middle of this plate.

The amphibia are provided with columellze similar to those of birds; but the cartilage is united to the under surface of the true skin, with- out any apparent application of muscles to alter its tension.

From the preceding circumstances, Mr. Carlisle is led, he says, to the following conclusions :—In man, and in most of the mammalia, the ﬁgure of the stapes is an accommodation to that degree of light- ness which seems a requisite condition; and that bone is especially designed to press on the ﬂuid contained in the labyrinth; the ulti- mate eﬁ'ect of which pressure is, an increase of the tension of the membrane closing the fenestra cochlea.

There does not, in Mr. Carlisle’s opinion, appear to exist any motion between the ossicula auditﬁs that bears any relation to the peculiar vibration of sounds. He rather conceives, that the different motions of these bones only affect the membrana tympani, so as to lessen the intensity of violent impulses. Sounds of less impetus, not