Page:Proceedings of the Royal Society of London Vol 1.djvu/210

Rh requiring such modulation, are transmitted by the vibrations of the integrant parts of these bones, unaccompanied by muscular action.

This reasoning, Mr. Carlisle says, is suggested by the columellae in the aves and amphibia; for, since many birds accurately imitate a variety of sounds, it may be inferred that they hear as acutely and as distinctly as mankind.

The muscles of the ossicula auditﬁs appear to be of the involun- tary kind; their peculiar stimulus is sound, and the chorda tympani is a gangliated nerve. If the above supposition is true, the muscles may be considered as all acting together ; especially as it is well known that some persons who hear imperfectly are more sensible to sounds when in a noisy place; as if the muscles were then excited to action.

It cannot, Mr. Carlisle thinks, be allowed, that the pressure of the watery ﬂuid in the labyrinth is necessary to produce the sensation of hearing, since birds hear without any such mechanism : such pres- sure, however, would give increased tension to the fenestra cochleae; and, as the membrane of that fenestra is exposed to the air con- tained within the cavity of the tympanum, it appears adapted to re- ceive such sounds as pass through the membrana tympani, without exciting consonant motions in the ossicula auditﬁs.

' In order to investigate the truth of the above opinions, Mr. Car- lisle had water, at the temperature of his body, dropped from a small vial into the meatus externus, the tragus being previously pulled to- wards the cheek. The ﬁrst drop produced a sensation like the report of distant cannon; and the same eﬁ'ect succeeded each drop until the cavity was ﬁlled.

In this experiment the vibrations of the membrana tympani must, he says, have been impaired, if not destroyed; yet the motions of the membrane produced by each drop of water aﬁ'ected the air contained in the tympanum, sufﬁciently to produce a sensible impression.

That something like this occurs in many kinds of sounds, is, Mr. Carlisle thinks, more than probable; and as the cochlea consists of two hollow half cones, winding spirally, and uniting at their apices, it follows that the sounds affecting either of the cones must pass from the wide to the narrow end; and the tension of the parts, in either case, will necessarily aid the impression.

Mr. Hatchett, after mentioning the experiments made by several eminent chemists on the substance generally called Tannin (but which he thinks would be better expressed by the word Tan), observes, that the results of those experiments have established, that tan is a peculiar substance, naturally formed, and existing in many vegetable bodies, such as oak bark, &c.; but that no one has ever