Contributions to physiology and pathology / by J. Moorhead.

  • Moorhead, J.
Date:
1865
Catalogue details

Licence: Public Domain Mark

Credit: Contributions to physiology and pathology / by J. Moorhead. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    upon to exercise tlie sense of hearing in atmospheric air. Sonorous undulations in this subtle medium being comparatively faint and delicate, necessitate a more elaborate development of the ear in all air-breath- ing1 animals than in those destined, like fishes and O . • other aquatic tribes, to appreciate sound excited in water. This higher evolution of the instrument, which is merely a beneficent adaptation of means to ends, consists in its simplest state of a tympanum traversed by a columella which has no muscular re- lations, and terminated by a thin but strong mem- brane—the membrana tympani. The latter, as is well known, is on a level with the surface of the head, and may be easily seen in lizards, tortoises, and other reptiles. It is formed by the general integument becoming attenuated so as to produce a delicate and susceptible membrane capable of freely vibrating in unison with the faintest sonorous undulations of air. Vibrations, being thus excited in the membrana tym- pani, are directly transmitted to the columella (for the latter is closely adherent to the membrane), and thence to the membrana fenestrse ovalis, to be con- ducted onwards by the perilymph to the expansion of the auditory nerves. That this is the course normally pursued by the vibrations is rendered most obvious by the arrangement which obtains in serpents. In this order, owing to the special provision to enable the creatures to enlarge the mouth to its greatest extent, the space otherwise occupied by the tympa- num is completely filled by the large muscles attached to the jaws ; and the only remnant of this cavity is the columella, which lies in the midst of muscular masses, and extends from the membrana tympani to the fenestra ovalis. This, wliich is the most elemen- tary condition of a tympanic apparatus, must obvious- ly fulfil its essentia] function,—namely, the reception and concentration of the delicate sonorous impulses of air.