RAND-BOOK OF ACOUSTICS.
arches of Corti which rest on these fibres, serve the purpose of transmitting this vibratory motion to the terminal appendages of the nerve, each arch being connected with its own nerve ending.
As already mentioned, there are about 3,000 of these arches in the human ear, which would give about 400 to the octave. When a simple tone is sounded in the neighbourhood of the ear, the radial fibre of the Basilar Membrane in unison with it, is supposed to take up its vibrations, which are then transmitted by the arch of Corti in connection with it, to the particular nerve termination with which it is in communication.
For a complete discussion of this theory, the reader is referred to Helmholtz's " Sensations of Tone," Part I, Chap. vi.
The Human Ear may be divided, for descriptive purposes, into three parts : the External, Middle, and Internal ears.
The External ear consists of the Lobe, and the tube or Meatus leading inwards, which is closed by the Tympanum.
The Middle ear contains a chain of three small bones, the Malleus, Incus, and Stapes, which serves to connect the Tympanum with the Fenestra Ovalis. The air in the cavity of the Middle ear is in communication with the external air, by means of the Eustachian Tube.
The Internal ear consists essentially of a membranous bag of exceedingly complicated form, filled with a liquid—Endolymph. This bag floats in another liquid—Perilymph—contained in a bony cavity, which is separated from the cavity of the Middle ear by the membranes of the Fenestra Ovalis and Fenestra Rotunda.
The nerves of hearing ramify on the walls of this membranous bag, and their ultimate fibres project into the Endolymph therein contained.
The alternate condensations and rarefactions of the sound waves which enter the External ear, strike against the Tympanum and set it vibrating. These vibrations are then transmitted by means of the small bones of the Middle ear to the Fenestra Ovalis, and from this again through the Perilymph and Endolymph to the minute terminations of the auditory nerve, which lie in the latter hquid.
One particular part of the membrane of the Internal ear, in which lie the Fibres of Corti, is specially modified, and is supposed to be the region of the ear which serves to discriminate the pitch and quality of musical sounds.