HANDBOOK OF ACOUSTICS - online book

A complete view of Acoustical Science & its bearings on music, for musicians & music students.

Home | Just The Tune | Order | Contact




ON THE EAR.
'21
and is termed the Cochlea. It is so placed with reference to the other parts of the ear, that the Scala Tympani is closed at the lower end by the Fenestra Eotunda (F. E., fig. 16) while the Perilymph of the Scala Vestibuli is continuous with that of the Labyrinth. On the other hand, the Endolymph which fills the Scala Media is in communication with that of the Sacculus.
Besting on the upper side of the elastic Basilar membrane are the arches or rods of Corti. Each of these rods consists of two filaments, joined at an angle like the rafters of a house. Altogether there are some three or four thousand of them, lying side by side, stretching along the whole length of the Scala Media like the keys of a pianoforte. A branch of the auditory nerve, entering the Modiolus, gives off fibres which pass through the Lamina Spiralis, their ultimate ramifications probably coming into close connection with Corti's rods.
Not much is known for certain of the functions of the various parts of the Internal ear. We have seen how the vibrations of the sounding body are transmitted to, and imitated by, the membrane of the Fenestra Ovalis. These vibrations are necessarily taken up by the Perilymph, which bathes its inner surface, and hence com­municated to the Endolymph of the Membranous Labyrinth and the Scala Media. It was formerly thought, that both of these organs were necessary to the perception of sound, but the re­searches of Groltz have shown that the special function of the Labyrinth, is to enable us to perceive the turning of the head. Thus, the only part of the Internal Ear which is engaged in transmitting sound vibrations to the auditory nerves, is the Cochlea.
The Basilar Membrane of the Cochlea consists of a series of radial fibres, lying side by side, united by a delicate membrane, and it is believed by Helmholtz, that the faculty of discriminating between sounds of different pitch is due to these radial fibres. The Basilar Membrane gradually increases in width, that is, its radial fibres gradually increase in length, as we pass from the Fenestra Ovalis to the apex of the Cochlea, being ten times as long at the latter, as at the former. Helmholtz likens them to a series of stretched strings of gradually increasing lengths, the membranous connection between them simply serving to give a fulcrum to the pressure of the fluid against the strings. He further assumes that each of these fibres is tuned to a note of definite pitch, and capable of taking up its vibratory motion. He considers that the