BASILAR MEMBRANE, a tissue lying under the organ of Corti in the cochlea of the internal ear of the higher vertebrates. It consists of transverse fibres which are not free but are packed together tightly side by side and embedded in a cementing sub stance and covered on both upper and lower surfaces by a layer of cells. As it approaches the apex of the spiral of the cochlea, it becomes gradually wider ; Keith's measurements of the membrane in man gave a width of o. I 7mm. at the base and 0.4omm. at the apex of the cochlea. The average length of the membrane is 35 millimetres. Its function in hearing has been the subject of much controversy and no satisfactory theory has as yet been developed. According to the "resonance" theory of hearing, commonly associ ated with the name of Helmholtz, the transverse fibres are a gradu ated set of resonators, each of which, by reason of its mass and length, the tension exerted on it by the spiral ligament and the loading produced by the columns of lymph lying between it and the two windows of the cochlea, is attuned to a tone of definite pitch and is thrown into sympathetic vibration which excites the hair-cells connected with the auditory nerve when the appropriate tone is transmitted to the cochlea. The partial vibrations of which a complex vibration is composed excite different fibres, and the ability of the ear to analyze complex tones is thus explained. Max Meyer's theory is that the membrane responds by forced, not by sympathetic vibration, that the frequency rather than the place of excitation of the hair-cells determines the pitch heard, and that the longitudinal extent of the membrane involved in the vibration, and thus the number of cells stimulated, is the correlate of loud ness. Other theories assume that the membrane vibrates as a whole. Ewald held that it vibrates as a whole with nodes or lines of rest in different places according to the pitch of the tone. Still another theory attributes no particular function to this membrane but claims that the essential tissue concerned in the excitation of the hair-cells is the tectorial membrane. (See HEARING.) BIBLIOGRAPHY.-Sir Thomas Wrightson, An Enquiry into the AnaBibliography.-Sir Thomas Wrightson, An Enquiry into the Ana- lytical Mechanism of the Internal Ear (1918) ; G. Wilkinson and A. A. Gray, The Mechanism of the Cochlea (1924)• (F. A. P.)