The function of the middle ear in lizards: Crotaphytus collaris (iguanidae).

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AbstractThe anatomy of the middle ear is outlined for lizards in general and described in detail for Crotaphytus collaris (Iguanidae). By use of the alternating potentials of the cochlea, the general function of the middle ear in the reception of aerial sounds was determined by comparing the ear's sensitivity when the middle ear mechanism was intact with the sensitivity remaining when the columellar shaft was severed, thus eliminating the connection between the stapedial footplate and the more peripheral parts. This comparison was made under two conditions, with the sound applied in the usual way at the external auditory meatus and with the sound introduced through a tube sealed over the oval window. The loss of transmission observed under the first condition is complicated by the presence in the sound path of the nonfunctioning tympanic membrane and by an interaction of sound waves reaching both round and oval windows of the cochlea, but the second condition, in which these complications are avoided, reflects the true performance of the middle ear as a mechanical transformer. This transformer action was measured as 35 db on the average for tones between 300 and 4000 Hz, which is the region of best hearing for this lizard.Further analysis of the operation of the tympanic mechanism was carried out by measurements of sensitivity before and after severing the connection between the internal process and the quadrate bone. These observations showed that the internal process makes little or no contribution to the ear's sensitivity. Other experiments performed with mechanical stimulation showed that the columellar system acts as a reduction lever, and the measurements gave a general indication of the amount of this lever action.The pressure increase provided by the columellar lever, together with a still greater increase produced by the areal ratio between the effective surface of the tympanic membrane and the surface of the columellar footplate, can account in large part at least for the total transformer action. This action makes it possible for acoustic energy to be conveyed with great efficiency from the air to the relatively dense and stiff sensory elements of the cochlea. This problem of the matching of impedances between two media, and its solution by the use of a mechanical transformer, has long been studied in mammals, and especially in the cat, and the solution provided by the lizard's middle ear seems altogether as satisfactory as that provided by the structurally different ossicular mechanism of the mammals.