The 4000-Dip: Anatomic Considerations

Abstract

In a number of sectional series of temporal bones of malformed fetuses and newborn, the enchondral layer of the cochlear capsule was found to perforate, in a broad stream, the periosteal layer and to meet beyond the latter the so-called cartilage (chondral) bar. This third interruption of the capsular layers—the first and second being the internal acoustic meatus and the cochlear aqueduct—is located in the basal cochlear turn corresponding to the 4000 dip of the audiometric curve. Lack of shielding by the periosteal layer, combined with the disturbance of the enchondral layer caused by the abrupt change of direction, may create here an area of diminished protection for the underlying membranous parts.

This interpretation, based on observations in the osseous capsule, is offered as a counterpart to the explanatory attempts regarding the 4000-dip, which consider happenings in the membranous portions exclusively.

Investigation of this “enchondral outflow” in normals of different ages is desirable.