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A method was evolved by which the organ of Corti could be examined in its entirety with the scanning electron microscope, the organ meanwhile retaining its spiral form. This made it possible to assess traumatic effects on the cochlea and qualify lesions in terms of extent, localization and pattern. It was also found possible eventually to cut the same specimen into sections for cellular and subcellular studies. The number of guinea pigs examined totalled 91, divided into three groups. The first group was used to study the anatomy of the organ of Corti with special reference to normal variations and artifacts. Unmistakable indications were found that the longest stereocilia of the inner hair cells are linked to the tectorial membrane. The animals of the second group were exposed to pure tones of high intensity, whereupon, lesions of the organ of Corti were described according to intensity, time, and frequency. Three different types of otologic drills were used to perform mastoidectomies on temporal bones and on the cadaver. The noise produced was analyzed as to intensity and frequency range. It was found that the drill with the lowest rpm (and highest torque) produced the highest noise intensities, at levels which can be traumatic to the human organ of hearing. The animals of the third group were exposed to the amplified noise produced by otologic drills of three different types. The resulting lesions in the organ of Corti were examined by the method described for scanning electron microscopy and compared. In spite of the wide variation in individual lesions, patterns of degeneration of three different types could be distinguished. The high-speed and the very-high-speed drill inflicted less damage on the organ of Corti than the low-speed drill. It is therefore advised to refrain from using the latter drill in prolonged operations.