The terminal zone of the external auditory meatus in a variety of mammals.

Standard texts describe the human tympanic recess as a shallow depression. Khanna and Tonndorf (1975) made silicone rubber casts from the auditory meatus of cats and found the recess to be narrow and deep. Guided by this finding, casts were made from 13 different mammals. At the terminal zone of the auditory meatus, profound variations were found, the consequence of three changing parameters: (1) angle between roof of the canal and tympanic membrane; (2) shape of the isthmus; (3) shape of the recess. For all specimens, values were assigned to each of these variables and a composite graph plotted. A reasonably good correlation was found to exist: the larger the angle, the higher and narrower will be the isthmus and the more narrow and deep the recess. The acoustic implications of these findings will be assessed in a separate study.