air-filled sacs of the floor and external walls of the tympanic cavity in most mammals. They are formed of tympanic bone, an angular bone homologous to the lower jaw in reptiles. Primitive mastoid cells protect the auditory ossicles and tympanic cavity. Increasing complication of the mastoid cells converts them into acoustic resonators or absorbers. Four types of eardrums are distinguished: spherical, chambered, large-cellular, and spongy-small-cellular. The functions of each type are determined by the acoustic properties of the animals’ habitat and characteristics of their orientation.