24

Middle Ear

Ossicles of odontocetes and mysticetes are large and dense, but have wide species variations

in size, stiffness, and shape (Reysenbach de Haan 1956, Belkovich and Solntseva 1970,

Solntseva 1971, Fleischer 1978). In odontocetes, a bony ridge, the processus gracilis, fuses the

malleus to the wall of the tympanic and the interossicular joints are stiffened with ligaments and a

membranous sheath. Mysticete ossicles are equally massive but have none of the high frequency

related specializations of odontocetes. The ossicles are not fused to the bulla and the stapes is

fully mobile. The mysticete middle ear cavity is substantially larger than that of any odontocete.

Thus, the mysticete middle ear consists of a large, open cavity with massive ossicles that are

loosely joined; i. e., a characteristically low frequency ear.

The middle ear cavity in both odontocetes and mysticetes is lined with a thick, vascularized

fibrous sheet, the corpus cavernosum. Computerized tomography (CT) and magnetic resonance

imaging (MRI) data suggest the intratympanic space is air-filled in vivo (Ketten 1994). If so, a

potential acoustic difficulty for a diving mammal is that changing middle ear volumes may alter

the resonance characteristics of the middle ear, and, in turn alter hearing sensitivity. Studies are

underway with free-swimming beluga whales (S. Ridgway, personal communication) to test

whether hearing thresholds change with depth. In light of the extensive innervation of the middle

ear corpus cavernosum by the trigeminal nerve, one novel task proposed for the trigeminal in

cetaceans has been to regulate middle ear volume (Ketten, 1992), which could also explain

exceptionally large trigeminal fiber numbers in both odontocetes and mysticetes (Jansen and

Jansen 1969, Morgane and Jacobs 1972).

There is no clear consensus on how cetacean middle ears function. Both conventional

ossicular motion and translational bone conduction have been proposed for cetaceans (Lipatov

and Solntseva 1972; Fleischer 1978; McCormick et al. 1970, 1980). Based on experiments with

anesthetized T. truncatus and a Pacific white-sided dolphin, Lagenorhynchus obliquidens,

McCormick et al. (1970, 1980) concluded that sound entering from the mandible by bone

conduction produces a "relative motion" between the stapes and the cochlear capsule. In their

procedure, immobilizing the ossicular chain decreased cochlear potentials, but disrupting the

external canal and tympanum had no effect. Fleischer (1978) suggested the procedure

introduced an artificial conduction pathway. From anatomical studies, he concluded sound from

any path is translated through tympanic vibration to the ossicles which conventionally pulse the

oval window. McCormick's theory assumes fixed or fused tympano-periotic joints; Fleischer's

requires a mobile stapes, distensible round window, and flexible tympano-periotic symphyses.

Both conclusions may have been confounded by experimental constraints: McCormick et al.

(1970) had to disrupt the middle ear cavity to expose the ossicles, while Fleischer's data were

subject to post-mortem and preservation artifacts. In addition, neither theory is completely

compatible with the wide structural variability of cetacean middle ears. The question of middle

ear mechanisms in cetaceans therefore remains open.