HEARING 207 



atmospheric waves. But, of course, eardrums designed for receiving this 

 type of wave would be useless to Cetaceans, whose ears must be specially 

 modified to their aquatic environment. An auditory apparatus designed 

 for atmospheric vibrations cannot be used in the water and our first 

 problem is therefore to investigate how acoustic isolation of the ears is 

 achieved in Cetaceans. Our second problem is that of the reception of 

 high pitched tones, which makes very special demands on the construction 

 of the auditory apparatus. Before we discuss the special modifications 

 which enable whales to receive, and to receive very acutely, sounds in 

 water, we must first take a closer look at our own ears. 



Our external ear - the pinna - picks up atmospheric vibrations and 

 propagates them along the air in the external auditory canal, a fairly wide 

 passage, surrounded partly by the cartilaginous concha and partly by the 

 bone of the skull. The external auditory canal is sealed oflTby the eardrum 

 about an inch from its beginning, and the eardrum is set into vibration 

 by the air, in the same way as, for instance, the diaphragm of a micro- 

 phone vibrates when we speak into it. The pinna, the auditory canal and 

 the eardrum jointly make up the external ear (Fig. 107). Behind the ear- 

 drum lies the middle ear, surrounded on all sides by cranial bone. It 

 consists of an air-filled space, the tympanic cavity, which communicates 

 with movith and throat by the Eustachian tube. In this way the pressure 

 inside and outside the eardrum is always equal, thus allowing it perfect 

 freedom of vibration. In a number of mammals the tympanic cavity is 

 evaginated, i.e. it shows a conspicuous globular swelling surrounded by a 

 shell-like protuberance of the tympanic bone, the so-called bulla tympani, 

 which may serve to increase the intensity of incoming sounds, though its 

 exact function has not yet been fully understood. 



The vibrations of the eardrum are transmitted by a chain of auditory 

 ossicles to the membrane covering the opening of the inner ear, the so- 

 called oval window. The auditory ossicles - malleus (hammer), incus 

 (anvil) and stapes (stirrup) - articulate by means of joints. The handle of 

 the malleus is attached to the eardrum, its head being linked by a small 

 joint with the body of the incus. A process of the incus is joined to the 

 stapes, whose footplate is fitted into the oval window, thus communicating 

 its vibrations to the membrane. Behind the oval window, there is a vestibule 

 which communicates with the auditory sense organs and the semi-circular 

 canals of the inner ear, both embedded in the hardest part of the temporal 

 bone, i.e. the petrosal. The inner ear or labyrinth contains a fluid to 

 which the vibrations of the oval window are transmitted. The vibrations 

 are then picked up by a complicated system of auditory receptors and 

 conducted by the auditory nerve to the brain, where we become conscious 

 of them as sounds. 



