2.04 



THE QUARTERLY REVIEW OF BIOLOGY 



time the osseous portions of the inner 

 ear have undergone certain changes, 

 which brought about a diminishing utility 

 for the tympanic membrane. We know- 

 that whales possessed at one time some 

 sort of external ear, for a vestige of the 

 auricular cartilage buried beneath the 

 skin has been observed in some porpoises 

 and the two outer ear muscles are still 

 retained by baleen whales. The external 

 auditory tube has been completely closed 

 in the whalebone whales. This must 

 have been accompanied by corresponding 

 physiological adjustments to prevent the 

 accumulation of an excess amount of wax 

 in the blind tube. In many of the por- 

 poises the outer orifice of the external 

 auditory tube is so minute that one can 

 not introduce the lead of a pencil, but the 

 lumen expands in a trumpet-like manner 

 as it approaches the ear drum. The 

 tympanic bulla, periotic, malleus, incus, 

 and stapes comprise the osseous portions 

 of the organ of hearing; they lie outside 

 the cranial cavity in a recess bounded 

 by the squamosal, exoccipital, and basi- 

 occipital. 



Whales have acquired an organ of hear- 

 ing in which resonance must play an 

 important part, for the malleus is rigid; 

 the tensor tympani muscle, whose function 

 in land mammals is to make taut the ear 

 drum, is vestigial; and in some species the 

 stapes is immovable in the vestibule. 

 The tympanic bulla is the relatively 

 dense and heavy sounding box fastened to 

 the periotic by two thin pedicles, which 

 can be set in vibration. Vibrations set 

 up in these pedicles produce a correspond- 

 ing amount of motion in the malleus, 

 whose anterior process is likewise fused 

 with the bulla between these pedicles, 

 and it in turn transmits these vibrations 

 to the incus and stapes. Another mechan- 

 ical arrangement is thus acquired to take 

 over the function ordinarily performed by 



the ear drum. Water-borne sound vibra- 

 tions transmitted to the air contained in 

 the tympanic bulla cause it to function as 

 a sounding box, and its vibrations reach 

 the cochlea by way of the ossicular chain 

 and the vestibule. Dr. George L. Streeter 

 at the request of the writer studied the 

 brain of Tursiops truncatus and found that 

 these porpoises have a highly developed 

 central nervous system for the reception 

 and disposal of cochlear stimuli, which 

 stands out in contrast to the less well 

 developed optic mechanism. The optic 

 colliculus was found to be not more than 

 one-fourth as large as the acoustic collic- 

 ulus. 



The organ of hearing as a whole and the 

 soft structures associated with it appear 

 to be especially well adapted for resisting 

 temporary heavy pressure at considerable 

 depths. The eustachian tube and lining 

 membrane of the tympanic bulla communi- 

 cate with several sinuses or reservoirs 

 capable of distention with air. If these 

 reservoirs are blown full of air and the 

 lining membrane of the tympanic bulla 

 distended before the animal dives, the 

 external pressure exerted by the water on 

 the tympanic membrane could be equalized 

 by muscular compression of the associated 

 air sinuses since the latter are all connected 

 with the tympanic cavity. 



A humpback whale (Megaptera nodosa) 

 has an external auditory tube zi inches in 

 length. The outermost portion of this 

 tube passes through the blubber and ends 

 blindly about 5^ inches from the external 

 orifice, the tube itself decreasing in 

 diameter from T V of an inch to almost 

 nothing. Then comes an interval of 

 3 inches where the tube is completely 

 closed. The innermost portion of the 

 tube widens to a diameter of more than an 

 inch and the walls are invariably collapsed. 

 The tympanic membrane is in many 

 respects the most remarkable structure 



