AQUATIC MAMMALS 



poise G. L. Streeter (in Kellogg, 1928) found to be four times the size 

 of the optic colliculus. 



The middle and inner ear of the Cetacea are sufficiently modified for 

 us to be sure that the function is altered. Presumably the ear of the 

 Sirenia has also undergone modification, but little or nothing is known 

 about it. It is also permissible to surmise that in the Pinnipedia some 

 change is going on, perhaps of two sorts, for the auditory bulla of most 

 genera of seals is considerably inflated, while that of the sea-lion is 

 not, appearing shrunken and rugose. Gray (1905) has reported on the 

 former. He stated that the inner ear of this mammal is larger than in 

 any other except the walrus. He found that two otoliths of remarkable 

 size were present in the vestibule and as these were unlike those of any 

 mammal yet recorded he surmised that they must have some particular 

 physiological function. It is unknown whether otoliths are present in 

 the Cetacea. That the large vestibule of the Phocidae is not a simple 

 corollary of the aquatic life is indicated by the fact that in the Cetacea 

 this is particularly small. 



The ear bones of the Cetacea are characterized by extreme hardness. 

 Kellogg (1928) said "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 

 corresponding amount of motion in the malleolus, whose anterior pro- 

 cess is likewise fused with the bulla between these pedicles, and it in 

 turn transmits these vibrations to the incus and stapes." The mal- 

 leus is rigid and the stapes is immovable in the vestibule. Kellogg fur- 

 ther said that "water-borne vibrations transmitted to the air contained in 

 the tympanic bulla cause it to function as a sounding box, and its vi- 

 brations reach the cochlea by way of the ossicular chain and vestibule." 

 Kernan and Schulte (1918) stated that Denker (1902) thoroughly dem- 

 onstrated vibration of the ossicular chain to be impossible, but the latter 

 merely claimed that vibration cannot be activated by the tympanic mem- 

 brane, for this is too lax for any such function. Kernan and Schulte 

 also mentioned what is well known clinically, that an increase in the con- 

 duction of sound through the bones of the head accompanies dimin- 

 ished function of the middle ear (this is the condition in the author) . 



The ear bones of the Mysticeti and Odontoceti are of two distinct 

 shapes and this is accompanied by other differences. In the former the 

 ear drum looks like the finger of a gray leather glove, moderately pliable 

 when fresh but inelastic, the part representing the finger tip extending 

 distad into the auditory tube. In the Odontoceti the tympanic mem- 



[72] 



