HEARING 213 



squamosal and the occipital, thus providing far closer contact ^vith the 

 skull than it does in Odontocetes (Fig. 112). 



One of the most striking characteristics of the Cetacean ear-bone is 

 probably the loose way in which it fits into the skull. In all other mammals, 

 the bones surrounding the different auditory organs fit very closely into 

 the other bones of the skull and form an important part of the wall of the 

 brain case. In Odontocetes, on the other hand, the ear-bone is connected 

 so loosely by ligaments to the rest of the skull that in a fresh specimen of, 

 for instance, a porpoise, it can be freely moved with the index finger. In 

 Mysticetes, though the mastoid process fits between the bones of the 

 skull, it is joined to them merely by connective and not by bone tissue. 

 This is the reason why the ear-bone is so easily wrenched out of the other 

 skull bones, and why such bones are sometimes found washed up on the 

 beach. It also explains why most fossil skulls lack the bone, and why 

 fossil ear-bones are so often found by themselves. The great Dutch expert 

 on Cetaceans, Dr A. B. van Deinse of Rotterdam, has described and 

 classified a number of such ear-bones discovered during excavations in 

 Achterhoek and Twente. 



This extremely loose connexion between Cetacean ear and other skull 

 bones is one of the main factors in producing the acoustical isolation which 

 is essential for directional hearing under water. What little connective 

 tissue there is will transmit few if any sound vibrations. Acoustical isola- 

 tion is, moreover, achieved in other ways as well. In the first place, 

 the ear-bone is very hard and massive and hence much heavier than the 

 other bones of the skull, as is immediately apparent if one picks it up in 

 one's hand. Precisely because it is so heavy it will not resonate with the 

 lighter bones - at least not for frequencies above 150 cycles - so that the 

 vibrations transmitted to the rest of the skull via the blul^ber cannot reach 

 the ear-bone itself. 



Secondly, the middle ear is completely surrounded by cavities filled 

 with albuminous foam (Figs. 109 and 113). The cavities are in fact 

 evaginations of the tympanic cavity, with which they communicate by 

 an opening, so that the pressure in both is equal. In Mysticetes, these 

 cavities are generally restricted to the immediate neighbourhood of the 

 ear-bone, but in Odontocetes they may also run on beneath the skull, in 

 two (apical and lateral) directions. Moreover, in Odontocetes the cavities 

 themselves lie embedded in a great deal of fatty tissue, and in Mysticetes 

 in a large mass of hard connective tissue, both of which may act as 

 acoustic isolators, though some investigators doubt if they can keep out 

 sound under water. In any case, it may be said that acoustical isolation 

 is mainly produced by the peculiar albuminous foam with which the 

 cavities themselves are filled. The cavities fair sinuses) ai"e surrounded 



