HEARING 215 



derives its name), but looks rather like a rectangular bone, sometimes with 

 a central hole. The bone is, however, not fused to the petrosal as was 

 formerly thought. The malleus is joined by a thin osseous strip to the 

 ear-bone, and thus much more rigidly held to that bone than it is in most 

 other mammals, where the connexion usually takes the form of a very thin 

 and supple ligament. Mice and bats, however, have a similar osseous 

 strip, which seems to indicate that it is an adaptation for the reception 

 of very high tones. (We all know that the tighter a violin string, the higher 

 is the note it produces.) In order to prevent what small vibrations there 

 are along the walls of the ear-bone from reaching the auditory ossicles by 

 way of this thin strip, the joint between malleus and incus is constructed 

 in an ingenious way so that vibrations of the ear-bone cannot be trans- 

 mitted to the incus, while those from the eardrum can. Fraser and 

 Purves have investigated this question very carefully, and have constructed 

 an excellent working model in which the special action of the joint can 

 be clearly seen. 



Despite their robust structure, the Cetacean auditory ossicles are 

 relatively small, particularly the stapes whose surface area is only one- 

 thirtieth that of the eardrum. On the other hand, the lever of the malleus 

 is very large, and hence the stapes is made to vibrate with thirty times the 

 intensity (amplitude) of the eardrum. The explanation for this is that the 

 pressure of sound waves is much greater, and the amplitude much smaller, 

 in water than it is in air. 



The study of the internal ear of any mammal, surrounded as it is by 

 hard bone, is difficult enough, but in Cetaceans the difficulties are almost 

 insurmoimtable, since here the petrosal is probably the hardest found in 

 the entire animal kingdom, and the most difficult to cut. Chisel and 

 bone-saw merely cause it to splinter, and Petrus Camper, who was not 

 put off by minor difficulties, said of the bone that 'it is just possible to file 

 it down, but it is a most laborious job'. Even so, different biologists have 

 on more than one occasion tackled this 'laborious job', in order to fathom 

 one of the whale's greatest secrets. All of them were struck by the fact 

 that the semi-circular canals are so remarkably small; in porpoises, for 

 instance, they are no larger than they are in hamsters - possibly one of 

 the reasons why so many famous anatomists - the great Camper included - 

 failed to locate them altogether. It must not be thought, however, that 

 mere size determines the efficiency or importance of this organ of equili- 

 brium since even the smallest canals can function very effectively. This 

 became quite clear during the transport of porpoises from Denmark to 

 Holland, for, whenever the truck swerved, the animals immediately tried 

 to regain their equilibrium by moving their flippers. 



In contradistinction to the semi-circular canals, the cochlea is well 



