214 THE EAR 



its nervous attachments) have broken down or the membrane 

 of the fenestra rotunda is not normal. Provided the organ of 

 Corti and its nervous attachments are intact and the round mem- 

 brane is flaccid, sounds may be heard by bone conduction, and 

 ordinary hearing is not impossible. The vibrations are trans- 

 mitted directly through the thick, exceedingly dense but elastic 

 bony walls of the aural cavity, and produce a movement of the 

 basihir membrane, etc. This can only take place if the membrane 

 of the round window is functioning properly, or if the stapes 

 moves normally in its oval window. If the openings were to 

 lose their elastic windows 'and not move to and fro with every 

 condensation and rarefaction, then the cochlea would be, to all 

 intents, a sealed cavity filled with fluid. Such fluid could not 

 oscillate ; it could be alternately compressed and released from 

 this extra pressure, but this slight molecular movement could 

 not stimulate the hairlets. 



Further, if both the stapes and round membrane were free to 

 move, hearing in the case of a diseased middle ear would not be 

 so good as when only one of the pair were free. This is because 

 part of the displacement of the cochlear fluid caused by the 

 vibrations of the surrounding bone is dissipated by moving the 

 stapes outwards. People may hear fairly well after the stapes has 

 become immovably fixed in thcfenestra ovalis. In cases where the 

 drum of the ear has been punctured, hearing may be improved by 

 fixation of the stapes, e.g. by application of a plug of cotton wool. 



When sounds are conducted to the inner ear by means of the 

 bones of the skull, in people with normal hearing, the intensity 

 of the sound is markedly increased if the movement of the stapes 

 is hindered. For example, on p. 423 of Part II. is given an experi- 

 ment where a vibrating tuning-fork is placed on the region of 

 the interparietal suture. When both ears are unobstructed and 

 normal, sound is heard equally by both. If the drum of one ear 

 and appended ossicles are hindered from taking a full excursion 

 by blocking the meatus with a finger, the sound appears most 

 distinctly at this ear. When both ears are treated in this way, 

 localisation is again median. A common entotic phenomenon is 

 the audibility of the pulse in an obstructed car. It may be due 

 to the transmission of the pulse-wave oscillation to the air of the 

 middle ear which acts as a resonator reinforcing the vibrations 

 and then transmitting them to the internal ear. It is more 

 probable, however, that the beat of the carotid artery is trans- 

 mitted through the parietal bone direct to the fluid of the cochlea. 



