266 OUTPOSTS OF THE INTELLIGENCE SERVICE 



medium. As fluid lills the space on one side of the membrane, 

 efficiency demands a fluid on the other side. One may also say 

 that the separation of the scala media from the scala vestibuli 

 enables (i.) damping to take place readily, and (ii.) a simpler and 

 more efficient balance-organ (labyrinth) to be constructed. 



Binaural Hearing. 



Quite apart from the undoubted fact that having two ears 

 enables us to hear more distinctly and allows of the damage of one 

 ear without completely shutting us out from the variations in 

 external sound, the possession of two listening points alone permits 

 of the localisation of sound. The head is oriented so that an equal 

 intensity of sound falls on each ear, i.e. if our ears are equally 

 sensitive, we face the sound — we are positively audiotropic 

 (Chap. XXXIII.). 



There remains one very important matter which should be 

 considered because of its diagnostic value to the physician, viz. 

 conduction of sound waves by the bones of the head. It is common 

 knowledge that sound vibrations travel more readily through 

 a solid than through a liquid or a gaseous medium. A watch, 

 placed sufficiently far away to be inaudible, can be heard ticking 

 if touched by a lath held between the teeth. If something goes 

 wrong with the mechanism of the ear, one wants in the first 

 place to locate the fault. Is the external ear, the middle ear or 

 the internal ear the seat of the trouble ? The test is usually made 

 by placing a vibrating body, such as a tuning fork, on one of the 

 cranial bones. If the sound is not appreciated, then the fault 

 lies within the internal ear. Either the organ of Corti (or 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 membrane 

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

 hearing is not impossible. The vibrations are transmitted directly 

 through the thick, exceedingly dense but elastic bony walls of 

 the aural cavity, and produce a movement of the basilar mem- 

 brane, 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. 



