198 THE HUMAN BODY 



Ordinarily, sound-waves reach the labyrinth through the 

 tympanum, but they may also be transmitted through the bones 

 of the head ; if the handle of a vibrating tuning-fork be placed on 

 the vertex, the sounds heard by the person experimented upon 

 seem to have their origin inside his own cranium. Similarly, when 

 a vibrating body is held between the teeth, sound reaches the end 

 organs of the auditory nerve through the skull-bones ; and persons 

 who are deaf from disease or injury of the tympanum can thus 

 be made to hear, as with the audiphone. Of course if deafness be 

 due to disease of the proper nervous auditory apparatus no device 

 can make the person hear. 



Function of the Cochlea. We have already seen reason to be- 

 lieve that in the ear there is an apparatus adapted for sympathetic 

 resonance, by which we recognize different musical tone colors; the 

 minute structure of the membranous cochlea is such as to lead us to 

 look for it there. Of the various structures making up the mem- 

 branous cochlea the basilar membrane seems to satisfy best the 

 requirements of an apparatus for registering sounds by sympa- 

 thetic resonance. It increases in breadth twelve times from the 

 base of the cochlea to its tip (the less width of the lamina spiralis 

 at the apex more than compensating for the less size of the bony 

 tube there) . Careful histological examination has shown that in- 

 stead of being a true membrane it is really made up of a large 

 number of transverse strands tightly stretched, and varying in 

 length as the space between the lamina spiralis and the wall of the 

 bony cochlea varies. 



Probably each strand vibrates to simple tones of its own period, 

 and excites the hair-cells which lie on it, and through them the 

 nerve-fibers. Perhaps the rods of Corti, being stiff, and carrying 

 the reticular membrane, rub that against the upper ends of the 

 hair-cells which project into its apertures and so help in a sub- 

 siduary way, each pair of rods being especially moved when the 

 band of basilar membrane carrying it is set in vibration. The 

 tectorial membrane is probably a " damper " ; it is soft and in- 

 clastic, and suppresses the vibrations as soon as the moving force 

 ceases. 



It is said that eleven thousand different tones can be distin- 

 guished in the whole range of the ear. The basilar membrane is 

 more than adequate to distinguish this number as it consists of 



