THE EAR, HEARING, TASTE AND SMELL 235 



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- 

 sidiary 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- 

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

 ceases. 



According to various estimates that have been made, from six 

 thousand to eleven thousand different tones can be distinguished 

 in the whole range of the ear. The basilar membrane is more than 

 adequate to distinguish this number as it consists of twenty-four 

 thousand strands. Fourteen thousand nerve-fibers communicate 

 with the hair-cells of the organ of Corti. 



We must suppose that compound tones entering the ear set the 

 fluids of the cochlea into vibrations whose form depends upon the 

 make-up of the tone producing them. These vibrations are 

 analyzed by the basilar membrane, the particular strands having 

 the vibration rates of the fundamental and the partials which are 

 present being set into sympathetic vibration and stimulating the 

 nerve-fibers with which they communicate. 



Auditory Perceptions. Sounds, as a general rule, do not seem 

 to. us to originate within the auditory apparatus; we refer them to 

 an external source, and to a certain extent can judge the distance 

 and direction of this. As already mentioned, the extrinsic reference 



