ts particular direction. Every
cone, in the central area of the retina, makes all the elementary
visual responses and gives all the possible color sensations; so it is
not strange that the number of visual {234} elements is small. On the
other hand, the ear, having no sound lens, has no way of keeping
separate the sounds from different directions (and accordingly gives
only meager indications of the direction of sound); but its sense
cells are so spread out as to be affected, some by sound of one
wavelength, others by other wave-lengths. The different tones do not
all come from the same sense cells. Some of the auditory cells give
the low tones, others the medium tones, still others the high tones;
and since there are thousands of cells, there may be thousands of
elementary responses.

Theory of Hearing

The most famous theory of the action of the inner ear is the "piano
theory" of Helmholtz. The foundation of the theory is the fact that
the sense cells of the cochlea stand on the "basilar membrane", a
long, narrow membrane, stretched between bony attachments at either
side, and composed partly of fibers running crosswise, very much as
the strings of a piano or harp are stretched between two side bars. If
you imagine the strings of a piano to be the warp of a fabric and
interwoven with crossing fibers, you have a fair idea of the structure
of the basilar membrane, except for the fact that the "strings" of the
basilar membrane do not differ in length anywhere like as much as the
strings of the piano must differ in order to produce the whole range
of notes. Now, a piano string can be thrown into "sympathetic
vibration", as when you put on the "loud pedal" (remove the dampers
from the strings) and then sing a note into the piano. You will find
that the string of the pitch sung has been thrown into vibration by
the action of the sound waves sung against it.

Now suppose the strings of the basilar membrane to be tuned to notes
of all different pitches, within the range of {235} audible
vibrations: then each string would be thrown into sympathetic
vibration whenever waves of its own vibration rate reached it by way
of the outer and middle ear; and the sense cells standing over the
vibrating fibers would be shaken and excited. The theory is very
attractive because it would account so nicely for the great number of
elementary tone sensations (there are over 20,000 fibers or strings in
the basilar membrane), as well as for