HEARING.
105
§ 3. Varieties of Air-waves and their Equivalents in
Consciousness.
We have seen that air-waves, originally set in motion by
the impact of one body upon another, impinge upon the ear,
through the mechanism of the tympanum and the labyrinth,
set up sympathetic vibrations in the elastic bodies which are
attached to the terminals of the nerves, and thus communi¬
cate a stimulation to the fibres themselves. The impulse so
imparted to the auditory centre is there subjectively cognized
as Sound. We have next to enquire what are the various
modes of aerial undulation, and what the corresponding
varieties of Sounds.
The first great distinction between air-waves is the one
already drawn of those which are produced by a single im¬
pact and those which are produced by the continuous vibra¬
tion of an elastic body. The latter are periodic and regular,
the former non-periodic and irregular. But if the ear is to
be differentially excited by these different stimulants, and so
to cognize them separately, it must have special organs for
the perception of each ; and the appendages of these special
organs must be set in sympathetic motion by one or the
other kind of stimulant respectively. Now we find in ex¬
ternal nature that some bodies are readily set in motion by
irregular and non-periodic waves, but that such bodies only
continue in motion for comparatively short periods, instead
of vibrating freely for a considerable time : while on the
other hand certain other elastic bodies are not set in motion
by non-periodic waves, but only answer to undulations whose