The original diagram
is thereby considerably complicated, as the simple path of connection
between K and M has added to it the arcs L S M and L S W M (Fig. 2).
[Illustration: FIG. 2.]
The surest test of the perfect condition of all the segments is afforded
by the repetition of sounds, syllables, and words pronounced by others.
Syllables and sounds, but no words, can be pronounced if W is missing
or the path S W or W M is interrupted; no syllables if S is missing or
L S or S M is interrupted. If L is missing, then nothing can be
repeated from hearing. If L M is interrupted, then syllables and words
are more easily repeated than simple sounds, so far as the latter are
not syllables. If L S is interrupted, then simple sounds only can be
repeated. All these abnormal states have been actually observed. The
proofs are to be found in Kussmaul's classic work on the disturbances
of speech (1877). Even the strange case appears in which, L M being
impracticable, syllables are more easily repeated than simple sounds.
If _a_ is interrupted before the acquirement of speech, and thus chronic
deafness is present in very early childhood, articulation may still be
learned through visual and tactile impressions; but in this case the
sound-center L is not developed. Another, a sound-touch-center, comes in
its place in deaf-mutes when they are instructed, chiefly through the
tactile sensations of the tongue; and, when they are instructed in
reading (and writing), a sound-sight-(or letter) center.
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