Further, it results from the abundance of clinical material, that the acoustic-center K must be divided into a sound-center L, a syllable-center S, a word-center W, each of which may be in itself defective, for cases have been observed in which sounds were still recognized and reproduced, but not syllables and words, also cases in which sounds and syllables could be dealt with but no words; and, finally, cases in which all these were wanting. 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. This last is, on the contrary, wanting to those born blind; and both are wanting to those born blind and deaf. Instead is formed in them through careful instruction, by means of the tactile sensations of the finger-tips, a center for signs of sound that are known by touch (as with the printed text for the blind). Accordingly, the eye and ear are not absolutely indispensable to the acquirement of a verbal language; but for the thorough learning of the verbal language in its entire significance both are by all means indispensable. For, the person born blind does not get the significance of words pertaining to light and color. For him, therefore, a large class of conceptions, an extensive portion of the vocabulary of his language, remains empty sound. To the one born deaf there is likewise an extensive district of conceptions closed, inasmuch as all words pertaining to tone and noise remain unintelligible to him.