(3) Periphero-Expressive or Articulatory Disturbances.
The centrifugal paths from the motor speech-center to the motor nerves of speech and to their extremities, or else these nerves themselves, are injured. Then occurs dysarthria, and, if the path is totally impassable at any place, anarthria. The hearing and understanding of words are not hindered, but speaking, repeating the words of others, and reading aloud are, as in the last case (2, c), impossible. In general this form can not be distinguished from the foregoing when both are developed in an extreme degree, except in cases of peripheral dysarthria, i. e., dyslalia, since, as may be easily understood, it makes no difference in the resulting phenomena whether the motor center itself is extirpated or its connections with the motor outlet are absolutely cut off just where the latter begins; but if this latter is injured nearer to the periphery, e. g., if the hypoglossus is paralyzed, then the phenomena are different (paralalia, mogilalia). Here belongs all so-called mechanical dyslalia, caused by defects of the peripheral speech-apparatus.
Of these five forms each occurs generally only in connection with another; for this reason the topical diagnosis also is often extraordinarily difficult. But enough cases have been accurately observed and collected to put it almost beyond a doubt that each form may also appear for a short time purely by itself. To be sure, the anatomical localization of the impressive and expressive paths is not yet ascertained, so that for the present the centripetal roads from the acusticus to the motor speech-center, and the intercentral fibers that run to the higher centers, are as much unknown as the centrifugal paths leading from them to the nuclei of the hypoglossus; but that the speech-center discovered by Broca is situated in the posterior portion of the third frontal convolution (in right-handed men on the left, in left-handed on the right) is universally acknowledged.
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).
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.