The command of language comprises, on the one hand, the understanding of what is spoken; on the other hand, the utterance of what is thought. It is at the height of its performance in free, intelligible, connected speech. Everything that disturbs the _understanding of words heard_ must be designated disturbance of speech equally with everything that disturbs _the production of words_ and sentences. By means of excellent investigations made by many persons, especially by Broca, Wernicke, Kussmaul, it has become possible to make a topical division of most of the observed disturbances of speech of both kinds. In the first class, which comprises the _impressive_ processes, we have to consider every functional disturbance of the peripheral ear, of the auditory nerve and of the central ends of the auditory nerve; in the second class, viz., the _expressive_ processes, we consider every functional disturbance of the apparatus required for articulation, including the nerves belonging to this in their whole extent, in particular the hypoglossus, as motor nerve of the tongue, and certain parts of the cerebral hemispheres from which the nerves of speech are excited and to which the sense-impressions from without are so conducted by connecting fibers that they themselves or their memory-images can call forth expressive, i. e., motor processes. The diagram, Fig. 1, illustrates the matter. [Illustration: FIG. 1.] The peripheral ear _o_, with the terminations of the auditory nerve, is by means of sensory fibers _a_, that are connected with the auditory nerve, in connection with the storehouse of sound-impressions, K. This is connected by means of the intercentral paths _v_ with the motor speech-center M. From it go out special fibers of communication, _h_, to the motor nerves of speech which terminate in the external instruments of articulation, _z_. The impressive nerve-path, _o_ _a_ K, is centripetal; the expressive, M _h_ _z_, centrifugal; _v_, intercentral. When the normal child learns to speak, _o_ receives the sound-impressions; by _a_ the acoustic-nerve excitations are passed along to K, and are here stored up, every distinctly heard sound (a tone, a syllable, a word) leaving an impression behind in K. It is very remarkable here that, among the many sounds and noises that impress themselves upon the portions of the brain directly connected with the auditory nerve, a selection is made in the sound-field of speech, K, since all those impressions that can be reproduced, among them all the acoustic images necessary for speech, are preserved, but many others are not, e. g., thunder, crackling. Memory is indistinct with regard to these. From K, when the sound-images or sound-impressions have become sufficiently strong and numerous, the nerve-excitement goes farther through the connecting paths _v_ to M, where it liberates motor impulses, and through _h_ sets in activity the peripheral apparatus of speech, _z_.