That a sub-conscious adjustment of the vocal organs may supplement the sense of hearing in the estimation of pitch is mentioned by Prof. Ladd. Speaking of the ability, by no means uncommon, to tell the pitch of any musical note heard, Prof. Ladd says: "Such judgment, however, may be, and ordinarily is, much assisted by auxiliary discriminations of other sensations which blend with those of the musical tone. Among such secondary helps the most important are the muscular sensations which accompany the innervation of the larynx and other organs used in producing musical tones. For we ordinarily innervate these organs (at least in an inchoate and partial way)--that is, we sound the note to ourselves--when trying carefully to judge of its pitch." (_Elements of Physiological Psychology._) Much more important in the study of the problem of tone-production are the adjustments of the hearer's vocal organs which were named the sympathetic sensations of tone. This peculiar auxiliary to the sense of hearing calls for the closest attention. Sympathetic sensations of tone are awakened in the hearer through the mere listening to the sounds of the human voice. Vocal tones impress the listener's ear in a manner entirely different from any other sounds. Not only are the tones of the voice heard, just as other sounds are heard; in addition to this, every vocal tone heard is mentally imitated, and this mental reproduction of the tone is referred in imagination to the hearer's own vocal organs. Besides hearing the vocal tone as a sound pure and simple, the listener is also informed of the manner of throat action by which the tone is produced. This mental imitation and judgment of vocal tones is not a voluntary operation. On the contrary it cannot even be inhibited. It is impossible for us to listen to the voices of those about us, even in ordinary conversation, without being to some extent aware of the various modes of tone-production. This idea of the mental imitation of voices may impress us at first as highly mysterious. Sympathetic sensations of tone have been felt and noted, probably ever since the human voice and the human ear were developed. Yet the process is purely sub-conscious. It is performed involuntarily, without thought on the part of the hearer, even without any consciousness of the process. The hearer simply knows how the voices to which he listens are produced. A throaty voice simply sounds throaty; the hearer feels this, and pays no attention to the source of the information. We take it as a matter of course that a nasal voice seems to come through the speaker's nose. Why a certain quality of sound gives this impression we never stop to inquire. The impressions of throat action conveyed by other people's voices seem so simple and direct that nobody appears to have thought to analyze the psychological process involved.