The nervous mechanism, by means of which vocal sounds are produced, is among the most complexly coordinated actions that regulate muscular movements.
Like respiration, vocalization at first seems a simple voluntary act, sounds of various kinds being produced at will by the individual. No doubt the respiratory muscles, which work the bellows of the voice organ, are under the control of the will so long as the respiration is not interfered with. The mouth and throat muscles, which shape the resonating tube, are also voluntary. But the intrinsic muscles of the larynx are only voluntary in a certain sense, while in another they are distinctly involuntary, as may be seen in spasm of the larynx; for they are, in part at least, controlled by impulses which arise at the organ of hearing and pass to some coordinating centre, which arranges the finer muscular movements necessary to produce a certain note. When we sing a note just struck on a musical instrument, we set the expiratory, the mouth, and the special vocalizing muscles in readiness, by a voluntary act, for the proper application of the air blast; but the exact tuning of the vocal cords is accomplished, in some measure at least, reflexly by impulses arriving from the ear at a special coordinating nervous centre, the education of which is in advance of that of the voluntary centres, and, therefore, can only be controlled by the latter in persons specially educated in singing. Some persons who can sing a given note with promptness and exactitude, without any effort, would find much difficulty in overcoming, by volition, the accuracy of this perfect reflex mechanism. In fact, a person with a naturally "good ear" finds it difficult to sing out of tune, even if he try. Though we feel that we have command over the pitch of the sounds produced in the larynx, we owe much of our accuracy to the aid given by our sound-appreciating organs and the nerve centres in connection with them.