The reader is referred to works upon anatomy for a hisIn some works upon tory of the action of the muscles. physiology, will be found descriptions of the acts of walking, running, leaping, swimming, etc.; but we have thought it better to omit these subjects, rather than to enter as minutely as would be necessary into anatomical details, and to give elaborate descriptions of movements, so simple and familiar. CHAPTER XVII. VOICE AND SPEECH. Sketch of the physiological anatomy of the vocal organs-Vocal chords-Muscles of the larynx—Crico-thyroid muscles-Arytenoid muscle-Lateral crico-arytenoid muscles-Thyro-arytenoid muscles-Mechanism of the production of the voice-Appearance of the glottis during ordinary respiration-Movements of the glottis during phonation-Variations in the quality of the voice, depending upon differences in the size and form of the larynx and the vocal chords-Action of the intrinsic muscles of the larynx in phonation-Action of the accessory vocal organs-Mechanism of the dif ferent vocal registers-Mechanism of speech. THERE are few subjects connected with human physiology of greater interest than the mechanism of voice and speech. In common with most of the higher classes of animals, man is endowed with voice; but, in addition, he is able to express, by speech, the ideas that are the result of the working of the brain. In this regard there is a difference between man and all other animals. It is the remarkable development and the peculiar properties of the brain that enable him to acquire the series of movements that constitute articulate language; and this faculty is always impaired pari passu with deficiency in the intellectual endowment. Language is one of the chief expressions of intelligence; and its study, in itself, constitutes almost a distinct science, inseparably connected with psychology. In connection with the study of movements, therefore, it is not necessary to discuss the origin and construction of language, but simply to indicate the mechanism, first, of the formation of the voice, and afterward the manner in which the voice is modified so as to admit of the production of articulate sounds. The voice in the human subject, presenting, as it does, a variety of characters as regards intensity, pitch, and quality, and susceptible of great modifications by habit and cultivation, affords a very extended field for physiological study. Of late years this has been the subject of careful investigation by the most eminent physicists and physiologists; but to follow it out to its extreme limits requires a knowledge of the physics of sound and the theory of music, a full consideration of which would be inconsistent with the scope and objects of this work. We shall content ourselves, therefore, with a sketch of the physiological anatomy of the parts concerned in the formation of the voice, and the mechanism by which sounds are produced in the larynx, without treating fully of their varied modifications in quality. It will not be necessary to treat of the different theories of the voice that have been presented from time to time, except in so far as they have been confirmed by more recent and complete observations, particularly those in which the vocal organs have been studied in action by means of the laryngoscope. Sketch of the Physiological Anatomy of the Vocal Organs. The principal organ concerned in the production of the voice is the larynx. The accessory organs are the lungs, trachea, and expiratory muscles, and the mouth and resonant cavities about the face. The lungs furnish the air by which the vocal chords are thrown into vibration, and the mechanism of this action is only a modification of the process of expiration. By the action of the expiratory muscles the intensity of vocal sounds is regulated. The trachea not only conducts the air to the larynx, but, by certain variations in its length and calibre, may assist in modifying the pitch of the voice. Most of the variations in the tone and quality, however, are effected by the action of the larynx itself and the parts situated above it. It is impossible to give a complete account of the structure of the larynx, without going more fully than is desirable into purely anatomical details. Some anatomical points have already been referred to under the head of respiration, in connection with the respiratory movements of the glottis ;' and we propose here only to refer to the situation of the vocal chords, and to indicate the modifications that they can be made to undergo in their relations and tension by the action of certain muscles. The vocal chords are stretched across the superior opening of the larynx from before backward. They consist of two pairs. The superior, called the false vocal chords, are not concerned in the production of the voice. They are less prominent than the inferior chords, though they have nearly the same direction. They are covered by an excessively thin mucous membrane, which is closely adherent to the subjacent tissue. The chords themselves are composed of fibres of the white inelastic variety, mixed with a few elastic fibres. The true vocal chords are situated just below the superior chords. Their anterior attachments are near together, at the middle of the thyroid cartilage, and are immovable. Posteriorly they are attached to the movable arytenoid cartilages; and by the action of certain muscles, their tension may be modified, and the chink of the glottis may be opened or closed. These ligaments are much larger than the false vocal chords, and contain a very great number of elastic fibres. Like the superior ligaments, they are covered with an excessively thin and closely adherent mucous membrane. According to M. Fournié, the author of a very elaborate and recent work on the voice, the mucous membrane over the borders of the chords is covered with pavement-epithelium, without cilia. There are no mucous glands in the membrane covering either the superior or the inferior chords. 1 See vol. i., Respiration, p. 358. * FOURNIE, Physiologie de la voix et de la parole, Paris, 1866, p. 129. It has been conclusively shown, particularly by the experiments of Longet, that the inferior vocal chords are alone concerned in the production of the voice. This author, who has made numerous experiments on phonation, has demonstrated, by operations on dogs, that the epiglottis, the superior vocal chords, and the ventricles of the larynx, may be injured, without producing any serious alteration in the voice; but that phonation becomes impossible after serious lesion of the inferior chords.' This being the fact, as far as the mere production of the voice in the larynx is concerned, we have only to study the mechanism of the action of the inferior ligaments and the muscles by which their tension and relations are modified. Muscles of the Larynx.-Anatomists usually divide the muscles of the larynx into extrinsic and intrinsic. The extrinsic muscles are attached to the outer surface of the larynx and to adjacent organs, such as the hyoid bone and the sternum. They are concerned chiefly in its movements of elevation or depression. The intrinsic muscles are attached to the different parts of the larynx itself, and, by their action upon the articulating cartilages, are capable of modifying the condition of the vocal chords. The number of the intrinsic muscles is nine, four pairs and a single muscle. In studying the situation and attachments of these muscles, it will be useful at the same time to note their mode of action. This has been experimentally demonstrated by Longet, who has studied the isolated action of the different muscles by galvanizing the nervous filament distributed to each one, either in the living animal, or in animals recently killed. In this way he has been able to show the mechanism of dilatation of the larynx during inspiration, and to indicate the precise action by which the vocal chords are rendered tense or are relaxed. These experiments, by the positive charac LONGET, Traité de physiologie, Paris, 1869, tome ii., p. 728, et seq. |