Page:The American Cyclopædia (1879) Volume XVI.djvu/418

 398 VOICE and vocal chorda are separated from each other, and reduced to the form of a narrow, almost linear slit, when they are closely ap- proximated. By the varying movements of the arytenoid cartilages, the tension of the vocal chords may at the same time be increased or diminished. It is upon these variations in the width of the rima glottidis, and the position and tension of the vocal chords, that the pro- duction and modifications of the voice mainly depend. The first condition of the formation of a vocal sound is the forcible expulsion of air through the larynx. The voice may also be formed imperfectly in inspiration, but only for a short time and with a certain degree of difficulty. The natural time for the continuous production of a vocal sound is during expi- ration. The chest is first filled with air, and then by its prolonged and steady expulsion, accompanied by the simultaneous action of the laryngeal muscles and the vocal chords, the vocal sound becomes audible. The second condition is that the vocal chords be approxi- mated to a certain extent and thrown into a state of appropriate tension. Even with a for- cible expulsion of air, if the rima glottidis be widely open and the chords relaxed, no vocal sound is produced, or at best nothing more than a hoarse whisper, audible only at a short distance. But when the chords are closely approximated and at the same time rendered tense by the action of the laryngeal muscles, their edges are thrown into a state of rapid vibration by the passage of the air, and a vo- cal sound at once becomes audible. This is readily seen in the larynx of a living animal when exposed to view. It can also be ren- dered visible in the human subject when the glottis is illuminated by sunlight with the aid of a mirror, placed at the proper angle at the back part of the pharynx, as in the la- ryngoscope. But the vocal sound is not di- rectly produced by the vibration of the vocal chords. These chords are not, like the strings of a guitar or violin, simply extended between two opposite points and capable of vibrating freely in the interval. On the contrary, they are bands of elastic tissue attached throughout by their external surface, as well as at both ends, to the surrounding tissues, and only pro- jecting inward, as more or less flattened or rounded ridges, at the level of the rima glottidis. In the opinion of some physiologists, it is the thin fold of mucous membrane on the internal surface of the vocal chords which forms the real vibrating edge in the vocal effort, while the chords themselves only constitute the necessary elastic basis for its attachment. Still it is evi- dent that the approximation and elastic tension of the chords supply the primary and essential physical condition for the formation of the voice. The immediate cause of the produc- tion of the vocal sound is the vibration of the column of air itself, while passing out through the rima glottidis. The mechanism of the larynx as a vocal organ, therefore, seems to be analogous to that of a reed instrument, in which a column of air, passing forcibly through a narrow slit, bounded on one or both sides by a thin elastic plate of wood or metal, first causes the edge of the plate to vibrate with a sufficient rapidity, and is thus itself thrown into a state of sonorous vibration. In in- struments of this kind, the tone or pitch of the sound produced varies with the length and width of the opening, the size of the vibrating plate, the force of the column of air, and the rapidity of the vibrations. With a wide open- ing, a large vibrating surface, and a moderate- ly rapid passage of air, the vibrations will be comparatively slow and the sound produced of a deep or grave tone. With a narrow opening and a more rapid current of air, the vibrations will be increased in frequency, and the note produced will be acute or high-pitched. The same thing takes place in the formation of vocal sounds in the larynx. When the vocal chords are only partly approximated, their ten- sion incomplete, and the vibrations allowed to take place along their whole extent, the voice has a grave sound ; when they are closely ap- proximated, reducing the rima glottidis to a comparatively short and linear slit, and thrown into a high degree of tension, the voice be- comes acute. Every variation between the two extremes of high and low notes is pro- duced in this way, by alterations in the width of the rima glottidis and the length and tension of its vibrating edges. These variations are also aided by changes in the position of the entire larynx ; the organ being usually carried downward toward the chest in the production of the lower notes, and upward toward the head in forming the more acute sounds. The character or quality of the voice is also con- siderably modified by the conditions of tension or relaxation, moisture or drvness of the mu- cous membrane of the larynx, mouth, and fauces, and the variation in form of the cavities both above and below the glottis; but the actual production of the vocal sound invariably takes place in the larynx, its modifications from the above causes being always secondary. From what has been said, it is evident that the formation of the voice depends upon the action of the laryngeal muscles, which move the arytenoid cartilages upon their articula- tions, and thus increase or diminish the width of the rima glottidis and the tension of the vocal chords. These muscles are animated by the recurrent laryngeal nerves, branches of the pneumogastric nerves, which come off from their parent trunks after these have entered the cavity of the chest, and then retrace their course from below upward, through the deep- er parts of the neck, until they reach the level of the larynx. Hence their name of " recur- rent" laryngeal nerves. If these nerves be divided by an operation, or compressed by an abscess, aneurism, or tumor, so as to interrupt the nervous communication, the voice is en- feebled or lost, according to the degree of in-