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and posterior angle of the arytenoids. By pulling the latter point downward and backward it separates the arytenoid cartilages, particularly at their anterior extremity, where the cords are attached. In this action it is aided by a small muscle connecting the posterior surfaces of the arytenoid, namely, the posterior arytenoid, which tends, when the two arytenoid cartilages are held apart, to rotate them, so that the vocal processes are separated.
The narrowing of the glottis is executed by the lateral cricoarytenoids which run upward and backward from the anterolateral aspect of the cricoid to the muscular processes of the arytenoid cartilages. They pull the muscular processes forward, and thus rotate the arytenoid cartilages so as to approximate the vocal processes to one another, while any tendency toward pulling apart the bodies of the cartilages, owing to the downward direction of the muscle, is overcome by the posterior arytenoid muscle and those muscular bands which pass from the posterior surface of the arytenoid cartilages to the epiglottis and the upper part of the thyroid cartilage, the external thyro-arytenoid, and the thyro-ary-epiglottic muscles (Henle). The other fibres, which pass directly from the arytenoid to the thyroid cartilages -internal and external thyro-arytenoid muscles—in the same direction as the vocal cords, complete the closure by helping to press together the vocal processes, and by approximating the cords themselves. In spasmodic closure of the glottis, all these latter muscles act violently together, and have been grouped by Henle as the constrictor of the glottis.
Relaxation of the vocal cords accompanies voluntary closure of the glottis, as in holding the breath, when the false vocal cords are said to have a valvular action. The muscular fibres which run from the arytenoid cartilages to the thyroid, nearly parallel to the true vocal cords, are those concerned in the act of relaxation when the cords are active. They pull forward the arytenoid cartilages, and at the same time draw the upper part of the cricoid slightly forward. These muscles have the allimportant action of adapting the edges of the cords and the neighboring surfaces to the exact shape most advantageous to their vibration.
The tightening of the vocal cords is caused by a single muscle, the crico-thyroid, which, on the outer side of the larynx, passes downward and forward from the lower part of the thyroid to the anterior part of the cricoid cartilage. It pulls the anterior part of the cricoid cartilage upward, causing it to rotate round an axis passing through its thyroid joints. The upper part of the cricoid, which carries the arytenoids, moves backward, the attachments of the vocal cords are separated, and the membranes are thus put on the stretch.
The requirements necessary for the production of voice are the following:
1. Elasticity of the vocal cords and smoothness of their edges ; freedom from all surface irregularity, such as would be caused by thick mucus adhering to them, or by any abnormality.
2. The cords must be very accurately adjusted, and closely ap
proximated together, so that they almost touch evenly throughout their entire length.
3. The cords must be held in a certain degree of tension, or their vibration cannot produce any vocal tone, but only a raucous noise.
4. The air must be propelled through the glottis by a forced expiration. The normal expiratory current is too gentle to give the necessary vibration. After the operation of tracheotomy, the air escapes through the abnormal opening, and sufficient pressure cannot be brought to bear on the cords, so no vocal sound can be produced, and the person speaks in a whisper, unless the exit of air through the tracheotomy tube is prevented by placing the finger temporarily upon the opening.
PROPERTIES OF THE HUMAN VOICE. In the voice we can recognize the properties noted in other kinds of sound. These are quality, pitch and intensity.
1. The quality of vocal sound is almost endless in variety, as is shown by the vocal capabilities of different individuals. The quality of any musical sound depends upon the relative power of the fundamental tone, and of the overtones that accompany it. The less the fundamental tone is disturbed by overtones, the clearer and better is the voice. This difference in quality of the human voice depends upon the perfectness of the elasticity, the relation of thickness to length, surface smoothness, and other physical conditions of the cords themselves, and the exactitude with which the muscles can adapt the surfaces. For singing well, much more is necessary than good quality of tone, which is common enough. The muscles of the larynx, thorax, and mouth must all be educated to an extraordinarily high degree.
2. The pitch of the notes produced in the larynx depends upon--first, the absolute length of the vocal cords. This varies with age, particularly in males, whose vocal organs undergo rapid growth at puberty, when vocalization is uncertain from the rapid changes going on in the part; hence the voice is said to crack. The vocal cords of women have been found by measurement to be about one-third shorter than those of men, and people with tenor voices have shorter cords than basses or baritones. Secondly, on the tension of the cords: the tighter the vocal cords are drawn by the crico-thyroid muscles, the higher the notes produced ; and the well-known singer Garcia believed he observed with the laryngoscope the vocal processes so tightly pressed together as to impede the vibration of the posterior part of the cords, and by this means they could be voluntarily shortened.
3. Intensity or loudness of the voice depends on the strength of the current of air. The more powerful the air blast the greater the amplitude of the vibrations, and hence the greater the sound produced. The narrower the chink of the glottis, and the tighter the parallel cords are stretched, the less is the amount of air and the weaker is the blast required to set them vibrating; and vice versâ, the looser the cords and the wider apart they are, the greater the volume and the force of the air current necessary for their complete vibration. Hence it is that an intense vibration or loud note can be produced much more easily with notes of a high pitch than with very low notes, and we find singers choosing for their telling crescendo some note high up in the range of their voice.
The human voice, including every kind, extends over about three and a half octaves. Of this wide range a single individual can seldom sing more than two octaves. The soprano, alto, tenor, and bass forming a descending series, the range of each one of which considerably overlaps the next in the scale.
During the ordinary vocal sounds, the air, both in the resonating tubes above the larynx and in the windpipe coming from below, is set vibrating, so that the trachea and bronchi act as resonators as well as the pharynx, mouth, etc. This may be recognized by placing the hand on the thorax, when a distinct vibration is communicated from the chest wall. Such tones are, therefore, spoken of as chest notes. Besides the chest tones of the ordinary voice, we can produce notes of a higher pitch and a different quality, which are called head notes, since their production is not accompanied by any vibration of the chest wall. The physical contrivance by means of which this falsetto voice is brought about is not very clearly made out. The following are the more probable views: (1) It has been suggested that in falsetto only the thin edges of the cord vibrate, the internal thyro-arytenoid muscles keeping the base of the cord fixed; while with chest tones a greater surface of the cord is brought into play.
(2) The cords are said to be wider apart in falsetto than in chest notes, and hence the trachea, etc., ceases to act as a resonator. (3) Or the cords may be arranged so that only one part of them, the anterior, can vibrate, and thus they act as shortened cords, a "stop" being placed on the point where the vibrations cease, by the internal thyro-arytenoid muscle.
The production of a falsetto voice is distinctly voluntary, and is probably dependent upon some muscular action in immediate relation to the cords, for it is always associated with a sensation of muscular exertion in the larynx, as well as with changes that take place in the conformation of the mouth and other resonating tubes.
NERVOUS MECHANISM OF VOICE. The nervous mechanism, by means of which vocal sounds are produced, is among the most complexly coördinated 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 coördinating 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,