This subject has been so exhaustively studied that nothing new can well be discovered regarding the muscular structure of the vocal organs. In all probability the reader is sufficiently acquainted with the anatomy of the larynx and its connections. Only a very brief outline of the subject is therefore demanded. The muscles concerned with breathing call for no special notice in this connection.
The special organ of voice is the larynx. This consists of four cartilages, with their connecting ligaments,—the thyroid, the cricoid, and the two arytenoids, and of nine so-called intrinsic muscles,—two crico-thyroid, right and left, two thyro-arytenoid, two posterior crico-arytenoid, two lateral crico-arytenoid, and one arytenoideus. The inner edges of the thyro-arytenoid muscles form the vocal cords. The hyoid bone, serving as a medium of attachment for the tongue, may also be considered a portion of the larynx. By means of the extrinsic muscles the larynx is connected with the bones of the chest, neck, and head.
While the muscular structure of the vocal organs is thoroughly known, the actions of the laryngeal muscles in tone-production have never been absolutely determined. This much is definitely established: Vocal tone is produced when the vocal cords are brought together and held on tension, and the air in the lungs is expired with sufficient force to set the vocal cords in motion. The tension of the vocal cords can be increased by the contraction of their muscular tissues, the two thyro-arytenoid muscles; further, increased tension of the cords can also result from the tilting of the thyroid cartilage on the cricoid, by the contraction of the crico-thyroid muscles.
It is also definitely proved that the pitch of the vocal tone varies with the state of tension of the vocal cords; increasing the degree of tension raises the pitch, decreasing the tension lowers it. As to the relative importance of the different groups of muscles in varying the tension of the vocal cords, nothing has been definitely proved.
In addition to the variations in pitch resulting from variations in the tension of the vocal cords, there is also much ground for believing that the pitch may be raised by shortening the effective length of the vocal cords. This is apparently accomplished by the rotation of the arytenoid cartilages; but the specific muscular contractions concerned in the rotation of the arytenoids have not been located.
It is generally asserted by vocal theorists that the quality of the vocal tone, on any one note, is determined mainly by the influence of the resonance cavities. Dr. Mills says on this point: "When it is borne in mind that the vocal bands have little or nothing to do with the quality of the tone, the importance of those parts of the vocal apparatus which determine quality... becomes apparent." (Voice Production in Singing and Speaking, 1906.) This theory that the quality of the tone is determined solely by the resonance cavities is directly contradicted by Prof. Scripture. He proves that changes in tone quality result from changes in vocal cord adjustment. This subject is more fully treated in the following section. Even before this matter had been definitely settled by Prof. Scripture, there was a strong presumption in favor of the vocal cord adjustment theory. Howard advanced this idea in 1883. Several empirical observations support this theory. Most important of these is the fact that a single tone, swelled from piano to forte, goes through a wide variety of changes in quality. Stockhausen's mention of this fact has already been noted.
This fact tends to cast some doubt on the value of laryngoscopic observation as a means of determining the laryngeal action. Under the conditions necessary for examination with the laryngoscope it is impossible for the singer to produce any but soft tones in the head quality of voice. Most of these tones, if swelled to forte, would change from the head to the chest quality. It is probable that this change in quality is effected by a corresponding change in the vocal cord adjustment, as the conditions of the resonance cavities remain the same. But this cannot be determined by laryngoscopic observation.
So far as the actions of the laryngeal muscles are concerned, no difference can be defined between the correct vocal action and any improper mode of operation. Sir Morell Mackenzie examined a large number of people with the aid of the laryngoscope; of these, some were trained singers, others, while possessed of good natural voices, had had no vocal training whatever. Many variations were noted in the notes on which changes of register occurred. But it could not be determined by this mode of examination whether the subject was a trained singer or not.
If there is one specifically correct mode of operation for the vocal cords, this correct action has never been determined from the anatomy of the organs. No doubt there is some difference between the muscular actions of correct tone-production and those of any incorrect operation of the voice. But the nature of this difference in muscular action has never been discovered by means of dissections of the larynx, nor by laryngoscopic observation.
The Acoustic Principles of Tone-Production