Figure IV
The thyro-arytenoideus muscles are attached in front to the interior angle of the thyroid cartilage, and at the back to the arytenoid cartilages. To their outer edges the vocal cords are attached and the space between them is called the glottis. The crico-thyroids are attached to the outer surfaces of the cartilages. The lateral and posterior crico-arytenoids have very complex attachments, which can be seen in Figure III. The arytenoideus connects the rear surfaces of the two arytenoid cartilages. In Figure IV a drawing of the larynx is presented, looked at from above, the surrounding parts having been dissected away to allow a clear view of the muscles just considered.
It is impossible to state with absolute certainty just what office the various intrinsic laryngeal muscles have in the production of tones of different pitches and qualities. Most of our information regarding this subject has been obtained by means of the laryngoscope. People interested in singing are now very generally acquainted with this little instrument, although it was invented by Manuel García so recently as 1855. While it is generally used only by physicians, anyone who wishes to can easily procure one and by its means observe the actions of his own vocal cords. The laryngoscope consists of a little mirror fitted to a handle at an angle of about 100 degrees. When used by a physician, it is held in the back of the subject’s throat, the tongue being pulled forward and to one side. A ray of strong light is reflected into it from another mirror, which the observer straps to his forehead. This ray of light is again reflected by the laryngeal mirror so as to illuminate the vocal cords. At the same time the observer sees in the laryngeal mirror the image of the vocal cords and so studies their movements.
While laryngoscopic observation has thrown much valuable light on the operations of the laryngeal muscles, it has not by any means cleared up all the mysteries pertaining to this peculiarly intricate subject. All the muscles concerned are very small. Each one contains a vast and intricate number of tiny fibres, extending in widely varying directions. As each one of these sets of fibres can be contracted with a greater or less degree of strength than those most intimately connected with it, the possibility of variety in the combined actions of the laryngeal musculature is seen to be almost unlimited. Many conflicting theories have been offered to explain the details of the laryngeal action, but a comprehensive review of the subject is not called for here. Our purpose will be served by stating the most generally accepted theory, without committing ourselves as to its accuracy or sufficiency. This theory is as follows:
In quiet breathing all the laryngeal muscles are in a state of relaxation, with the possible exception of the posterior crico-arytenoids. These muscles draw the arytenoid cartilages apart, and so open the glottis. For the production of a tone the glottis is closed by the contraction of the arytenoideus, which pulls the arytenoid cartilages together. The tension of the vocal cords is regulated by two sets of muscles: first, the thyro-arytenoids, to which the cords are directly attached; second, the lateral crico-arytenoids, which rotate the arytenoid cartilages, bringing their forward spurs together.
The pitch of the tone is determined in two ways: first, by the tension of the vocal cords; second, by their effective length. In different parts of the vocal range the manner of adjustment for pitch varies. For the lowest notes of the voice, the chest register, the cords vibrate in their full length. As the pitch rises in singing an ascending scale passage in this part of the voice, the tension of the vocal cords is gradually increased by a corresponding increase in the strength of the contraction of the thyro-arytenoids. When the limit of the chest register has been reached, a further ascent in pitch is brought about by the gradual shortening of the vocal cords. This is effected by the contraction of the lateral crico-arytenoids, which rotate the arytenoid cartilages inward, as has just been described. The forward spurs of these cartilages are slightly curved in shape, so that, as they continue to rotate, their point of contact is gradually brought further and further forward. As the vocal cords are held tightly together behind this point, the portion of the cords left free to vibrate is shortened with each material increase in the tension of the lateral crico-arytenoids.
With each shortening of the vibrating parts of the cords the pitch is correspondingly raised. The portion of the vocal range thus produced is called the medium register. The highest note of the medium register is reached when the forward spurs of the arytenoid cartilages are in contact at their tips. Beyond this point there can be no further shortening of the vocal cords in this way. The remaining notes of the compass, known as the head register, are secured through a shortening of the effective length of the vocal cords at their forward ends, as well as by a further increase in their tension. Both these actions are accomplished by the contraction of the thyro-arytenoids.
Figure V
Figure V shows a laryngoscopic view of the larynx, as it appears in quiet breathing. It will be observed that the vocal cords are widely separated, and the glottis is opened to its full extent. A similar view of the larynx during the production of tone is given in Figure VI. The vocal cords are closely approximated and the glottis is narrowed to a tiny slit-like opening.