抄録
The purpose of the present study is to investigate how the position and the shape of the vocal cord are altered by contraction of the intrinsic laryngeal muscles, and thus to determine the role of each intrinsic laryngeal muscle in adjusting the vibrator, i. e. the vocal cord.
In excised canine larynges, changes in position and shape of the vocal cord were investigated with still pictures and motion pictures taken from above as well as from inner side when each single muscle was electrically stimulated and also when two muscles in various combinations were activated. Then the larynges were rapidly immersed in cold alcohol solution of -30° and fixed in the stimulated condition. They were dehydrated with a freeze substitution method, and later served for histological examinations.
The results appear to justify the following conclusions:
1. When the cricothyroid muscle is stimulated, the vocal cord moves to paramedian position. The thyroarytenoid muscle and lateral cricoarytenoid muscle adduct the vocal cord beyond the midline. The arytenoid muscle adducts the vocal cord to the paramedin position. It does not, however, affect shape of the membranous portion of the vocal cord significantly. The posterior cricoarytenoid muscle abducts the vocal cord to lateral position.
2. The posterior cricoarytenoid muscle makes the glottal area widened and the other muscles make it narrow.
3. The level of the upper margin of the vocal cord is raised by contraction of the posterior cricoarytenoid muscle, whereas the other muscles lower the level of the cord.
4. The cricothyroid, lateral cricoarytenoid and posterior cricoarytenoid muscles elongate the vocal cord, whereas the thyroarytenoid and arytenoid muscles shorten the vocal cord.
5. The tension of the vocal cord is increased by contraction of the cricothyroid, lateral cricoarytenoid and posterior cricoarytenoid muscles. Contraction of the thyroarytenoid muscle loosens the mucous membrane although it makes the muscle itself tense.
6. The thyroarytenoid muscle thickens the vocal cord, whereas the cricothyroid, lateral cricoarytenoid and posterior cricoarytenoid muscles make the vocal cord thin.
The findings described above indicate that the laryngeal muscles play important roles to change the shape of the vocal cord as well as to determine the position of the vocal cord. The results of the present investigation give bases to discuss laryngeal adjustments which produce different patterns of vocal cord vibration in various ways of phonation.
