Motion pictures were taken with an ultra-high speed cinematographic apparatus to investigate vocal cord vibration in the cases of unilateral recurrent laryngeal nerve paralysis, bilateral sulcus vocalis, bilateral vocal cord polyps, unilateral cyst of the vocal cord, epithelial hyperplasia of the bilateral vocal cords, and cancer of unilateral vocal cord. The ultra-high speed films were analyzed frame by frame with a computer. Taking the layered structure of the vocal cord into consideration, the mechanism of abnormal vibration in each pathology was discussed, and schemata of the pathological movements on frontal section of the vocal cords were drawn.
The conclusions are as follows:
1) Factors to make glottic closure incomplete are immobility of the affected vocal cord, retractation of the margin of the vocal cord and the inversion of an abnormal mass in the glottis.
2) The irregular vibration of the vocal cord is caused by imbalance between the subglottic pressure and the elasticity of the vocal cord.
3) Asymmetry of the physical quality of the two vocal cords results in asymmetrical movements.
4) The factors to make the amplitude of vibration small are an increase in mass and stiffness of the vocal cord and a loss of the subglottic pressure. The amplitude of the intact cord can be affected with an abnormal mass of the contralateral pathological cord.
5) The factors to reduce wave-like movements are collaps of the layered structure of the vocal cord and a loss of subglottic pressure. Wave-like movements are also affected by pathologies of the vocal cord on the opposite side.
6) The factors to cause phase differences within one vocal cord are ununiformity along the length of the vocal cord, and a marked imbalance between subglottic pressure and elastisity of the vocal cord. The phase differences also appear when interferences from the opposite side are varying along the lengh of the vocal cord.

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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.

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