It is an important notion in voice physiology that the vocal fold has a layered structure. This paper describes a tutorial summary of our series of investigations of the layer structure of the vocal fold. (1) The vocal fold consists of the mucosa and the vocalis muscle. The mucosa, in turn, consists of the epithelium and the lamina propria. The lamina propria can be subdivided into three layers : the superficial, intermediate and deep layers. (2) From a mechanical point of view, these layers can be divided into three sections : the cover consisting of the epithelium and the superficial layer of the lamina propria, the transition consisting of the intermediate and deep layers of the lamina propria, and the body consisting of the vocalis muscle. (3) The layer strucfure varies along the length of the vocal fold. The cover is the thickest at the midportion, and it becomes thinner towards the auterior and the posterior ends. The in termediate layer is the thinnest at the midportion and it becomes very thick near the ends, forming two masses which are called the anterior and the posterior macula flava. The deep layer of the lamina propria is the thickest in the posterior portion. (4) The layer structure is immature in young children. In newborns the entire lamina propria is rather uniform and loose. It is after adolescence that maturation of the layer structure is observed. (5) The layer structure differs significantly among various animals. (6) The effect of laryngeal muscle contraction upon the mechanical properties can differ from layer to layer. In particular, the cover and the transition get slackened by the contraction of the vocalis muscle while the body becomes stiffened. (7) Different patterns of vibratory movements of the layers result from different combinations of the crico-thyroid and vocalis activities.
It seeme most likely that the essential principle of the vocal fold vibration is a wave like motion of the membrane of the vocal fold itself. For easy travelling of the membrane, elasticity and viscosity which the vocal fold membrane has, are thought to be the most important. From the viewpoint of this idea, we have carried out two experimental studies. One is to observe the vibration from the tracheal side. In the experiment using an excised larynx, it is very difficult to observe the lower surface of the vocal fold while vibrating, because the tube which is used to send the air flow from the trachea obstructs the direct observation of the lower surface vibration. Hence, we separated the trachea and larynx completely and then made a room at the supraglottic side. Instead of sending the air flow from the trachea, we sucked the air from the supraglottic room. Thus we obtained the air flow from the subglottis to the supraglottis. In addition, we obtained a completely free space at the subglottic side in order to observe and photograph the vibration directly from the tracheal view. Remarkable vibrations are always obtained under any condition, at the upper surface of the vocal fold. It is very interesting that no closed phase is obtained at the lower surface vibration of an excised larynx and that the higher the pitch, the smaller the movement obtained at the lower surface vibration. The other is to study on the lubrication of the vocal fold during phonation. Dryness of the fold affects on the efficiency of of the phonatory function of the larynx. In addition, the quality of the mucous fluid is very important to obtain on adequate experimental phonation with reasonable air flow rate and subglottic pressure. From the facts mentioned above, it should be emphasized that from the viewpoint of phonodynamics, the vocal folds must have at least three layers, body, cover and mucous fluid layer.
A ultrasonic method makes it possible to observe the movement of the cover and body of the vocal fold simultaneously. (1) Variant M-mode display The equipment is almost the same as that used for ultrasonoglottography. Using the gate system of echo selector and callipermarker, effective depth of laryngeal vibration and phase difference of motion of the cover and body of the vocal fold can be measured. The effective depth has a tendency to increase as the loudness increases and to decrease as the pitch increases. The phase difference between two waves has a tendency to increase as the loudness and/or pitch increase. (2) Ultrasonic real time imaging Using the real time imaging by ultrasonic pulse method, the movement of marginal and inner layer of the vocal fold during phonation is visualized as a two-dimentional image. We can observe gross movement of the arytenoid, muscular layer and free margin of the vocal fold in horizontal section simultaneously. An average value of the effective depth is about 0.4 mm.
This paper discusses models of the vocal cords, which can be categorized into a drivingpoint impedance (or terminal) type and a multi-mass system type. The former typically involves a so-called one-mass model and also a two-mass model of the vocal cords, and the latter represents the difference approximation of motion equations and also the finite element method. Models for the air flow through the larynx are also considered in connection with the models of the vocal cords. The mechanism of self-excited oscillation of the vocal cords is described in terms of the degeneration of natural modes of vibration which results from the glottal flow. The intention is to clarify the distinction between the self-excited oscillation and the free oscillation of the vocal cords. A self-excited oscillator inherently involves some of non-linear factors with respects to the amplitude of oscillation. In the vocal-cord oscillator, they are the collision of the bilateral vocal cords, the non-linear elasticity of cord tissue, and the non-linearity in the aerodynamics. These non-linear factors must properly be involved in the computer models of the vocal cords. Finally, general cosiderations of the computer simulation of cord-vibration are discussed.
In order to investigate the nature of motor defects of the speech musculatures in dysarthric patients due to cerebral vascular accidents, non-verbal oral movements were studied in relation to overall intelligibility of speech, coordinative movements of speech organs and swallowing movements. The results led us to the following conclusions. 1. The degree of individual isolated oral movements were correlated with efficacy of articulatory, coordinative and swallowing movements. 2. A considerable number of the subjects, particularly the subjects whose articulation were moderately disturbed, showed unconscious accompanied movments in other parts during an intended oral movement. 3. The types of the accompanied movements were often common to different subjects. 4. In spite of the lower grade of oral movements ability, the subjects who exhibited more accompanied movements got fairly well marks in swallowing. This seemed to indicate that the accompanied movements reflected the lower motor patterns of speech organs in some way. 5. It is important to estimate the degrees of the accompanied movements and the isolated movements for the evaluation of the ability of oral movements.
A study was undertaken of the process of improvement in an infant suffering from expressive aphasia with central dysarthria and his problems to be solved. The investigation involved an aphasic male infant whose condition was caused by a traffic accident, and was conducted from 6yrs 9 m to 7 yrs 4 m. The child had lost consciousness for 50 days after the accident, which also caused contusion of the brain and fractures of the left leg. When he first visited our center 9 months after the accident, most of the pathological symptoms had already recovered, except for a partial paralysis of the right arm and leg. The main complaint was expressive aphasia with central dysarthria. The patient could speak words voluntarily and the only vowel he could pronounce was /a/ with a strained and breathing voice. Both voluntary writing and letter copying were impossible, although he could hold a pencil with the right hand. No abnormality was found either in E.E.G. or CT-scan. Special emphasis in therapy was put on preparation for entrance into elementary school, which was to come 10 months thereaffer. Four months after the therapy was started, the patient was able to speak 2-word sentences and the problems of articulation had mostly disappeared. At this point, studies of school subjects such as mathematics and Japanese were added to the speech therapy. The patient still has some problems to be solved concerning studying of school subjects, although he is able to enter an elementary school.