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.