intrinsic muscles of the larynx
The cricothyroid muscle bridges over the lateral portion of the cricothyroid interval. It takes origin from the arch of the cricoid cartilage, whence its fibers spread out in an upward and backward direction, and are inserted into the inferior horn and lower margin of the thyroid lamina. The cricothyroid muscles are the chief tensors of the vocal ligaments.
Each posterior cricoarytenoid muscle springs, by a broad origin, from the back of the cricoid cartilage, and its fibers converge to be inserted into the muscular process of the arytenoid cartilage.
It is an abductor of the vocal fold; and the pair of muscles therefore widen the rima glottidis.
The transverse arytenoid muscle is the only unpaired muscle in the larynx. It is a thin, flat band that passes from the back of one arytenoid cartilage to the back of the other.
The oblique arytenoid muscles are a pair of feeble slips that lie on the back of the transverse muscle, and cross each other like the limbs of the letter X. Each arises from the muscular process of one arytenoid cartilage, and passes obliquely to the apex of the other. Some of the fibers are inserted into the apex of the arytenoid, but most of them are prolonged into the aryepiglottic fold. There, they are called the aryepiglottic muscle, which proceeds upward to be inserted into the margin of the epiglottis.
These muscles help to close the inlet of the larynx during swallowing; for the arytenoid muscles draw the arytenoid cartilages closer together, and the aryepiglottic muscles draw these cartilages toward the epiglottis.
Each lateral cricoarytenoid muscle springs from the upper border of the side of the cricoid arch; its fibers run backward and upward, and converge to be inserted into the muscular process of the arytenoid cartilage. The superficial surface of the muscle is covered by the lamina of the thyroid cartilage and the upper part of the cricothyroid muscle; its deep surface is applied to the cricovocal membrane,
The lateral cricoarytenoid muscles are adductors of the vocal folds, and therefore reduce the width of the rima glottidis.
The thyroarytenoid muscle is a muscular sheet that springs from the deep surface of the thyroid lamina close to the anterior border, and passes backward to be inserted into the anterolateral surface of the arytenoid cartilage. It pulls the arytenoid cartilage forward, and slackens the vocal fold.
The uppermost fibers of the thyroarytenoid muscle, when they reach the arytenoid cartilage, curve upward into the aryepiglottic fold. They constitute the thyroepiglottic muscle, and join with the aryepiglottic muscle to be inserted into the edge of the epiglottis.
Some of the deepest fibers of the thyroarytenoid – those stretching from the thyroid cartilage to the lateral side of the vocal process – form a bundle called the vocalis muscle, though it cannot be separated from the rest of the muscle except by artificial means. It lies along the lateral side of the vocal ligament and the upper part of the cricovocal membrane, and some of its fibers arise from the vocal ligament.
Actions of the laryngeal musclesTension of the vocal folds is produced by the contraction of the cricothyroid muscles. The anterior parts of the muscles pull the upper border of the cricoid arch upward, and the posterior portions draw the cricoid cartilage backwards, thereby increasing the distance between the angle of the thyroid cartilage and the vocal processes of the arytenoid cartilages. When the cricothyroid muscles cease to contract, the relaxation of the vocal folds is brought about by the elasticity of the ligaments. The vocalis and the thyroarytenoid muscles must be regarded as antagonistic to the cricothyroid muscles. When they contract they pull the arytenoid cartilages towards the angle of the thyroid cartilage and still further relax the vocal folds; and when they cease to act, the elastic ligaments of the larynx again bring about a state of equilibrium.
The width of the rima glottidis is regulated by the arytenoid and cricoarytenoid muscles. The posterior cricoartyenoid muscles draw the muscular processes backwards, rotating the arytenoid cartilages on vertical axes so that the right and left vocal processes and folds are swung apart and the rima glottidis is widened. The lateral cricoarytenoid muscles act in the opposite way. They draw the muscular processes forwards so that the vocal processes and folds move medially to close the anterior part of the rima. At the same time the arytenoid muscles draw the arytenoid cartilages towards each other to close the posterior part.
But the thyroepiglottic and aryepiglottic muscles, aided by the arytenoid muscles, have a different function to perform. During swallowing, they close the inlet of the larynx by drawing the arytenoid cartilages together and pulling them up into close contact with the tubercle of the epiglottis. This sphincter-like action is the essential feature in the closing of the laryngeal inlet.
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