The larynx, or voicebox, is composed of two major cartilages, the ring-shaped cricoid at the top of the trachea, and the thyroid, also known as the Adam’s apple. The larynx hangs from a U-shaped bone called the hyoid bone to which the base of the tongue is attached.
Located at the back of the thyroid on each side are two sets of “horns,” the upper, or superior horns, and the lower, or inferior horns. The upper horns attach to the hyoid bone. The lower horns attach to the cricoid cartilage allowing the thyroid “a rocking articulation.”
On the top edge of the cricoid are two small cartilages know as the arytenoids, to which are attached the vocal ligaments and the internal muscles of the larynx. These muscles open and close the vocal folds during breathing and phonation and help to adjust dynamics and pitch.
Another cartilage that deserves mention is the epiglottis, a leaf-shaped cartilage that is attached to both the thyroid and the arytenoids and serves as a “lid” to the trachea during swallowing.
The muscles of the larynx are named for the cartilages to which they attach. The one with which singers are primarily concerned is the thyroarytenoid muscle or vocalis. The vocalis forms the body of each vocal fold. When it contracts the vocal folds shorten and thicken, thus lowering pitch. In addition to shortening the vocal folds, contraction of the vocalis can also keep the glottis closed longer and increase loudness.
When the crycothyroid muscle contracts it causes the thyroid cartilage to rock forward slightly. This in turn causes the vocal folds to stretch and thin, thus raising pitch.
The muscles that close the vocal folds for phonation are the lateral cricoarytenoids and the interarytenoids. When the lateral cricoarytenoids contract, they bring the vocal processes of the arytenoid cartilages together closing most of the glottis. For complete closure the interarytenoids must also contract to close the posterior portion of the glottis. Failure of the interarytenoids to contract allows air to escape resulting in a breathy tone.
The posterior cricoarytenoid muscle is responsible for opening the vocal folds for inhalation. It acts in opposition to the lateral cricoarytenoids and also can control pitch and loudness by acting as a check on the vocalis and cricothyroid muscles.