On each side, the mucosa of the middle cavity bulges laterally through the gap between the vestibular and vocal ligaments to produce an expanded trough-shaped space (a laryngeal ventricle) (Fig. 8.221A). An elongate tubular extension of each ventricle (laryngeal saccule) projects anterosuperiorly between the vestibular fold and thyroid cartilage and may reach as high as the top of the thyroid cartilage. Within the walls of these laryngeal saccules are numerous mucous glands. Mucus secreted into the saccules lubricates the vocal folds.
When viewed from above (Fig. 8.221C,D), there is a triangular opening (the rima vestibuli) between the two adjacent vestibular folds at the entrance to the middle chamber of the laryngeal cavity. The apex of the opening is anterior and its base is formed by the posterior wall of the laryngeal cavity.
Inferior to the vestibular folds, the vocal folds (true vocal cords) and adjacent mucosa-covered parts of the arytenoid cartilages form the lateral walls of a similar, but narrower, triangular opening (the rima glottidis between the two adjacent vocal folds). This opening separates the middle chamber above from the infraglottic cavity below. The base of this triangular opening is formed by the fold of mucosa (interarytenoid fold) at the bottom of the interarytenoid notch.
Both the rima glottidis and the rima vestibuli can be opened and closed by movement of the arytenoid cartilages and associated fibro-elastic membranes.
The intrinsic muscles of the larynx (Table 8.19) adjust tension in the vocal ligaments, open and close the rima glottidis, control the inner dimensions of the vestibule, close the rima vestibuli, and facilitate closing of the laryngeal inlet. They do this mainly by: acting on the cricothyroid and crico-arytenoid joints, adjusting the distance between the epiglottis and arytenoid cartilages, pulling directly on the vocal ligaments, and forcing soft tissues associated with the quadrangular membranes and vestibular ligaments toward the midline.
The fan-shaped cricothyroid muscles are attached to the anterolateral surfaces of the arch of the cricoid cartilage and expand superiorly and posteriorly to attach to the thyroid cartilage (Fig. 8.222).
Each muscle has an oblique part and a straight part:
The oblique part runs in a posterior direction from the arch of the cricoid cartilage to the inferior horn of the thyroid cartilage.
The straight part runs more vertically from the arch of the cricoid cartilage to the posteroinferior margin of the thyroid lamina.
The cricothyroid muscles move the cricothyroid joints. They pull the thyroid cartilage forward and rotate it down relative to the cricoid cartilage. These actions lengthen the vocal folds.
The cricothyroid muscles are the only intrinsic muscles of the larynx innervated by the superior laryngeal branches of the vagus nerves [X]. All other intrinsic muscles are innervated by the recurrent laryngeal branches of the vagus nerves [X].
There is a right and a left posterior crico-arytenoid muscle (Fig. 8.223). The fibers of each muscle originate from a large shallow depression on the posterior surface of the lamina of the cricoid cartilage, and run superiorly and laterally to converge on the muscular processes of the arytenoid cartilage.
The posterior crico-arytenoid muscles abduct and externally (laterally) rotate the arytenoid cartilages, thereby opening the rima glottidis. These muscles are the primary abductors of the vocal folds. They are innervated by the recurrent laryngeal branches of the vagus nerves [X].