To see the remaining muscles, we’ll remove this half of the thyroid cartilage, together with the cricothyroid muscle. Here are the internal laryngeal muscles. To begin understanding them, we’ll take them all out of the picture for a moment.
Here’s the cricoid cartilage, here’s the arytenoid cartilage. Here’s the conus elasticus. The vocal ligament is up here. This is the mucosa of the vestibule. The first muscles to add are the two crico-arytenoid muscles. Here’s the posterior one, here’s the lateral one.
They both converge on the muscular process of the arytenoid cartilage. The posterior crico-arytenoid muscle arises from here on the back of the lamina of the cricoid cartilage.
The lateral crico-arytenoid muscle arises from the upper border of the cricoid cartilage. The posterior crico-arytenoid pulls the muscular process backwards. This rotates the arytenoid cartilage, thus widening the vocal opening.
The lateral crico-arytenoid pulls the arytenoid cartilage forwards and laterally, drawing the vocal ligament towards the midline.
The next two muscles that we’ll see, the thryro-arytenoid and transverse arytenoid muscles, act to shorten and narrow the vocal opening.
We’ll add the thyro-arytenoid muscle to the picture first. Here it is. The thyro-arytenoid muscle arises from here on the inner aspect of the thyroid cartilage. It inserts here in front of the lateral border of the arytenoid cartilage.
Next we’ll add the transverse arytenoid muscle. Here it is. The transverse arytenoid muscle, also called the arytenoideus, is a sheet of muscle that bridges the gap between the posterior surfaces of the two arytenoid cartilages.
Let's see how these two muscles work. Contraction of the thyro-arytenoid muscle rotates the arytenoid cartilage inward and pulls it forward, along with the cricoid cartilage. This action slackens the vocal ligament, and shortens the vocal opening from font to back.
Contraction of the transverse arytenoid muscle brings the two arytenoid cartilages closer together, thus closing the posterior part of the vocal opening.
The sphincter action of the last two muscles is augmented by a pair of slender muscles that pass upward toward the epiglottis, the ary-epiglottic muscles. These begin behind the transverse arytenoid muscle, cross the mid-line, and extend upward and forward a little below the ary-epiglottic fold.
Acting together, the thyro-arytenoid, transverse arytenoid and ary-epiglottic muscles act as a sphincter that can completely close the larynx. We close our larynx every time we swallow, cough, and hold our breath.
The most medial part of the thyro-arytenoid muscle, which is attached to the vocal ligament, has a special function. It’s known as the vocalis muscle. It makes fine adjustments to the tension of the vocal ligament.