- The cricoid
- The thyroid
- The paired arytenoid cartilages
- The epiglottis
- It regulates the volume of air during respiration with its contractile abilities.
- It prevents air flowing from the mouth to enter the trachea, again making the horse an obligate nasal breather.
- It prevents aspiration of food into the lungs by folding back to block the larynx while swallowing.
- The Thyrohyoids: Paired, they're long and extend backwards from the body with their end tips attaching to the thyroid cartilage of the larynx.
- The Basihyoid (also called Thyrohyoid): Unpaired, but with its lingual process, a forwards projection of a bony rod. In the space between the jaw bars, at a point just in front of the jowls, we can feel the underside of the Basihyoid.
- The Keratohyoids (or Ceratohyoids): Paired rods of bone, they project upwards and forwards from the ends of the body to meet…
- The Stylohyoids (sometimes called the Tymponohyoids): Paired long, flat, delicate bone shafts and also the largest of the Hyoids, being approximately 7–8” long (17–20cm). Projecting backwards, they attach to the skull with cartilage at the petrous bones.
A hinged system, these delicate bones also support the walls of the larynx and join together to form a sling that slides back and forth as well as up and down, allowing the horse to swallow. To illustrate, the top part of the Hyoid sling (the tops of the Stylohyoids) sits inside the cup–like recesses of the petrosal bone (one on either side of the skull), forming a ball–and–socket joint. Incidentally, the petrosal bone is intended by nature to "float," never becoming ossified to its surrounding bone. Moving on, the external auditory meatus is formed by its upper wall, and within it, sit the small bony ear ossicles that form the hearing complex and the semicircular canals that govern balance. These Stylohyoids bone extend down to attach to the rod–like Keratohyoid in a knee–like joint. In turn the Keratohyoid attaches to the Basihyoid, all together forming the swing or sling structure. Connected to this sling are the thyroid cartilages of the larynx. In this way the Hyoid apparatus supports the larynx, meaning that the larynx is suspended from the two ear regions like swing. Now just above the larynx, and attached to it, is the esophagus, both of which are attached to and supported by the tissue that forms the back of the pharynx, the space behind the mouth cavity which mouth contexts must pass through to their intended tube destinations. So when the horse swallows, this causes the Hyoid sling to swing forward and to flex at the knee–joints, so the Basihyoid not only swings forward, but upward, too.
Because of the structure and attachments of the sling then, the Hyoids are particularly vulnerable. For example, while none of the muscles activating this are directly attached to the ears, pulling on the ears (or "ear twitching") can not only tear the muscle that move the ears, but actually rupture the basal cartilage of the ear, pulling it away from its attachment to the temporal bone which surrounds the petrosal portion. In turn, this can inflame and irritate the upper part of the Stylohyoid with the petrosal bone, causing them to fuse together through exostosis. This prevents the Stylohyoid from moving in its ball–and–socket joint with the petrosal bone, causing the horse permanent problems with swallowing and difficulty breathing. Likewise, the tongue is attached to the Basihyoid, so hard pulls on the tongue can sprain the joint between the Stylohyoid and Keratohyoid which may irritate it enough to cause exostosis of the petrosal–stylohyoid joint, too. For example, yanking on the tongue or ear can even make the larynx re–seat crookedly or lock the entire mechanism, then how will the horse swallow? Additionally, the round tendon of the Digastricus muscle has a synovial sheath where it passes between the forked tendon of the Stylohyoideus bone. This sheath can become irritated by yanking of the tongue, or even tying the tongue down (often seen with racehorses), causing pain each time the horse swallows. Also because of the structure and attachments of the Hyoid Apparatus, horses who suffer from bad teeth, poor riding, or tack gadgets or from little freedom to use of his head and neck will often have compromised movement since this "under muscle chain" becomes stiff or prohibited from functioning. In a similar way, strong tension on one rein can inhibit good motion of the hind leg on that side.
- The mouth, with a return to the mouth blocked by the Palantal Drape.
- Two internal nares where the nasal cavity empties into the pharynx.
- Two Eustachain Tubes with their Guttural Pouches.
- The esophagus, the food tube.
- The larynx, the air tube.