The Hyoid Apparatus 

I learnt something new this week. 

Have you ever heard of the hyoid apparatus? Me neither.

The hyoid is a series of eleven tiny bones, about half the thickness of a biro, which make up a shoe shape, and is situated between the cheeks of the horse. The name comes from “shaped like the letter upsilon (Y)”, which actually describes it’s appearance well.

Here’s the fascinating part. Every single muscle in the horse’s body eventually connects to the hyoid. 

The next fascinating thing about the hyoid apparatus is that it is the only bone in the body not connected to another bone, instead held in place by ligaments. Incidentally, it sits lower in humans, which allows us to form sounds for speech.

The main use of the hyoid apparatus is to support the tongue. However, when dentists manipulate the tongue to rasp the teeth they often upset the balance of the hyoid, knocking it off centre. 

You can tell if the hyoid is misaligned by looking at your horse square on. If one eye is sitting lower, or not facing forwards, giving the face a slight wonky appearance, then the hyoid has been disturbed. Otis’s right eye was fractionally lower than his left, but after his red light therapy session (more on that in a couple of days) I was impressed to see that his eyes were level again.

The importance of having a correctly aligned hyoid bone is mostly observed in ridden work because the hyoid is very influential on balance. A horse with a wonky hyoid will be slightly crooked, prefer one rein, and struggle to accept the aids.

I found this interesting article, and have quoted it below because I don’t think I can better the explanation as to how the hyoid is linked to the ridden aids.

There is a connection from the riders legs to the tongue, neck, jaw and poll. Via the hyoid apparatus, the serratus muscle, which makes the connection between the rib cage and the base of the neck, can be influenced by the lower legs and consequently involves the muscle directly connected to the tongue. This sustains the coordination of stride and breathing. When the horse blocks his atlas-axis mobility, this is the whole muscular system and skeletal chain that the rider must unblock. The connection from the mouth with the rider is seen when the horse makes a soft clicking of the bit with motion of the jaws and with his lips closed. This action is a reaction to the acceptance of the riders aids, legs and hands. Since the horse is a nose breather, and the cantering stride is in time with breathing, a misuse of a bit can interfere with a normal function of swallowing. For placing the horse on the aids correctly, it connects the muscle and ligament of the tongue towards chewing the bit, allowing the contact into the rein and adjusting the neck vertebrae from convex toward the ground, to concave, producing an arched neck. 

Faults with the tongue are always linked to the hyoid apparatus. When adjusting a bridle, the horse must have sufficient room to keep his jaw mobile. If the horse flicks or sticks his tongue out, it is a sign that bitting or the use of the aids are associated with pulling hands. In conclusion, the tongue and larynx are both fixed to the hyoid apparatus. Any tongue movement results in laryngeal movement and would interfere with breathing. The hyoid apparatus is also as we just described, a crucial part of the anatomy involved in the effect of the aids from seat, legs and hands. When a rider can say his horse is in front of his legs and on the aids, and/or on the bit, it is because the muscles connected to the tongue via the base of the hyoid are relaxed and offer a soft chewing and swallowing of the bit.

So you can see that having a displaced hyoid apparatus will affect the whole of the horse. Signs of a dislodged hyoid, are the visual one I mentioned earlier, chewing more on one side, resentment of the bit, headshy, difficulty flexing at the poll, has sensitive to the touch, and the upper and lower incisors not aligning. Horses with an out of place hyoid are also said to suffer from headaches, which could lead to head shaking and other associated behaviours.

As I’m sure you’re now thinking about how to prevent upsetting the hyoid apparatus, let’s look at management techniques. Encouraging the atlo-axis joint to flex reduces tension and helps stabilise the hyoid apparatus, so feeding from the ground is highly beneficial. Making sure bits are a good fit and nosebands aren’t too tight so stopping tension there, and also allowing the horse room to move his tongue, open his mouth and swallow whilst ridden. Rolkur and having the horse overbent is a big no no for the hyoid as well as the numerous reasons we should all be aware of. Instead, ensure jaw flexion originates at the base of the neck Instead of the head just tucking in. Oh and of course any dental problems will affect the area, so you want your horse to have regular dental checkups.

I thought the hyoid apparatus was very interesting, and I’m surprised I hadn’t heard of it before, but one googled I found hundreds of articles to satisfy the inner geek! 

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