Heads Above The Rest!; Exploring The Science and Art Of The Equine Head for Sculpting: Part 5

Introduction

Hello there! Let’s continue our quest of discovery about the equine head from both an anatomical and artistic point in this 20–part series. As artists who work in equine realism, our work benefits from an understanding that’s based in biology so we better grasp the whys of equine structure. Every bit about the animal has its own “biologic” and to understand that means we gain better insights into structure and function. Armed with this insight then, our creative choices have more confidence, substance, and thoughtfulness. Of course then, the equine head is rich in biological context. In fact, so much so that it’s actually defined by its biological context alone. There’s nothing about the equine head that’s superfluous or ornamental—it’s an elegant study of pure functional economy. As it needs to be! Indeed, the equine is built to do three things exceptionally well—eat grass, be dexterous, and run fast for a long time…and that he does in spades! No other animal can do what he does in that unique combination, and so efficiently. And his head contributes a great deal to these abilities, and reflect them clearly in its structure. So let’s learn more! In this installment then, we’ll continue with his mouth and throat since they're also distinct features of this animal. Here we go!…

Mouth And Throat

The mouth is formed by the lips, cheeks, hard palate, soft palate, tongue and the membranes below it, and the line of the mouth which is the actual entry into the mouth cavity. The corners of the mouth are where the two lips form a junction, and protrudes from the side of the head. The tongue lays on the floor of the mouth cavity between the bars of the mandible, helping to push food back into the pharynx. The side cheek is also fleshy, mobile and elastic. The skin of the lips is fleshy and wrinkled, owing to their elasticity, and are supported by the Obicularis oris, the muscle of the mouth unattached to bone, giving the lips great flexibility and squishiness.

His upper lip is of particular interest, being nearly prehensile and extremely flexible and mobile. It has a firm, fleshy, boxy bulb on the front, sometimes grooved by a subtle medial depression, and moves more or less as a whole unit, often expressing his mood. The lower lip is very fleshy and mobile and from it hangs the chin, a fibromuscular pad. The lower lip and chin can also be distorted by surrounding muscles to express his emotional state.

Also unique to equines is a large flap of tissue, the soft palate, the Palantal Drape, which blocks off the pharynx from the oral cavity. It’s a muscular, tough, stretchy sheet of flesh that extends from the hard palate in front of the pharynx, with lower ends tacked down near the larynx at the base of the epiglottis, forming an effective wall between the oral cavity and the pharynx. It’s equivalent to the uvula in people, the little “punching bag” at the back of our throat, but only in horses, it grew into an enormous, specialized design. Functionally, the Drape prevents the horse from inhaling mouth contents so water and food are squeezed through one–way slits in the middle and along the sides of the drape. So when we look at the back of horse’s mouth, we don’t actually see down his throat like we would with us, a dog, or a cat. What we see is the pink, fleshy sheet of the Palantal Drape. This makes the equine an obligate nasal breather—he can neither breathe through his mouth nor pant as a method of thermoregulation, but can only breath through his nostrils. The Drape also helps to prevent the horse from vomiting. Only in the worst, most extreme situations does a horse vomit, yet all the same, the Drape diverts the flow through the larynx and into his lungs (and sometimes through his nose)—possibly a fatal condition. Yet to prevent vomiting from happening in the first place, nature created a powerful one–way valve at the top of his stomach. The reasons for this over–riding need to prevent vomiting are unclear. Perhaps it’s to inhibit the regurgitation of food—and possible aspiration into the lungs—when such a large animal with a sloshing gut–full of liquid slurry must sustain a gallop to escape from predators. 

As for his hard palate, it also changed to develop a bit of a vault and pronounced stepped ridges that act as a conveyor belt to move chewed food back to the grinding molars and then to the back of the throat for swallowing. His chewing muscles also enlarged, becoming powerful and meaty as clearly seen on his jowls and cheeks. Subsequently, the horse’s facial muscles are nominal at the profile of the skull, robust on the sides with the cheeks and jowls, to again thin out underneath. Isn’t it interesting how his facial muscles directly reflect the importance of chewing for this animal?

The equine tongue plays important roles both in digestion and respiration, too. The biggest muscle in the head, it lays on the floor of the mouth, anchored at its root by the Hyoids, mucus membranes, and the muscles of the lower jaw. The muscles of the larynx, Palantal Drape, and pharynx may be essential for keeping the airway open, but the tongue is crucial, too. Specifically, it narrows the laryngeal opening when its root is pulled backwards in the swallowing motion thereby helping to divert food into the esophagus. Essentially then, it also helps to maintain the correct position of the larynx. 

The nasal passages join to the larynx via the pharynx which is the opening at the back of the throat where the nasal cavity and oral cavity meet before separating into the trachea (air tube) and the esophagus (food tube). Separated from the oral cavity by the Palantal Drape, the pharynx is a muscular cavity about 6” long (15cm) in an adult horse. Its upper portion is sometimes called the nasopharynx, which protects the entrance to the auditory tubes, the oropharynx (which contains tonsilar tissue), and the laryngopharynx. Unlike the larynx and trachea, the pharynx lacks a rigid support of bone or cartilage and so depends on the muscles of the Hyoids, Palantal Drape, and tongue for structure.

The larynx, often referred to as the “voice box,” joins the pharynx and trachea. It’s a short tubular structure made up of five cartilages that form a rigid framework that articulate together as a coordinated system. These cartilages are:

• The cricoid
• The thyroid
• The paired arytenoid cartilages
• The epiglottis

Of particular interest is the epiglottis, a cartilaginous, flap–like, moveable structure that overhangs and lays at the upper part of the voice box and covers the opening of the larynx. When a horse swallows, the walls of his pharynx contract to push the food into the esophagus. However, foodstuff still has to cross the open gap of the pharynx. This can be a problem because the esophagus (food tube) is situated above the larynx (air tube), creating a risk that food might be pushed into the lungs. Yet, the special little epiglottis prevents this from happening. How? Well, when swallowing, muscle contraction and the actions of the swallowing systems themselves automatically close the epiglottis, protecting the lungs from food or water. In a nutshell then, the epiglottis performs three key functions:

• 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.

As for swallowing mouth contents then, when the horse is breathing, the free edge of the soft palate is usually under the epiglottis and the laryngeal entrance is open. But when swallowing, muscles raise the tongue, pressing contents against the hard palate. The root of the tongue is pulled backward, the laryngeal entrance is narrowed, and the soft palate is elevated to the rear wall of the pharynx. Increased pressure in the pharynx forces the contents into the esophagus where involuntary contractions take it to the stomach. So this means that when breathing, not only does the pharynx and soft palate form a smooth, uninterrupted passageway for the flow of air into the trachea, but when swallowing, the pharynx and soft palate move so that food is directed into the esophagus rather than the trachea. 

Yet perhaps the most overlooked system in the equine head is the Hyoid Apparatus, a group of small bones that really are some of the most important bones in the equine body. Typically left out or glossed over in anatomical or sculpting references, this system is critical for equine biomechanics, health, and psychology. Why? Because together they form a mechanism directly involved with swallowing, balance, movement, hearing, taste, and well–being. In people, the Hyoids are unattached and “float,” but in equines, they're attached by cartilage, muscle, tissue, and ligaments in part to the tongue, ear, and skull. Located inside the head towards the back, laying at an angle between the base of the ears and between the rami of the mandible, they are:

• 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.

[Note: Sometimes the paired Tymponohyoids are named as separate components, but they’re most often regarded as cartilaginous extensions of the Stylohyoids. Also sometimes paired Epihyoids are named, but sometimes not. They’re tiny in the horse.)

Even more, the Hyoid Apparatus in the horse is also attached to the scapula and sternum! When the horse lost his clavicle during evolution, this forced the Omohyoideus muscle to find a new attachment to stabilize the Hyoids. To do so, it connected to the inside of the scapula of the foreleg system. This means that some of the horse’s ability to balance as well as to swallow are directly linked to his foreleg! What's more, it also means his tongue and larynx are also directly associated with his forelegs! Likewise, the Sterntothyroideus and the Sternohyoideus muscles both originate off the Hyoid Apparatus to attach to the top of the sternum, linking the Hyoid Apparatus directly to his torso. These three muscles create a connection through the Pectorals and to the abdominal muscles to the pelvic muscles, creating an "under muscular chain" between the animal's mouth and his hind legs!

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.

Now when not ridden, the horse naturally protects these connections, especially the connection to the shoulder, through coordinated motion. However, these precautions can be over–ridden by irresponsible riding or tack contraptions that can jerk the carefully synchronized Hyoid Apparatus. For example, when a fore foot is fixed and the neck is cranked hard and rapidly by the rider in the opposite direction, the Hyoids can be damaged. Predictably then, many domestic horses exhibit some form of damage or degeneration to the Hyoid Apparatus and its bones (especially the lower ones) because many people don’t understand the damage potentially caused when manipulating the tongue, ears, head, neck, or forelegs, especially when those movements are out of synch with the animal’s natural coordination. Truly, the animal was meant to function holistically, and no part of his body operates alone, but in tandem with every part of his anatomy.

Anyway, altogether then this means that air, food, and water are channeled through the open space of the pharynx by seven openings, those being:

• 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.

So, in sequence, when breathing, air passes through the nostrils into the nasal cavities, over the hard palate, past the turbinates, to the sinus cavities where it passes the Eustachian Tubes and Guttural Pouches into the pharynx, past the soft palate, through the epiglottis and into the larynx then into the trachea (windpipe) through the bronchi and down into the lungs where the diaphragm and ribs create inspiration then back again during exhalation. When swallowing, food is gathered first by the lips, nipped by the incisors, passes to the grinders by the tongue and ridged hard palate where its chewed into the texture of fine cornmeal. Then it's squeezed through the slits of the Palantal Drape, goes through the pharynx and over the epiglottis and into the esophagus then down to the stomach and intestines where digestion happens. Coordination of these systems is complicated, demanding a careful orchestration for certain parts, such as the epiglottis, to physically change shape to accommodate inhalation, exhalation, and swallowing. For this reason, disruption or damage in any part of this finely–tuned system can be dangerous, even fatal.

As for the equine sense of taste, it’s closely linked to smell, just like with us. While the horse’s sense of taste is largely unknown, it’s believed to be quite good and, paired with smell, can detect certain toxic plants or tainted water. Indeed, some domestic horses deftly pluck out those parts of hay that are spoiled, eating only those morsels still edible with great precision. Horse also prefer certain foods and often have favorite treats they truly enjoy. 

Conclusion To Part 5

Knowing what goes on inside the horse's head is just as important as knowing its outsides. There's so much we take for granted about this animal and about our aims in equine realism. Too often we simply think about duplicating a horse correctly without much thought as to why all those body parts are shaped that way. Yet expanding our understanding in this regard not only allows us to better express his cranial topography, but also lets us learn its context to inform our creative decisions better. Things simply become more important to get right since we understand its Biologic. Not taking anything for granted also broadens our understanding by asking us to consider often–ignored features, like the Hyoids, which are just as critical for fully grasping the truth of this creature as any muscle or bone. Not only do they provide insight into the animal's physique, but also his psychology, and that helps us avoid depictions of harm or distress. We should always remember that no part of this creature is without an important formative story that can be directly relevant to what we do with our clay. 

So in Part 6, we’ll move onto his hearing and ears, both essential elements to his behavior and body language as well as character and expression. So until next time…gobble up insights about equine cranial function!

“You can accelerate your development by giving yourself a fresh set of challenges, or the same set viewed from a different angle, every day. Explore a different path – if it’s a dead end, explore another.” ~ Paul Foxton