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

Heads Above The Rest!; Exploring The Science and Art Of The Equine Head for Sculpting: Part 4

Introduction

Hello again and welcome back to this 20–part series that anatomically and artistically explores the equine head. Very few resources discuss the equine head from these combined perspectives, yet both are necessary to understand if we hope to create not only authentic realism, but responsible realism. Truly, by understanding the whys for his head structure we can come to insightful creative decisions that promote the honesty of our Voice. Plus understanding these whys deepens our appreciation for this remarkable animal in ways not possible by simply looking at him with uninformed eyes.

So in this Part 4, we’ll discuss his nostrils and sinus, things that are part and parcel to the species itself. Truly, no other animal is quite like the equine—he’s utterly unique in the animal kingdom, and one of these ways he's so different is how he breathes. So let’s get to it, shall we?…

Nostril and Sinus

The nostrils can be a bit fiddly to sculpt because of their mobility and fleshy nature. Yet they're important to get right being such prominent features of the head. As for function, the nostrils—and in particular the sinuses—serve to warm or cool, and moisten incoming air while filtering out pollutants before reaching the lungs.

The nasal passages are divided into two halves by the cartilaginous nasal septum that provides the initial framework for the nostrils and sinus. The septum projects forward from the end of the nasal bone to attach to the premaxillæ, above the incisors, bordered by the nasal bone, the maxillary, and hard palate. Anatomically then, the nostrils fill in the large cavity on either side of this septum and nasal bone, channeling air directly into the sinuses through the length of the head and into the lungs. 

The shape of the nostrils is achieved by two comma–shaped cartilages projecting from the front, the Alar cartilages (also called the “comma–cartilages”). The Alar cartilage is easily palpated, and has an obvious “head” ending with a “tail” which is palpable and often seen as a slight bump, even at rest, beneath the end of the lower nostril rim. Therse two comma–cartilages are placed back–to–back when seen from the front, and they attach to the nasal septum and the lateral cartilage of the nasal bone with which they articulate. So when viewed from the front, the Alar cartilages form an “x” owing to their connected back–to–back portions. Their stiff cartilaginous construction prevents the nares from collapsing during inspiration and provides a rigid infrastructure for the muscles that activate them. 

As for the posterior rim, it’s comprised of gooshy flesh and is more swooping, elastic, and uniform in shape. It connects at the top of the Alar cartilage, where it forms the upper fold, and the tail of the Alar cartilage at the bottom, where it forms a depression at rest. However, during exertion, the nostril may be so flared that this upper fold will open up while the bottom depression will stretch to flatten out and widen. 

The nostril (or nare) has two compartments, the “true” nostril and the “false” nostril (or nasal diverticulum), a curious fleshy feature. The true nostril leads directly into the nasal cavity whereas the false nostril is a pouch above it, oriented towards the nasal bone and forming a dead–end at the junction between the premaxillary and the nasal bone. Muscles activate the false nostril which can therefore capture air, or contort and bulge in interesting ways during certain airflow conditions or during communications such as snorting. It’s thought to collect dirt and debris before entering the airway. When the nostril is flared, this false nostril also flares, often creating an elevated curvy bulge of flesh outlining the nostrils along the side of the nasal bone. 

A thin canal from the eye’s lacrimal gland descends under the mucus membranes of the nasal cavity, the nasolacrimal duct, to a small opening in the nostril. In this way, excess fluids of from the eye are drained through the nostril. So that’s not necessarily snot inside the nostrils, but excess tears, meaning that he doesn’t really bathe us in boogers when he snorts on us, but mostly with tears.

Altogether then, the equine nostril has no bones or bony connection to the skull, being only cartilage and flesh attached loosely by fibrous connections. The nostrils then are highly elastic, especially the fleshy posterior rim, adopting all sorts of sizes, shapes, depressions, bulges, flares, closures, shifting, and ridges in response to exertion level, moment, or mood. They can also dilate a great deal for inhalation and exhalation. For example, flaring into a rather large circular or oval shape during physical exertion, or into a pinched shape to snort and blow, and almost closing when swimming. Also note the network of fleshy wrinkles between the comma cartilages, when seen from the front, and how they change when the nostrils change shape. Sometimes delicate wrinkles surround the posterior rim, too, often becoming more pronounced with nostril dilation or distortion. When strongly dilated, a gentle furrow can rise up between the front rims along the top at times, right by the upper fold where the rims meet at the top. Indeed, the face is quite fleshy around the nostrils and usually distorts with nostril distortions. As for planing, from the front, at rest, the nostrils angle inwards at the upper fold and protrude outwards at the bottom of the posterior rim. From the top, at rest, the front rim angles inwards while the back rim protrudes outward. However, when distorted these planes can become modified depending on circumstance, so we need to pay close attention to how they’re planed specific to the kind of nostril we sculpt so inspect nostrils of similar natures from multiple angles when sculpting. 

The only part that remains somewhat consistent is the comma–cartilage owning to its stiffer construction and their middle connection. Also, where the two rims meet at the top fold is slightly more fixed while the posterior rim can be independently moved and shaped by connecting facial muscles. However, nostril motion is also synchronized with muzzle motions, particularly the mouth and upper lip, and can be thus stretched, tweaked, made asymmetrical, scrunched, or puckered. That said, the nostrils can be moved independently on either side, creating some interesting expressions and distortions. That’s to say horses don’t use their nostrils just to breath, but to also communicate (such as puffs, snorting, and blowing), scent the air (watch how they delicately quiver and dilate), pinch (when swimming, or clearing the nasal passages with that all–too–familiar snot blow), and to convey emotion (note how they morph in sympathy to what he’s feeling). This fleshiness also means that when the nostril is flared, its flute doesn’t manifest as a solid, smooth, triangular, tube of flesh as is often misrepresented in sculpture, but as a convoluted series of bulges and dips consistent to its anatomy, overlaying musculature, and air flow. Nostrils can also be flared or moved independently, depending on the situation or the horse’s mood or reaction. Nostril shape can vary with each individual, too, and it can even be a function of breed type. For example, Arabians tend to have more oblique, horizontally–placed nostrils whereas Quarter Horses often have more up–and–down, vertically–oriented ones. So we should pay attention to nostrils based on the depicted exertion, gesture, individuality, moment, breed type, and emotion embodied in our sculpture.

As for his sense of smell, little is known though it can be deduced with some confidence that it’s superior to ours. Indeed, this sense also changed during his evolution, gaining large surface areas within the nasal cavities as his head elongated for his new teeth. And large they are—it’s believed that if the sensory mucous membrane in these long nasal cavities were spread out, they’d cover his entire body surface!

The horse’s olfactory receptors are located in the mucus membranes within the upper portions of the nasal cavities. When odor molecules enter the nasal passage, they contact the protein and lipid surfaces of the mucous membranes to stimulate the microscopic tufts of hair projecting from the receptor cells. By sniffing, even more hairs are stimulated. The olfactory cells have two branches, one that covers the surface of the olfactory mucosa and another that leads directly to the brain. The twin olfactory bulbs (scent–dedicated areas of the brain) are situated at the front of the cerebrum (one on each lobe) and are directly connected to the receptors in the nasal passages through the main olfactory nerves. Curiously, the olfactory bulbs in the horse are one of the only structures in the brain that don’t overlap; the receptors of the right nostril are directly connected with the right olfactory bulb and the left one is directly connected to the left olfactory bulb. 

What's more, at the back and lateral sides of the nasal cavity exist four delicate bones called the turbinates—a pair of dorsal (upper) turbinates and a pair of ventral (lower) turbinates. Thin honey–combed bones, scroll–like in shape and covered in thick mucus membranes, they help to increase the surface area to which the tissues are exposed to air, providing a broad surface area for discriminating scent. The first two are the dorsal and ventral turbinates, and the fifth, in the back, the ethmoturbinate, is rich in olfactory nerves and transmits the scent stimuli to the brain. All five are rich in nerve and blood supplies, and are thickly covered with mucus and fluid–producing glands to also warm or cool, moisten the air, and filter out particles. These three chonchæ further divide the nasal passage into three airway channels, the dorsal meatus, middle meatus, and ventral meatus which channel airflow directly to the olfactory nerves. The ventral meatus is the largest of the three and a direct pathway from the nostrils to the pharynx.

But the horse really has two olfactory systems! Specifically, he has a specialized smelling organs called the Organs of Jacobsen (or vomeronasal organ) (VNOs). In fact, nearly all animals have VNOs, and only people and cetaceans (such as whales and dolphins) are among the few species without them. In horses, the VNOs are about 5” long (12cm), tubular, and cartilaginous. They’re lined with mucous membranes and contain sensory fibers of the olfactory nerve. The VNOs expand and contract from stimulation from strong odors, and have their own pathways to the brain, acting almost like independent sensory organs. Functionally, the VNOs are thought to detect and analyze pheromones, the chemical signals produced by other horses, especially to identify another horse’s sexual status. In this way, they can be considered a sexual organ, mostly to help stallions identify a mare in season.

Correspondingly, the presence of VNOs suggest that the “flehmen” response (loosely translated as “testing," or "to bare the upper teeth") may actually be an analysis of scent. After several moments of olfactory analysis, a horse draws in a scent and curls up his upper lip, closing the nasal passages and holding the scent particles inside. Then an upward head posture may allow these scent particles to linger within the VNOs for a longer time to allow a better analysis. Flehmen is practiced mostly by stallions, however mares can exhibit the behavior as well. Gelding appear to flehmen the least, implying that gelding a horse may compromise his motivation to analyze pheromones. But flehmen isn’t only reserved for sexual communication. Indeed, unfamiliar or pungent smells may trigger the response as well. The flehmen reaction may also allow a precise analysis of territorial marking such as “stud piles” left by stallions. Yet flehmen isn't something unique to equines as cows, goats, sheep, deer, antelope, even cats, can exhibit this behavior.

The horse is constantly using scent to identify threats, evaluate his habitat, pick suitable food and water—and refuse medicated feed no matter how carefully it's been doctored with molasses! He also uses smell to identify and interact with herd mates…even with us. In fact, it’s thought a horse can identify a specific person from 100 paces! And if we coat a foal's nostrils with menthol cream, he typically cannot find his mother! And all this olfactory sensitivity is good since scent is extremely helpful by providing information about something that’s hidden from view, travels great distances, and remains for a long time. This makes it a highly effective means of communication, threat assessment, and identification.

Anyway, as the horse’s skull grew in size, it had to avoid doing so purely by bony substance alone; otherwise it would’ve become too heavy. Therefore, the nasal cavity also has a series of enormous paranasal sinus cavities on each side of the skull—these sinus cavities are the rostral maxillary, caudal maxillary, and frontal sinuses plus the sphenopalatine and ethmoidal spaces. Literally hollow areas inside the skull, they help to add moisture, coolness or warmth, and filtering capacity to the air, or reclaim moisture to minimize fluid loss when exiting the lungs (though it’s believed they play no part in scent detection). They’re also thought to act as internal “air conditioners” that draw heat out of the blood flowing to the brain to avoid fatal over–heating which is important for a galloping horse.  

    

The frontal sinuses occupy the dorsal (top) part of the skull, between the eyes. There are two, one on each side, divided by a bony septum. These communicate with the inside of the conchæ, forming the concho-frontal sinuses. Drainage into the nasal passages is through the caudal maxillary sinus. The maxillary sinuses lay within the maxilla, above the tooth roots. Each is divided into two components, the rostral maxillary sinus in front and the caudal maxillary sinus behind; they don’t connect. In addition, each of these is subdivided into a medial (inside) and lateral (outside) component by an incomplete bony wall that carries the infraorbital canal containing nerves and blood vessels. The close proximity to the tooth roots mean that as the teeth erupt with age, the maxillary sinuses become larger.

During exertion then, the horse dilates his nostrils, pharynx (and nasopharynx), and larynx to intake more air. What’s interesting is that the motion of his body, particularly at the gallop, is synchronized with breathing. To explain, during the suspension phase of the gallop, when his head is up and his gut is shifted backwards, he’ll inhale, then during the extension phase, when his head is down and his gut is shifted forwards, he’ll exhale. And the more rapid his strides, the more rapid his breathing automatically becomes. If we watch a galloping horse closely, for example, we can see this synchronization in the movement of his nostrils. And exerted breathing is no easy feat for the horse! Why? Well, when horses exercise, pulmonary resistance approximately doubles, with 50% of the total resistance originating within the narrow nasal passages. Plus, during major exertion (like galloping), the wind streaming through the horse’s nasal passages rushes in at the unprecedented, astonishing speed of 400 mph (Bennett, 1999). Now consider this…no wind that fast exists on the planet since current wind records are 280 mph in Antarctica and 318 mph in an F5 tornado. The volume of air is substantial as well. For example, a Thoroughbred at a full gallop requires 636–681 gallons (3kl) of air per minute. That’s a lot going on through the humble nostril! What’s more, volumes of up to 79 gallons (300 liters) of blood are pumped at high pressure through small lung capillaries surrounding 10 million air sacs to supply oxygen to the working muscles at the gallop. The horse truly is a marvel of bioengineering, isn’t he?

Helping along these mechanics, the sinus also contains the eustachian tubes which extend from the sides of the pharynx to the ears (or “bulla” in horses, which holds the ear), and can be seen as slits on either side, at the back of the throat. This is where the outer ear (outside the ear drum) is exposed to atmospheric pressures while the inner ear (behind the ear drum) is subject to pressures inside the throat or body, which can be quite different. For this, the eustachian tubes equalize pressure between the inside and outside of the head to maintain an ideal, constant pressure level. When we climb a mountain, for instance, the atmospheric pressure lowers and our eardrum bulges outward due to the internal pressures from inside our own body. So if we didn’t “pop our ears” our eardrums would burst. Also, every time we swallow, we use our Eustachian tubes automatically. The sucking motion creates a negative pressure behind the ear drums, which get sucked inward. To compensate then, the swallowing mechanism (comprised of the Hyoids) activates the eustachian tubes to equalize the pressure with each swallow. This is why we literally cannot swallow without “popping our ears.” Well, the same applies to the horse.

Nevertheless, remember that the wind streaming through a galloping horse’s nasal passages rushes in at that tremendous speed and volume, presenting some imposing physics the horse’s head has to mediate. Indeed, the vacuum created by this intake of air would quickly burst the horse’s eardrum, no matter how efficient the eustachian tubes. Therefore, the eustachian tubes are assisted by a feature unique to equines—the Guttural Pouches. Comprising a two–chambered stretchy pouch, they exist in “Vyborg’s Triangle,” branching off each Eustachian tube; there’s one pouch on each side of the pharynx at the back of the throat, separated by the stylohyoid bone. It appears that horses, mules, and donkeys have the largest pouches. Previously, researchers thought they might enhance the vocal cords or the ear drums, or even make the horse’s head more buoyant for swimming. Yet recent research has shown that the Gutteral Pouches assist the eustachian tubes by acting as additional “eddies” for pressure equalization thereby avoiding a fatal vacuum inside the horse’s head during extreme exertion. Each pouch can hold about 17–20 oz. of air (502–591 cubic cm)—quite a sizeable amount compared to the horse’s skull, which makes sense for such a large animal that depends on sustained speed for survival.

What’s more, because air entering or leaving the airway also passes through the Pouches, any pathogen has ready access to the rich mucus lining that provides an ideal petri dish. However, the pouches also happen to be made of epithelial tissue fortified by a hefty immune system, and so much so than some scientists even consider the Guttural Pouches as a gland of the immune system.

More recently still, the Pouches are thought to also help the sinuses cool the blood going to the brain (Baptiste, et. al. 2000). In other athletic animals such as cheetahs or gazelles, brain cooling depends mostly on the artery of the neck, the carotid artery, and the network of vessels surrounding it, the carotid rete mirabile. Hot blood from the heart flowing into the carotid artery pours into smaller arteries surrounded by cool blood returning from the head. Equines lack this rete plus they also have a body surface area per unit body mass is lower than in us. Yet despite profuse sweating, a moving horse generates a tremendous amount of internal heat which must be dissipated to avoid fatal internal temperatures, especially for the brain. Interestingly, several major arteries and veins that supply the head are also associated with the inner lining of the Pouches. The current addendum then is that internal heat is removed from the brain by the transfer from the blood to the air and out the pharynx via the Pouches (Baptiste, et. al. 2000). So thanks to the Pouches, and other heat dissipating mechanisms, a galloping horse will usually heat up internally to only about 102˚F (38.72˚C) rather than fatal higher temperatures.

This brings us to another curious structure of the horse: The relationship of his “nose tubes” with his larynx. The intake of air is clearly important to a sustained running animal. Yet particular to the equine, evolution placed the internal nares tubes directly over his larynx. This means that the horse literally has air–injection directly into his lungs! What’s more, the horse greatly dilates his nostrils, nasopharynx, and larynx to intake even more air during exertion. On top of that, remember that his stride is synchronized with his breathing, so the more rapid the strides, the more rapid his breathing becomes, automatically. The horse is truly built to run.

Conclusion To Part 4

Fascinating stuff, huh? Isn’t the equine an amazing creature? So much more than we take for granted! This beast is so ubiquitous, it's easy to forget his place in the evolutionary history of this planet as well as his own evolutionary and biomechanical story. We forget and overlook much—too much. Truly, there’s nothing about this beast that isn’t a marvel of bioengineering, having had 55 million years to hone him into a finely–tuned running (and eating!) machine. No other animal can match him in terms of size and mass, speed, stamina, toughness, and agility. Indeed, he’s the only large fermenting herbivore that can maintain a high speed, sustained gait with such athleticism, dexterity, endurance, and strength. These are all features that define the equine, that speak to his biological niche, but there’s more to explore! So in the next installment we’ll continue with his mouth and throat, things that entail swallowing and breathing. So until next time…keep sniffing around for biological facts!

“My heart and soul have been surprised over and over again, with every work of art I explore. It has helped me understand that ‘The Soul Loves the Truth.’” ~ Kathleen Carrillo