Thursday, July 27, 2017

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 20part 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

Share/Bookmark

Tuesday, July 4, 2017

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

Share/Bookmark

Tuesday, June 27, 2017

Heads Above The Rest!; Exploring The Science and Art Of The Equine Head for Sculpting: Part 3


Introduction

Here we are again, this time at Part 3 of this 20part series discussing the equine head from an anatomical, biological, and artist point of view. Combining these three perspectives not only gives us a better means to shape our sculptures, but also gifts us with the contextual underpinning that lends meaning to those structures. In turn, this meaning feeds back into our understanding to reconfirm our ongoing explorations whether they be research or artistic. Once this feedback loop is generated, our learning and skills accelerate, pushing our potential into unexpected avenues of expression. Ultimately, we not only gain a deeper respect for this animal's reality, but also a richer sense of purpose and satisfaction in our work. And these two benefits are certainly welcome in this convoluted and demanding art form!

So far then we’ve explored his evolutionary backstory and how his gut and teeth changed to accommodate his new habitat. In this installment then, we’ll focus on his eyes since they needed modification as well. So let’s get started!…

Eyes

To accommodate his new grazing and arrhythmic lifestyle (daily and nightly) on the open plains, the horse had to have new eyes, too, something he’d come to depend on heavily for survival. Indeed, a large portion of his cerebrum is dedicated to visual stimuli while one–third of the sensory input into his brain is believed to originate from his eyes, a testament to how much he depends on those peepers! 

The eyes sit inside the ocular orbits on either side of the skull, bulbously protruding farther out than our own. The last upper molar (in the back) lays below the eye. Equine eyes slightly angled inward at the front corner, slightly angled outward along the upper rim, and slightly inward along the lower rim, tipping them on three basic planes (which we'll get to in a later part). These planes can vary a bit among individuals or breeds, as well as how much the brows are pronounced—some are less defined and softer while some are more protruding and marked. A complex structure, there exist some basic eye structures we should know:
  • Palpebral fissure: The slit of the eye created by the meeting of the upper and lower lids, about 2” long (5cm).
  • The medial canthus (pl. canthi): Front corner of the eye, or the front of the palpebral fissure. “U” shaped to form a recess, and sometimes termed the “lacrimal lake” (or lacus lacrimalis). Tears mostly drain through the medial canthus, pass through the nasolacrimal ducts in the skull, through the ventral turbinate fold, and drain out of the nostrils near the border between the outer nostril skin and the inner mucus membrane of the nasal cavity. With close inspection, we can see the tiny hole of this duct under the upper wing of the “comma” cartilage, usually where the inner nostril “pink” begins. However, in the mule, this duct opening is present on the lateral portion of the nostril floor, or even the lateral wall of the nostril. (Note: Tears contain moisture and nutrients for the cornea as well as Vitamin A, enzymes, and growth factors essential for corneal and general eye health.)
  • The lateral canthus: Back corner of the eye: the back of the palpebral fissure. Rounded, with an open angle when the eye is opened, tears also drain here.
  • Extraocular muscles: Within the orbit are several muscles that attach to the sclera to move the eye within the socket, in all directions. These muscles are controlled by the cranial nerves which come directly from the brain. Interestingly, the horse can automatically retract the globe back into its socket if he’s triggered by pain, stress, fear, or disease such as tetanus, sometimes causing the third eyelid to cover the cornea. This reaction is induced by the retractor muscle connecting the back of the globe to the inner surface of the orbit. This is why a terrified horse, for example, has that curious flattened, sunken look to the eye ball.
  • The eyeball itself (bulbus oculi) sits inside the orbit connected by various muscles and suspended and protected by a pad of fat in the back; there’s no post–orbital cranial wall separating the back of the orb with the brain case and interior of the skull. (This is why a starving horse will appear hollow–eyed, or with age, the “Salt Cellar” will deepen and the eye will sink into the orbit a bit more, giving older or starving horses a distinctive sunken look about the eyes.) The equine orb isn’t round in shape like our eyes, but shaped like a large, oblong egg with a bulge for the cornea and associated structures, with the lower part of the globe slightly flattened. The average size of an equine eyeball is about 2” (5cm) in diameter and about 1.7” (4cm) long.  
  • Cornea: The clear bulge of fluid (aqueous humor) above the iris and pupil which protrudes in a more pronounced bulge beyond the curve of the sclera. The average thickness in the middle is about .40–.60” (1–1.5cm) and the outside aspect is about .30” (.76cm). It’s margin (the limbus corneæ, or just limbus) connects to the sclera in a shallow groove. However, the sclera overlaps the cornea more in the front aspect than behind, and more on the top and bottom rim than at the sides, which is why the “grey line” isn’t always symmetrical. It’s one of the most sensitive tissues in the horse’s body, with nerves concentrated on its outer layer. 
  • Ciliary body: Produces the aqueous humor.
  • Anterior chamber: The fluid–filled space between the cornea and the iris. (The posterior chamber is the space between the iris and the lens which isn’t seen.) The fluid is derived from blood and nourishes the cornea. 
  • Uvea: Comprised of the iris, ciliary body, and the choroid, the vascular uveal tract helps to produce the aqueous humor, helps it drain from the eye, is involved with the immune response, and nourishes the eye itself. This tissue is delicate and easily damaged; inflammation of the uvea is termed uveitis, which is a serious condition in horses.
  • Choroid: Containing many capillaries and blood vessels, this is the primary blood supply to the retina. The triangular tapetum is also found in the dorsal portion which amplifies light in low light conditions.
  • Iridocorneal angle: A junction or angle made by the cornea, iris, and ciliary body that drains the aqueous humor from the eye to the blood. It pools there and then gets flushed out or absorbed.
  • Conjunctiva: Important to the eye’s immune system, this is a membrane that lines the inner eyelids, third eyelid, and the sclera. It produces tears, too, and protects the eye. It can be pigmented near the limbus.
  • Precorneal tear film: Produced by glands within the eye, this coating gives the eye an optically smooth surface and helps to nourish the eye structures. It drains to the corner of the eye through the nasolacrimal duct and out of the nose.
  • Vitreous chamber: The large, vitreous–filled chamber between the lens and the retina. 
  • Retina: The most complex structure in the eye, it has a ten–layered structure, converting light energy into chemical energy that generates the electrical signal the brain recognizes. Interestingly, as per unit of weight, the retina is the most metabolically active tissue in the body based on its oxygen consumption. The retina is also what contains the cones (photopic, or day vision) and rods (scotopic, or night vision) that discriminate color and light. The retina also contains a lot of large ganglion cells that conduct visual impulses quickly, which is why horses can detect movement so quickly and adeptly. 
  • Optic nerve: Comprised of the retina’s nerve endings, the optic nerve of the horse is unique due to a large proportion of large diameter axons (Brooks, 2002), indicating a strong sensitivity to motion detection and sensitivity in dim light. These fibers converge into a trunk to emerge from behind the orb. It passes through the fat pad behind the orb and within the Retractor bulbi to pass directly to the optic foramen. The optic nerve is about 1” long (2.5cm). After this, it crosses the nerve from the other eyeball. The nerve is sheathed in the membranes of the brain.
  • Optic disk (also called the optic papilla or optic nerve head): The equine’s optic disk has ganglion cell nerve fibers. 
  • Nictitans membrane (or third eyelid, palpebra tertia, nictitans, or nicitans membrane, or sometimes abbreviated as “TE”): Constructed of a semilunar fold of conjunctiva (the same delicate membrane that lines the lids), it’s a triangle mass of soft tissue, with a T–shaped shield of hyaline cartilage embedded within, that’s located at the medial canthus at the front of the eye. It can be completely darkly pigmented, partially pigmented, or unpigmented pink, and it contains a gland that produces tears (the nictitans gland). When the eye is open, this third eyelid is retracted to manifest as a sheet of skin at the medial canthus. However, when the lids blink, it sweeps across the orb in a rapid, almost horizontal motion across the eye’s surface, removing debris from the eyeball and distributing more tears. It’s activated by the muscles that close the eye lids which act on the fat into which the deep part of the cartilage lays. The TE is unique to the horse and only a few other animals. 
  • Lacrimal caruncle (sometimes referred to as the lacrimal caruncula, caruncula lacrimalis, or spelled caruncula lachrymalis): A rounded knob of flesh about the size of a small pea in the anterior corner of the eye that drains the excess fluids from the eye. It also has specialized skin cells that produce sebaceous secretions. It can be darkly pigmented, partly pigmented (“mottled”) or unpigmented pink. In other words, it’s that little bulb at the corner of the front of the eye sometimes referred to as the “tear duct.”
  • Lacrimal punctum: Two lacrimal puncta exist in the medial (inside) portion of each eyelid. Together they collect tears produced by the lacrimal glands which is conveyed through the lacrimal caruncle to the lacrimal sac and then through the nasolacrimal duct of the nostril.
  • Upper and lower lids: Protecting the eye with reactionary closure, the lids shut fast and firmly. The lids are thin and vascular, and serve important functions: they protect the eye, help to distribute tears over the orb, keep the cornea from drying out, help to control the amount of light pouring into the pupil, and help to move tears into the lacrimal puncta. They’re divided into four basic layers—the skin, the eyelid muscles, the fibrous tarsal plate (or tarsus) and the innermost palpebral conjunctival layer. The upper lid is convex and larger with a straighter curve while the lower lid is convex and usually has a deeper curve (though sometimes straighter in certain individuals or breeds), forming a biconvex opening for the orb. Depending on the depth of its curve, the eye can look rounder and bigger such as with the Arabian, more almond–shaped such as with the Andalusian, “snake–eyed” with the Teke, or smaller with the comparatively larger head of the Shire. The upper lid is more mobile than the lower lid which is relatively stationary. The upper lid also has long stiff eyelashes which cross each other like a lattice while the lower lid has only a few eyelashes. 
  • Eyelashes: Being sensitive, they can trigger a blink reflex to protect the eye. 
  • Whiskers: Surrounding the eye are a few long whiskers used as feelers for eye protection. They’re more profuse around the lower rim. And don’t forget the moles from which they erupt! 
  • Sclera: Comprising approximately 75% of the globe, this is the white portion of the eye that comes into view when the horse rotates his eye, or “eye white,” and is often used for expression by artists. It’s a dense membrane made of interlacing bundles of white fibrous tissue (mostly collagen), but it also has a few elastic fibers knitted in. This membrane encases most of the orb, being thickest in the rear to thin along the sides to thicken again at its junction with the cornea. It’s usually white, but may have a bluish tinge in particularly thin areas, or may be heavily pigmented brown surrounding the iris. It has a rich blood supply, often with capillaries showing, and laced with fine blood vessels, most notably the circular venous plexus (also called the plexus venosus scleræ, formerly called the Schlemm’s canal), which sits near the border between the sclera and the cornea. The transition between the opaque sclera and the transparent cornea creates a transition zone like a shallow groove along the border (rima cornealis or sulcus scleræ), into which the cornea is seated like a jewel in a setting. It’s this transition between the clear cornea and the white sclera that creates that “grey line,” or limbus, encircling the iris, which is most prominent at the medial and lateral sides. (Note: This grey line essentially represents the insertion of the pectinate ligaments that connect the sclera and cornea together.) Furthermore, the conjuctiva of the orb, which begins in the limbus, is pigmented around the limbus in some horses, creating that blotchy, mottled, or ruffled pigmented border between the cornea and the sclera. All muscles responsible for moving the eyeball within the socket attach to the sclera.         
  • Iris: The colored tissue of the eye that’s visible through the transparent cornea surrounding the pupil which constricts or dilates according to light conditions. It’s not smooth and flat, but has small folds, ruffles, and furrows that run from the pupil like spokes on a bicycle tire, and radially, like ripples on water. Some of these folds are permanent and some are temporary, caused by the constriction of the iris. It’s separated into a pupillary zone and a peripheral ciliary zone, which can be seen on the iris as an irregular circular line surrounding the pupil (the collarette), which is created by the slight overlapping of these two areas. The pupillary zone usually is a darker color and lined by a pigmented frill, an extension of the posterior pigmented epithelium. The iris in horses is usually colored various shades of brown (sometimes with a metallic sheen), but blue, amber, golden, hazel, white, greenish, and mottled colors can also occur (and, again, sometimes with a metallic sheen). It should also be noted that the iris is slightly oval–shaped, not round, creating a distinct curve of white sclera when the eye is rotated forwards, backwards, upwards, or downwards. Also, the iris cannot move independently of the sclera since everything moves together as one unit.
  • Lens: Inside the eye, it’s a biconvex, transparent structure behind the iris and suspended by the cilliary muscles within the orb. (We can see the lens behind the pupil.) It has tiny muscles to change its shape to alter focusing abilities at different distances. It’s pigmented yellow to limit the transfer of very short, high–energy wavelengths to protect the retina. 
  • Pupil: The void in the iris through which light passes to hit the lens and retina. It appears clear, dark, or “mirrory” in normal light due to the light–reflecting iridescent tapetum lucidum behind the retina. It’s an elongated, horizontal oval when contracted and a rounder oval when dilated. Therefore, it shouldn’t resemble a human or dog eye. As a general guide, the equine pupil is set on a horizontal plane in alignment with the canthi in the resting position. However, deviations from this alignment occur when the orb rotates when the head is raised or lowered as the eye works to focus on an object, keeping the angle of the pupil relatively level with the ground. What’s more, the position of the pupil indicates the eye’s rotation which must move in accordance with the entire globe, so don’t forget the sclera; the pupil itself cannot move or rotate within the iris independently. The pupils of foals are sometimes rounder than the more oval pupils of adults. 
  • Nigra bodies (or corpora nigra, corpora negra, granula iridis, or granula iridica): Normal in horses, these small dark folds or bundles of tissue are a unique feature of the equine eye, and while most abundant on the upper rim of the pupil, they can also be present to a lesser extend on the bottom rim. They’re believed to be a sunshade or visor for the pupil, guarding the ventral portion of the retina from excessive overhead sunlight while grazing. 
Being so important for survival, the horse’s eye enlarged to become one of the largest orbs in the animal kingdom, and developed a peculiar curvature along its outer surface. However, all equines have a similarly–sized globe, more or less, so it’s more the breed differences in the shape of the lids, the set of the orbit, the size of the head, and the peculiarities of the surrounding features that make an eye appear larger, rounder, smaller, “snake–eyed,” "toad eyed," etc. So, for example, an Arabian doesn’t have a bigger globe than a Clydesdale, only a different way in which the skull and flesh encase it. The Clydesdale also has a much bigger skull than the Arabian and very different lids, altering the look and relative size of the eyes even more. So it’s these factors that make the Arabian’s eye look larger, rounder, and "buggier," than that of the Clydesdale, which looks comparatively smaller, more almond–shaped, and more sunken. 

As for little Hyracotherium, he had eye sockets oriented more flatly and in the middle of his head, more like a bunny, and may have had little to no lateral vision. For the grassland lifestyle, however, the orientation and structure of his eyes changed, probably in response to his dependence on early predator detection. That’s because a predator’s best bet for bringing down a horse is ambush, and chances of a successful attack significantly increase within 50 yards (46m) or less. In response, the horse developed a sensory system keenly attuned to early detection to provide an essential head start, which is why a horse is quick to spook and ask questions later. That’s because once a horse starts running, it’s unlikely a predator big enough to bring him down can catch him. 

Of course all this visual keenness only works if the image is clear and in focus since a blurred image conveys much less or less accurate information to the brain. To keep the image focused properly then, the incoming light needs to be focused on the retina and the lens accomplishes this, sitting immediately behind the iris at the front of the eye. A lens with a fixed shape would focus objects at different depths depending on how far away they are. For example, objects that are further away focus closer to the lens. This is because all the lens does is bend (refract) the light by a certain amount. So the angle at which the light rays hit the lens determines the angle at which they leave the other side of it. Light beams from a distant object will be travelling nearly parallel to each other while those from a nearer object will be more divergent. In turn, the image of an object is focused when all the light rays coming through the lens from the object meet at a single point, referred as the “point of focus.” When this point of focus lands squarely on the retina, the object is in focus. And this is a pickle.

As a solution, there are two ways nature gets around this problem of having objects from different distances focusing at different depths within the eye. The first is an irregularly shaped eye ball so that light from a distant object falls on a closer part of the retina than light from a nearer object—bringing us back to that peculiar curvature along the eye's outer surface mentioned previously. With the eye in the correct position then both distant and close objects can be in focus at the same time. At present, it’s believed that this is primarily how the eye of a horse works. And the thing is, a horse spends most of his life with his head down eating so he doesn’t need to change his view of the world much. Plus this design is useful for seeing predators on the horizon at the same time as the grass at his feet, seeing far away and up close in full focus simultaneously. Humans however use a different method—nature’s second solution—which is to have a flexible lens with activating muscles. By changing the shape of the lens, we change the angle of refraction of the light passing through the lens and thus move the point of focus closer to or further away from the lens. This means we can bring distant or close objects into sharp focus at the twitch of a muscle, but it does sacrifice the ability to have objects at different distances in the same sharp focus. Being so, this system is more useful for animals that need highly accurate images of different parts of the world such as predators. That said, however, horses can still adjust the shape of their lenses though not to the extent we can. So in these ways, the equine exploits both of nature's solutions, again confirming how important sight is for this animal.

Over the millennia, evolution also produced equine eyes located on the sides of the head that protrude outwards with oval pupils, resulting in an almost 350˚ field of vision with only a narrow blind spot immediately in front of and below his nose and a few feet behind his tail (yet these areas come into view with a slight shift of his head). This arrangement provides a wide field of monocular vision estimated at about 175˚ vertically and 215˚ laterally on either side. "Monocular vision" means that the horse sees two different images, one from each eye, simultaneously. In other words, each eye works independently and sends its signal to separate sides of the brain. So what the horse sees in his left eye is different that what he sees in his right eye, and he sees both sides simultaneously. What this also means is that each eye (or side of the brain) must process the same information independently. So while a horse may first be spooky about a flapping ribbon on his left side to then calmly accept it, he again may be spooky about that same ribbon on the right. Nonetheless, this isn't because his brain doesn't transfer information from one eye to the other eye. In fact, recent research has found that visual information is indeed transferred between the eyes. Instead, it's hypothesized that what startles him in this circumstance is that he may not always realize it's the same ribbon when viewed from another angle. Regardless, this kind of monocular vision is handy for early detection of predators, which are most likely to attack from the side, or in the case of pack hunters, to attack from multiple sides. This is because a horse can almost see his tail with both eyes independently, which is why it’s nearly impossible to sneak up on a horse. That said, however, when we consider the front blind spot, the abilities of jumping horses seem all the more incredible, don't they? The horse loses sight of the obstacle when he’s a few feet away and has to rely totally on the rider, his calculations, and memory to tell him when to jump! Indeed, considering the biology of his vision, the trust this animal is capable of is humbling.

As for binocular vision, these two monocular fields of vision overlap in the front to produce about 60˚–71˚ binocular vision, or depth perception within a triangular space in front of his face. This is a far smaller field than ours, and one that may even require head motion to bring specific objects into binnocular focus. At distances less than 3–4’ (91–122cm), he has a limited ability to focus, and may even lose binocular vision altogether. So when an object comes within the 3–4’ binocular boundary, he may be forced to move his head to focus on it. That said, however, a mere shift of his head brings these areas clearly into view. When using his binocular vision then, a horse will raise his head and stare straight at an object “through the nose,” or if below his nose, he’ll drop or tuck his nose so his eyes can be pointed right at the object. We can often illicit this response by pulling a treat out of our back pocket and watch him visually investigate the tasty tidbit. We also see this head–up staring effect when horses approach a jump as they prepare for take–off. Curiously, too, the width of his head may play a role in altering the amount of binocular vision, though research is inconclusive. More still, while we're able to pick up details at a standstill, we'd find it hard to detect danger while we're moving, especially when running. However, this kind of detection is critical for the equine, which is why he tends to keep his head fairly still as compared to the rest of his body. Also consider all the information and detail coming into the equine's brain from both eyes, all at once. Therefore, his brain prioritizes what's being processed. This is why if a horse's attention is intently focused on something, we can inadvertently spook him if we "pop" into his field of vision.

Equine eyes also have a wide range of motion, helping to amplify vision and add expression. Specifically, the pupil and sclera indicate the position of the eye in the most obvious way. So we can see that the eyes can move together forward or backward (sclera simultaneously at the back or the front of the iris, respectively), or upwards or downwards (sclera simultaneously under or above the iris). In particular, the independent brain processing may have lead to another adaptation in equine eye movement. Now the horse can’t move his eyes as independently as a chameleon under normal conditions, of course, but each eye does have greater mobility in this regard than ours since each eye must process its own independent information. Specifically, equine eyes can also move to some degree like a “cat clock,” or side–to–side motion to increase the visual options when pinpointing potential dangers. They can also move in opposing up–and–down motion in similar fashion, often seen when the horse shakes his head.

Yet this does bring up the question of how well does the horse judge the speed of oncoming objects from the side. While it’s believed horses can use monocular depth cues for judging distance, monocular vision, especially with a predator attacking from the side or rear, might inhibit his ability to accurately judge just how much time he has to escape. That means his flight response has to be at full throttle every time, so when confronted with an oncoming spooky object, he tends to just take off. If he has the chance though, he’ll turn his head (or wheel around completely) to use his binocular vision. We can see this behavior, for example, when a horse turns to face us after we induced him to cavort away (like with a rock–filled plastic milk jug).  

Nevertheless, a horse cannot use his monocular and binocular vision simultaneously—he has to switch from one to the other. It’s thought he does this by altering his head position to refocus his eyes. For instance, he may hold his head in a neutral position when using monocular vision, but then hold it high to switch to binocular vision to investigate something far away. If a horse’s head is fixed down when he’s attempting to visually evaluate something then, by tack or by the rider’s hand, he may begin to adopt problematic behavior. Give him a proper chance to focus on the object in question then and he’ll likely calm down.

All said, other changes in the equine eye occurred to facilitate improved vision. The lens changed shape as did the location of the receptive ganglia on the retina, both allowing for sharper focus at long distances. Moreover, the equine eye developed a higher ratio of light–sensitive rods to color–identifying cones (9:1), compared to people (1:20). 

Rods connect to nerves in groups which reduces their resolution and sensitivity to color. However, they’re highly sensitive to light intensities and motion detection, making them useful for dim or no–light situations. In contrast, cones are individually connected to nerves to provide maximum visual resolution (or detail) and color detection. However, while they’re sensitive to color, they’re a poor means to see at night. This is because when light is removed, an object’s molecules no longer reflect their color–specific spectrum, making objects appear comparatively grey. This is why in low light conditions, or “moonlight intensities,” colorful objects appear grey to us.

Therefore nocturnal animals typically have a high percentage of rods enabling them to see in the dark, but making color vision less likely for them. Yet in humans and horses, the back of the lens, in the middle, is covered in cone cells, indicating that horses may be able to see a broad spectrum of color, too. Disseminating from this patch is a perimeter of mixed cones and rods and further out still, outside the edge of the retina, the rods become the dominant cell with no cones present. This means objects in the horse’s periphery vision have the least resolution, but are subject to the most motion detection that triggers a flight response.

As if that wasn’t enough, equines also developed a light–reflecting tapetum lucidum behind the retina to reflect light photons back onto the retina for a second chance to be read. It’s that iridescent layer of cells that cause “eye shine” or that “mirrory” effect in the pupil. What's more the equine lens is yellowish in color to filter out the shorter blue light wavelengths to diminish glare, giving the animal great clarity in bright light.

Overall then, while the horse does have cone cells, the percentage of his rod cells is greater than ours, indicating that he probably has better night vision, perhaps also implying he’s partly nocturnal, further confirmed by the presence of eye shine. It makes sense—most predators are more active at dusk or night. And thanks to the greater ratio of rods, horses are particularly sensitive to motion detection, especially in their peripheral vision. This is why fluttering or rustling objects often spook them—they simply perceive motion better—more "loudly"—than we can. So while we tend to interpret objects as shapes and colors, horses respond to movement, as appropriate for a prey animal subjected to ambush. Pair this with his uncanny memory, and we have an animal acutely adept at identifying alterations to his immediate environment, quickly and accurately. Remember, new objects might be a crouching, stalking predator! This is why a horse may spook at an unfamiliar new object—he’s not being stupid—he simply interprets it differently than we do. For this reason, too, sometimes changing a horse’s familiar environment may upset him more than simply putting him in a new one altogether. It’s also important to remember that those horses who were most reactionary were the ones who survived to reproduce. In this light, a horse that seems “wound too tight” actually makes sense from an equine point of view!

Nonetheless, all this means the rods are responsible for the good night and low–light vision horses have (about 50% better than us) while cones provide daytime vision and limited color vision. It’s also thought this cone–rod ratio allows horses better vision on cloudy days compared to sunny days. 

Despite all this, however, these structures can make it harder for a horse to adjust quickly to rapid light changes. Moving from a dark barn into bright sunlight, for instance, or from bright sunlight into a dark trailer, a dark barn, or into dark shadow can pose visual challenges to equines. In fact, it’s thought that equine eyes take longer to adjust between light and dark than other animals. Quite literally then, the inside of a trailer or darkened barn may first appear as a black hole to a horse! Yet when his eyes acclimate, his vision in low light is far superior to ours. This may also explain why sometimes a horse may hesitate before entering a dark trailer or barn on a brightly lit sunny day.

Regardless, it’s thought that horses have a visual acuity 0.6 that of people. (“Visual acuity” refers to the ability to see detail.) This means that if a person had a Snellen acuity of 20/20, a horse would have 20/33 vision. In other words, an object appearing 33 feet away to us looks only 20 feet away to a horse. This compares favorably with dogs (being 1.5 times more, dogs having 20/50 vision) and it’s a whopping three times more than cats (or 20/100). In other words, horses have a better field of vision designed to pinpoint movement at a distance whereas we have a narrower field of vision to see greater detail at a distance. However, as a result, it’s believed he cannot see very small objects like lettering in a book or individual beads on a necklace, no matter how closely they’re held to his eye. This may be why horses get into trouble with barbed wire or exposed nails—they simply cannot see the wire, the barbs, or nail heads well enough. Interestingly, too, it’s believed that many domestic horses are near–sighted (about one–third of the population) while wild horses appear to be mostly far–sighted (Giffin et al, 1998). Yet with diminished depth perception and a tendency towards far–sightedness, such horses can develop a horizontal astigmatism in which horizontal lines are more blurry than vertical lines. This may be why some horses run through fences—they just can’t discern the horizontal fence rails well enough. 

Now in the past, scientists thought horses had a retina that was “ramped,” not flat like ours. The concept of a “ramped retina,” first proposed in 1930, claimed that the horse must move his head up or down to focus on objects—high up for close–up objects and low for far–away objects. That’s to say the horse has built–in bifocal lenses that require him to move his head to exploit the different focal properties. This hypothesis has been refuted, however (Sivak, 1975). Instead, the horse was recently found to have a “visual streak” in the central retina in which the cone cells exist at a higher concentration. Therefore, the horse has better acuity when an object falls within this field but poorer on the peripheral retina. Subsequently, he’ll alter his head position to bring an object within that retinal area for sharper focus. Ironically then, this concept comes to the same conclusion as the idea of the “ramped retina”—that the horse must move his head to bring objects of interest into sharper focus—but only through a different mechanism. This may be why fixing the horse’s head down can cause behavioral resistance, tension, or problematic responses. 

As for color vision, it’s currently believed equines have dichromatic vision similar to a person with a red–green color blindness. Specifically, it appears equine color sensitivity shows a primary peak at 550 nm (greenish yellow) with a secondary peak at 439 nm (blue), meaning that horses probably see the world in shades of yellow, blue, grey, and green, but probably not so much in red, orange, purples, and violets (Brooks, 2002). Instead, dark red appears grey, bright red appears yellow–greenish, and greens are turned into a muddied yellow. 

Furthermore, because the brain must compare the signals from three cone types in trichromats (such as people), the relative signal strength from each cone is lessened (referred to as the “signal–to–noise ratio”). In comparison, dichromat horses may not have this signal–diffusing effect of the additional cone type so the color signals to the brain are stronger. This translates into better color discrimination in dim lighting (Roth et al, 2008). This may be why at low–light levels, a horse’s sensitivity to certain colors changes and amplifies. Specifically, as light dims, our ability to see color diminishes, eventually washing out into a series of greys. In contrast, the horse is able to pick out color at much lower light levels. One study even found that horses became more sensitive to greens and yellows in the middle range of light (such as at dusk or dawn) (Meszoly, 2003). It makes sense though—in the horse’s native grassland habitat, this kind of vision would allow a predator to be quickly identified amidst tall grasses. However, one study suggested that horses may have a cone sensitivity comparable to people if the reflective quality if the tapetum lucidum is taken into account (Roth et al, 2008).

Altogether then, equine vision may most likely be a combined product of both the workings of the lens and head position. It also appears the equine eye is more designed for movement detection, and for light detection at night and moderately adapted for color vision at night, things ideal for his evolutionary–induced survival on the open plains. Nonetheless, the horse still has one of the largest spectrums of color detection in dusk and dawn intensities (Roth et al, 2008). But in all fairness, the exact properties of equine vision are still being discovered and continually debated. Perhaps new technologies will shed new light on how the horse perceives his world.

Nonetheless, eyes truly are the “windows of the soul,” and, of course, the horse’s eye is supremely expressive, revealing his attention focus and mood changes, especially through his brows and eyelids. In this way, the sculpted eye helps to duplicate the soul of a living animal so necessary for that special anima. So we need to pay special attention to the eyes as we sculpt them in order to capture that enticing expression, character, and living quality so essential for our clay while maintaining technical accuracy consistent to the equine.

Conclusion To Part 3

Eyes are a lot more than meets the, well…um…them! In fact, we've only brushed the surface here! They truly are a complicated, beautiful, essential feature for the species which also provides us with endless options for expression. For this reason, understanding their “biologic” helps to guide our creative decisions with informed facts, and that increases our options and promotes the credibility of our work. And that's important! The eyes are probably one of the most important features of our sculpture we need to get right since we're such a visually–based species. Indeed, we look quickly to the eyes for cues and connection, don't we?

Understanding the properties of the equine eye also helps to demystify much of his behavior. So many things he does can seem bewildering without understanding, leaving some to wonder what the horse is thinking. Some even may believe it's because he's inherently stupid. Yet once we come to fathom the biological and evolutionary context of those senses do we come to more fully understand him, and that both humbles us and amplifies our appreciation of him, two prerequisites for improving our work in meaningful ways. It's also the first step towards "going to where he is," of starting to perceive the world from his point of view. When we can do that as artists, we can more faithfully portray his reality in clay—more purely convey his unique experience—by stripping away our own imposed preconceptions that can actually be rooted in misconception, or at least not tell the whole story.

In Part 4 then, we’ll continue our exploration with the nostrils and sinus, two more essential components to the animal’s biology. So until next…here’s looking at cha, kid!

“Once an artist gets it in his mind that it’s a blooming adventure, then, and only then, everything falls into place and starts to work.” ~ Joe Joseph P. Blodgett

Share/Bookmark
Related Posts with Thumbnails