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

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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

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Tuesday, June 20, 2017

Heads Above The Rest!; Exploring The Science and Art Of The Equine Head for Sculpting: Part 2


Introduction 

Welcome back to this 20–part series exploring the equine head. In Part 1 we introduced the basic evolutionary basis for its structure and now we’ll dive into structural details to better flesh out our understanding. The equine head is truly a marvel of bioengineering and to understand it is to literally understand what it means to be a horse. Fantastically, everything about the horse’s biology is embodied in his head, making its accurate portrayal in sculpture all the more critical for authenticity. That's to say, to sculpt equine realism doesn't mean to simply sculpt what we see—it means to sculpt our subject's biology in the full breadth of what that means. We're literally sculpting his evolutionary history as expressed by his unique physique. If we don't understand the hows and whys of that evolutionary history then, we'll not only miss out on a huge chunk of what it means to be this animal, but we'll risk making errors due to oversight or misunderstanding. So let’s continue our journey of discovery, starting with his gut, the feature that helps to define this curious animal…

Digestion

As we learned in Part I, as Hyracotherium ventured out onto the newly forming grassy plains, his body had to undergo a series of specific adaptations. One of these key adaptations was in the viscera—and yes—this has everything to do with his head, so hang tight…

In order to eat grass, the animal needed a digestive system that could process grasses rather than forest vegetation. Yet this is no easy task! Grass is a comparatively poor food source that possesses cellulose, the complex sugar found in fibrous plants. Grasses cannot be broken down by mammals without the aid of gut bacteria to break up this cellulose into volatile fatty acids the animal’s body can process. Yet this is a time–consuming method of digestion that requires a chamber for this organic matter to be stored to allow the symbiotic bacteria to work their magic. 

There are two different ways this relationship has been expressed in ungulates. One is ruminant digestion (or pre–gastric fermentation) and the other is cecal digestion (or post–gastric fermentation). Artiodactyls (even–toed animals) evolved ruminant digestion first, developing four stomach chambers. The first two chambers, the rumen and reticulum are where bacterial fermentation occurs. Then the food material is regurgitated and chewed again, known as “chewing the cud.” Upon being swallowed for a second time, the food matter passes through a special opening into the last two chambers of the stomach, the omasum and abomasums, where further digestion takes place. Pound per pound, rumination is the most efficient means for extracting nutrients from grasses largely because food matter ferments for the longest period of time. It takes about 70–90 hours for food to pass through a cow, for example. 

In contrast, Perissodactyls (odd–toed animals) evolved cecal digestion, or digestion in the cecum (also called the “hind gut” or “water gut,” equivalent to the human appendix) which grew to enormous size to provide a fermentation chamber. An adult horse of average weight will have a cecum of about 3–4 feet long (91–122cm) and with a capacity of about 7–8 gallons (26–30l). In comparison, the stomach of an adult horse takes up only about 10% of the digestive track while the cecum (and colon) comprises about 60% (and the small intestine about 30%). In this scenario, food is chewed and swallowed into a comparatively small stomach in which the foodstuffs are turned into a slurry with digestive juices and then passed into the cecum (via the small intestine) for bacterial processing. From the cecum, the materials then travel to the colons for further digestion. Because cecal digestion is a more effective digestive system in animals under 11 pounds, it’s believed Perissodactyla may have adopted it while still small during the Paleocene. However, cecal digestion extracts about 30% less energy from the same food stuff as does ruminant digestion. Nonetheless, the benefit is that it takes far less time to digest the food matter—only about 48 hours.

Consequently, this difference allows equines to flourish in ecological niches where few other animals could survive, particularly ruminants. Biological data reveals that wild equines usually target the worst, lowest quality and highest fiber roughage they encounter. For example, ruminates may eat only the leaves, but equines will eat the stems left behind (as observed between Plains zebras and Gnus). In North America, feral horses and cattle may compete for the same forage, but horses utilize territories not exploited by cattle such as those far away from water sources (especially in winter) and at higher elevations. Likewise, feral horses in North America don’t compete with the Pronghorn, a native ungulate, since this animal eats mostly shrubs, herbs, and woody forage. In fact, many wild equines thrive in areas characterized by such poor quality vegetation that ruminants appear to avoid the area altogether! For instance, wild asses and the Takh thrive in regions other large grazers deliberately avoid. Further studies support this observation by showing that unless a certain level of fiber is provided by the habitat, a ruminant will starve and so will avoid that habitat. 

So, what’s the reason? Time! A ruminant’s system can only process a limited amount of food in a fixed period, which happens to be rather long. Conversely, an equine’s response to poor vegetation is to simply eat more since his form of digestion can process nutrients far faster. So while a ruminant digestion is more efficient per unit of energy per unit of time, a horse can extract far more energy from a grassy habitat than a ruminant. There’s a very important reason why horses love to eat! 

Moreover, unlike ruminants, the horse’s digestive system allows him to “eat on the go,” one moment munching lunch while the next avoiding becoming one. He was the original “fast food junkie”! This digestive design is also ideal for migration because he doesn’t need to be stationary to chew his cud. So while cecal digestion is considered comparatively archaic, it actually panned out as an evolutionary advantage for Equus.

However, all this extra time a horse spends grazing amounts to about 15–20 hours a day, a bit longer than the 8–10 hours a day a ruminant spends. All this “head–down time” creates a greater vulnerability to predation, which has some interesting implications for how his head developed, which we’ll get to a little bit later…anyway, for now… 

Teeth

Yet the equine required more than a new digestive system to deal with abrasive grasses—he had to have teeth that could chew it while also coping with the gritty soil it grew in, and all without being milled down to nubs in short order. Unlike fruits, shoots, sprouts, and leaves, which either depend on being eaten to propagate or can suffer loss of some material without dying, the blades of grass are the plant itself—and plants enduring heavy herbivore depletion usually evolve various defensive strategies. With grasses, this involved the infusion of sturdy silica particles (or phytoliths) into the cell walls, essentially locking the nutrients in a silica skeleton. Similar to glass powder and working like sandpaper, it’s these silica particles that lend shape and stiffness to a blade of grass, even when dead. So to pulverize this silica skeleton to extract the nutrients—to the necessary consistency of fine cornmeal—a horse has to do a lot of chewing, something which would rapidly wear down his teeth due to those abrasive silica particles.

In response, the horse lost his short, gently–cusped, bunodont teeth for a design that could withstand this constant abrasive onslaught. As such, the bunodont dentin cusps became elongated into long prongs sheathed in hard enamel which then formed into slicing ridges. Hard cementum (once vestigial in the old bunodont teeth) began to encapsulate the entire tooth and fill in any spaces, particularly between the dentin prongs. Eventually, the equine developed continually erupting (hypselodont), long–crowned teeth (hysodont teeth) with long ridges made of blended dentin cusps (lophodont teeth) coated in tough enamel and encapsulated with hard cementum. As a result, the equine head grew in size and length, stretching forward below the eyes and deepening through the jaw to accommodate the new batteries of long, large grinders and long, big incisors along with the powerful chewing muscles needed to activate them. Yet this also placed the roots of the teeth incredibly close to the floor of the sinus cavities, leaving paltry little room for deviations caused by breed type such as the crushed nasal bones of “extreme–headed” Arabians. Indeed, such heads are often typified by tooth roots actually penetrating the sinus floor, causing pain, inflammation, behavior problems, and bleeding. (However, the opposite end of the spectrum, the straight, convex, or sub–convex head, usually doesn’t have the same problems with tooth rooting or breathing when it comes to their head shape and cranial axis.)

Ultimately then, the biological result was continuously erupting teeth of about 40–42 for males and 36–40 for mares. Specifically:
  • 12 incisors, 6 on the top and 6 on the bottom. 
  • 12 premolars, 3 on either jaw, top and bottom, part of the battery of grinding cheek teeth.
  • 12 molars, 3 on either jaw, top and bottom, part of the battery of grinding cheek teeth.
  • 4 canines (also called  “tushes," "fighting teeth," or “tusks.”): 2 on the upper jaw and 2 on the lower jaw, erupting between the ages of 3.5–5 years always in stallions, but rarely in mares, fewer than 28%, and usually with only one or two tushes. They erupt in the diastema, and those on the top jaw are set further back than those of the bottom jaw.
  • Wolf teeth: An occasional small, peg–like first premolar can be present which is thought to be gradually disappearing as the horse evolves, being considered functionless. They aren’t canines as the name would imply, but vestigial premolars of Hyracotherium. Fossils show that Hyracotherium had 3 incisors, 1 canine, 4 premolars, and 3 molars on each side of the jaw. Through evolution, the second, third, and fourth premolars and the 3 molars became the long–crowned, big grinders we know today. However, the first premolar, the wolf tooth, became smaller and cone–shaped, sitting just in front of the second premolar. It’s been reported that about 40–50% of modern horses are born with up to 4 wolf teeth, most commonly only 1 or 2, which may or may not be fully erupted. Wolf teeth can be found in both stallions and mares, and usually found only on the top jaw, commonly being absent or very small in the lower jaw. They can erupt as early as birth to 6 months, but usually by 1 year. They have shallow roots and may be dislodged naturally. While wolf teeth may generally not be a problem for the horse, they’re often removed if present, being thought to interfere with the bit and so causing discomfort and behavior problems.
  • Adult females normally have the same complement of teeth as the males minus the canine teeth.
The incisors are used for cropping grass and the molars grind it up in preparation for the digestive process. The incisors are more curved and column–like than the molars which are more rectangular and straight (the tushes are more pointed and curved than the molars). Equine teeth aren’t pearly white, but stained with brown, yellow, orange, rust, and grey due to thicker enamel and green stains from the grasses they consume. 

The teeth are also often used to determine the age of the animal with the distinct stages of growth they undergo. For example, when a foal is born, he’s generally toothless (though sometimes he’ll be born with four erupted incisors), but within the first week he’ll develop four incisors—two on top and two on the bottom of the front of the jaw, the central incisors. The second set of incisors—the intermediate incisors—erupt within the first few weeks. The final set of incisors—the corner incisors—erupt at about 6 months. Plus a few weeks after birth, the foal will also erupt 3 cheek teeth (or premolars) on each side, top and bottom, meaning six uppers and six lowers. 

These temporary teeth are often called “deciduous teeth,” “caps,” “baby teeth,” or “milk teeth,” and are smoother, shorter, smaller, and whiter than permanent teeth. The incisors of milk teeth are short and square–like in shape, and also have a “neck” or “step down” at the gum line which permanent teeth lack. The corners on a 1–year–old’s incisors don’t touch either, distinct from a 2–year–old whose incisor corners do touch. The baby teeth have short roots and are designed to be popped out by the erupting permanent adult teeth.  

All this means that in the first few weeks of life, a foal will erupt 16 baby teeth. Then by about 8–9 months, he’ll have a full set of 24 milk teeth—6 upper and 6 lower incisors and 6 upper and 6 lower premolars. At about this age, too, he’ll also erupt the first permanent molar set (2 on top and 2 on the bottom), erupting behind the baby premolars. By the time he’s a yearling then, he’ll erupt 24 to 30 teeth!

Then it’s during 2–3.5 years of age that largest turnover of deciduous to permanent teeth occurs. Specifically, he’ll shed 2 sets of milk incisors and 2 sets of milk premolars, all replaced by permanent teeth. Then by 3.5 years, he’ll erupt his second set of adult molars and the third set starts to erupt. That means he’ll erupt up to 24 permanent teeth in 1.5 years. Indeed, there’s a lot going on inside a horse’s mouth between 6 months and 5 years, with a full complement of baby teeth being replaced by permanent teeth plus 12 additional molars. Then at 5–6 years, most stallions and geldings erupt 4 canine teeth. That means at about 6 years, a horse will have his full set of permanent adult teeth. (Incidentally, teeth most commonly shed in the Fall.) 

So because baby teeth are shed in the order they arrive, their shedding can also help determine a young horse’s age. For example, the central upper and lower incisors are the first to go at about 2.5 years; the intermediates are shed at about 3.5 years, and the corner incisors at about 4.5 years. The rate of shedding the premolars is about the same—the first set is shed at about 2.5 years, the second set at 3 years, and the final premolars at about 4 years.

Even so, because teeth are constantly growing and being worn down, the pattern of enamel wear and the shape of the tooth itself can also be used to gauge a horse’s age. For example, after five years a horse’s age can be estimated by these tooth features:
  • Wear on the incisor cups: A “cup” is a dark pit in the center of the tooth surrounded by enamel. It tends to disappear in the central incisors at about 5.5 years, the laterals at 6.5 years, and the corners at 7.5 years. While the cups tend to disappear first on the lower incisors, they're deeper in the upper teeth and exhibit more variation in their waning wear schedules. The vestige of the enamel surrounding the lost cup is called a “mark.” 
  • Dental star: A secondary deposit starts to appear in the central incisors at 8–10 years, in the laterals at about 9–11 years and in the corners at 10–12 years. Stars are more flush than marks in topography and so aren’t as palpable on the tooth surface.
  • Shape of the incisors: The incisors change shape as the horse ages as wear takes its toll. When the horse is young (up to about 7 years), the teeth are relatively oval. They become more triangular as they erupt (about 9–13 years) to become rounder in horses older than 13 years.
  • Slope of the incisors: When seen from the side, a young horse’s teeth are comparatively upright, about 160˚–180˚, making his bite appear more blunt. As his teeth erupt, however, they become more sloped, decreasing to almost 90˚ in a very old horse, making them appear longer and thus the origin of the adage “long in the tooth.”
  • Hook: At about 7 years, a hook develops on the upper corner incisor to eventually be worn away by 9 years. In some horses, the hook reappears at about 13–14 years to disappear again. 
  • Galvayne’s groove: Stained dark brown, a groove gradually appears at the gum line on the upper corner incisors at about 10 years, running the entire length of the tooth at 20 years to then disappear at 30 years. However, not all horses have a Galvayne’s groove.
Nonetheless, determining a horse’s age can be a bit subjective since lifestyle can alter these effects. For instance, horses raised in sandy areas usually have more wear than those raised in lush environments. Horses with bad habits like cribbing and chewing can wear their incisors down rather quickly, too. 

All tooth development takes place in the maxilla and mandible, and they don’t really grow, but continuously erupt from the gums, so it’s a myth that horse teeth continuously grow for perpetuity since they do have a terminal length after which point no new tooth is produced. The working tooth surface is referred to as the “working crown” or “clinical crown,” with the remainder referred to as “reserve crown.” Of course then, this means the tooth we see as palpable in the skull isn’t the whole tooth. Because the teeth erupt progressively as more tooth is needed, their bulk is embedded in the skull to grow out and be worn down as the years pass. Indeed, the total length of the longest grinder molar in a 6–year–old horse (of about 15–17 hands) is about 7” (18cm), with only about 3/4 of its total length as root. Likewise, the total length of the longest incisor tooth is about 5.25” (13cm), with again, only about 3/4 of its length as root. Surprisingly, even the total length of a tush of that same horse, on average, is a surprising 5” (12.7cm), of which up to 2” (5cm) is root. 

And how they erupt is rather interesting—it’s triggered by a tooth trying to “find” its paired mate above it. For example, the second molar on the top left side will grow until it “finds” its mate on the bottom left side. Yet the rate at which equine teeth erupt is based on individual genetics and metabolism which can be influenced by management and environmental conditions. Nevertheless, the basic rate of tooth eruption is about 1/3 inch (.84cm) a year. Given that the horse has about 6” (15cm) of molar crown, this means the tooth will be functional for about 20 years.

What’s also particularly important for artists to notice is how tooth eruption over time affects a horse’s head shape. If we study closely, we’ll see that a horse’s head often starts out more wedge–shaped because of all the long, new teeth, often with hefty bars. “Tooth bumps” (also called “eruption cysts”) can occur in horses 2–5 years old, manifesting as lumps along the jaw bars. They’re caused by the pressure of the permanent molars (especially premolars 3 and 4), which are still under the gum line, trying to push the baby teeth out. However, as the teeth are worn down with age, the head usually becomes progressively more rectangular, becoming shallower through the bars as they thin and “collapse” around the shortening tooth roots. This is one of the reasons why the heads of senior citizens look quite different, being a bit more rectangular than the heads of younger horses.

The equine tooth itself is typified by a layering of materials of differing hardness which results in different rates of wear. Enamel is hardest and so wears the slowest, creating sharp ridges as the cementum and dentin are worn away faster. As the horse chews then, his teeth wear like self–sharpening knives, always maintaining the necessary edge to pulverize grasses effectively. And the more wear that happens, the sharper the horse’s teeth become. That’s important because parts of the silica skeleton not smashed won’t digest since even gut bacteria cannot penetrate the silica shell. Altogether, this layered structure can withstand the abrasive silica particles to efficiently turn grasses into the necessary consistency of fine, milled cornmeal to increase digestive efficiency and protect against intestinal blockage. 

For these reasons, it’s no small statement to say a horse is only as healthy as his teeth. Indeed, his lifespan is essentially decided by his teeth—how long can he grind his food into the critical consistency of fine cornmeal? In this way, his teeth are important not only for efficient digestion, but for his very life! The fine milled–cornmeal texture produced by his teeth is necessary to avoid lethal blockages in his digestive system since it’s vulnerable to obstruction through its various bottlenecks. For example, colic is a typical outcome of an intestinal blockage. In particular, any grass bits longer than 3/8–1/2” (especially 1/2–1”) can be lethal to a horse by causing such an obstruction. The horse doesn’t chew his cud nor can he vomit so whatever he swallows has a one–way trip. In this way, equine teeth not only make food digestible, but also safe to eat. Quite a big responsibility for such unassuming anatomy!

Interestingly, when he becomes incapable of pulverizing grass into the texture of fine cornmeal, he’ll begin to starve which is one of the sad reasons why seniors can become so thin and frail—it’s not because of their age per se, it’s because of their worn–down, spent teeth. In response, old horses often “quid,” or chew a wad of grass to extract the juices to then spit out the fibrous wad. They know the danger of improperly ground foodstuffs that could cause deadly blockages in their digestive tract. However, owners can intervene by providing fine or “pre-chewed” mashes to extend the life of their senior equine friends.

Being so, perhaps one of the most improved areas of equine health care involves dental procedures. Research has shown that when teeth are properly cared for, the horse can masticate his food better and, as a result, make safer and better use of the nutrients being ingested—and that translates into a longer, happier life. 

Yet his teeth also had to change orientation in his skull as well. In particular, they had to form into front incisors for nipping off grass when the head was in the grazing position and then large grinders in the back of his jaw to smash it. This telescoped the skull from below the eyes while also creating a long diastema, the space between the front incisors and the back molars (where the bit sits). For example, about 40 million years ago, the head of Mesohippus began to elongate, having a battery of grinding teeth increased to six (as his premolars adapted into grinders) on either side of his jaw, top and bottom—the same number in living horses today. 

Over time then, the head of Equus would become longer and larger, pulling the teeth out from under his eye orbits and deepening his jaws to make room for these new, bigger, high–crowned teeth accompanied by the large, powerful chewing muscles required to activate them. Also a benefit of this new, long head was the placement of his eyes relatively “higher” in relation to ground level, allowing him to scan the grass line for predators when in the head–down grazing position. In other words, his lips would be at ground level for nipping off grasses while his eyes were higher up to watch for threats.

Furthermore, there’s no joint at the mouth, around the chin. Instead, the mechanics of the skull dictate that the jaw opens at the joint between the mandible and the zygomatics, behind the eye, dropping the entire lower jaw when the mouth is opened. The biomechanics of his mandible joint also changed away from for and aft motion to instead facilitate side–to–side rotary or triangular motion, or shearing action, something still distinctive today. And because the horse’s lower jaw is narrower than the upper jaw, his upper teeth are offset and overhang the lower teeth, or in other words, the lower teeth are positioned to the inside of the upper teeth. 

His lips also adapted to grazing, which in the modern horse have an almost prehensile quality and are unusually sensitive and adept at selecting choice bits of food. Indeed, his lips are used for exploration of new objects and, in a way, can be likened to his “hand.” This design was produced by a shortening of his nasal bones to permit a fleshier and more flexible muzzle advantageous for selective grazing. Fortuitously, the shortened nasal bones also allowed his nostrils to enlarge and become fleshier, enabling greater intakes of air for sustained galloping.

Conclusion To Part 2

It’s an interesting evolutionary story, isn’t it? And we're just starting! As equine artists, we can get caught up in the structure of muscles and bones and—sure—we can get by with the whats. But if we neglect the whys for those muscles and bones, we'll have missed what it means to be equine in the first place. And it's that evolutionary backdrop which gifts us with the perspective needed to better acknowledge his full narrative, authentically and responsibly. Truly, there's a big difference between knowing structure and understanding it. To be an insightful equine artist then means we first need to be equine biologists. Every feature of his physiology has a story to tell, one that can enrich our depictions in untold ways. So to overlook his evolutionary biology is to miss the world from his perspective, yet isn't this one of the overarching stories we want to tell? If all we do is focus on the whats of anatomy, won't we have missed something far more important? Equine anatomy is much more than skin deep! Far bigger than mere structure! Ultimately, we're really telling the story of what it means to be equine, in the full depth of what that means. To consider this dimension then is to provide an opportunity for our grasp of structure to grow in sophistication and scope, and in ways that lend substance to our portrayals. As such, our work evolves beyond prosaic representation to become something that transcends this failing of realism to embody something more—equineness itself. In doing so, our work comes to speak for this animal in ways otherwise impossible, and in ways that not only imbue that elusive élan vital, but also offer an ode to viability often missed by those who lack such context.

So—no—we’re not done yet! There’s a lot more to explore as his body changed to adapt to his new niche, a whole host of modifications that speak to his biological truth. In Part 3 then, we’ll continue our discussion with his eyes, an important feature for the species, in more ways than one. So until next time…chew on all this tasty information!

“The trick is to keep exploring and not bail out, even when we find out that something is not what we thought.” ~ Pema Chodron

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