Tuesday, June 27, 2017

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


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!…


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

Related Posts with Thumbnails