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!…
- 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.
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.
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!
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!