Bio-logic: Equine Anatomy 101

Introduction

Deciphering the equine's structure and mechanics is no easy task. The animal is complicated and nuanced in design resulting in an interdependent network of mechanisms that stabilize and move him along. What's more, science is unraveling more about equine mechanics every day, adding to the complex mix and throwing in a medley of surprises to boot. 

Yet having a firm grasp of equine structure is fundamental to equine realism; otherwise we risk errors in the technical accuracy that underlies our work. For this reason, let's explore some of the basics of equine anatomy and biomechanics to help clarify the major systems of the animal. It's by no means comprehensive and there are a multitude of other features than influence his build and movement, but these ideas provide a good springboard for further study.

So let's go!…

Head

As a source for such rich expression and breed type, we naturally gravitate towards the horse’s head. But what’s most important for an artist to understand are the evolutionary mechanisms that created that animal's head in the first place. First and foremost then, the head is an essential component for life. Now that may seem ridiculously obvious, but consider how wrapped up artists get with rhetoric or stylizations regarding "breediness" of the head. So it’s a good idea to remember that the head is the singular source for:

• Thought
• Personality
• Air
• Water
• Food
• Sight
• Hearing
• Taste
• Smell
• Balance
• Vocalizations

The head is also the source for some elements of body language and acts as an important counter–balance to the body as well. So in a nutshell, the head is the primary pathway for essential biological functions that allow the animal to live. Plus, also consider that the equine head is purely functional neither carrying ornamentation to attract a mate nor appendages for battle or display such as horns, antlers, tusks, etc. It’s built for an animal meant for speed on the open plains—economical in design and no–nonsense. So it bears to reason that an artist should be adequately versed in equine evolution if for nothing more than to keep the biological importance of the equine head close to heart. 

As for its structure, the skull is comprised of two parts: 

• The upper jaw and cranium
• The lower jaw or mandible. 

The upper jaw is slightly wider than the lower jaw, and the back of the jaw aligns with the back of the zygomatic arches, in front of the ear. There’s only one joint in the head, for the jaw, which is located behind the eye, resulting in the entire jaw dropping when the joint is articulated; the mouth doesn't open like a trapdoor at the chin. The jaw can open and close with a limited amount of lateral play, and he chews in a circular motion, not up and down. And when he chews, the cavity behind the eye pops in and out (as do the side cheek muscles). That's the coronoid process of the mandible popping in and out of the temporal fossa synched with the chewing motion. 

The skull is mostly subcutaneous on the dorsal aspect and mostly subcutaneous muscle on the ventral part. For this reason, we need to sculpt the bony parts convincingly as bone and the fleshy bits as flesh; otherwise our head won't be believable. For instance, the hourglass–shaped nasal bone should appear hard while the cheek's buccinators should appear fleshy. In contrast, the ears, nostrils, and lower nasal portion are made of cartilage. The horse has approximately 36-40 teeth; six incisors on both top and bottom, three premolars on either jaw, top and bottom and three molars on either jaw, top and bottom. Erupting between the ages of 3.5–5 years, both genders have a two pairs of “tushes” (or canines), but these rarely erupt in mares. The tushes on the top jaw are set back farther than the tushes on the bottom jaw.

Generally speaking, the alignment of the bulb of the ear, under the eye and under the lower rim of the nostril form a straight line; the corners of the eye are angled at an approximate 40˚–44˚ angle to this alignment (drafters tend to have the more acute angle). Also, parallel to this ear–eye–nostril alignment is the angle of the mouth and the approximate angle of the teardrop bone of the jowls. However, the ear–eye–nostril alignment is only a general guide. In many Iberian or Draft breeds, the head is arched or sub–convex, in which the entire nasal portion of the skull drops downwards, giving the type a distinctive “ram head” appearance. In contrast, Arabians and some pony breeds have a dished head in which the nasal portion is lifted upwards. In contrast, stock breeds and Thoroughbreds tend to have straighter heads. Each horse’s head is unique and distinctive, rich with idiosyncrasis, but this alignment is a good template to get a bearing for sculpture. 

As for the eyes, they are truly the “windows of the soul," and, of course, the horse’s eye is supremely expressive, revealing his focus of attention and changes in his mood, especially through the brows and eyelids. The eye also helps us to duplicate the soul of a living animal so necessary for that special anima. It sits inside the ocular orbit on either side of the skull. The eyes are set laterally on the sides of the skull and protrude outward. As a result, he has nearly a total of 350˚ field of vision with only a narrow blindspot immediately in front of and below his nose, and few feet behind his tail, requiring him to move his head to see these blindspots. But in all fairness, the exact properties of equine vision are still largely unknown and continually debated. Perhaps new technologies will shed fresh light on how a horse sees his world. 

The equine eye is the largest globe of any land mammal. However, an important point to understand is that all horses have a similarly sized globe. It’s the breed differences in the shape of the lids, the set of the orbit, and peculiarities in other surrounding fleshy features that make an eye appear larger, rounder, almond–shaped, smaller, “toad eyed," etc. In other words, an Arabian doesn’t have a bigger globe than a Clydesdale, only a different way in which the skull and flesh encase it. The Arabian's skull is also much smaller than that of a Clydesdale, making his eye appear larger in comparison. In addition, the equine eye isn’t shaped round like a human eye, but oval or oblong, like a large egg, with the lower part of the globe slightly flattened. 

A complex structure, much of it beyond the scope of this discussion, but artistically speaking, there’re some specific eye structures of importance such as the following: 

• Nictitans membrane (third eyelid): A triangle mass of soft tissue with a T–shaped shield of cartilage embedded within it. When the lids blink, it sweeps across the orb, removing debris from the eyeball and distributing tears. This structure is unique to the horse and only a few other animals. 
• Lacrimal caruncle: A small dark pad in the anterior corner of the eye that drains excess fluids from the eye.
• Upper and Lower Lids: Protecting the eye with reactionary closure, these lids shut fast and firmly. The upper lid has as straighter curve while the lower lid has a deeper curve.
• Eyelashes: Being sensitive, they trigger a blink reflex to protect the eye.
• Whiskers: Surrounding the eye are a few long whiskers used as feelers for eye protection.
• Medial Canthus: Front corner of the eye.
• Lateral Canthus: Back corner of the eye.
• Cornea and Aqueous Chamber: Forms the round shape of the eye ball.
• Sclera: Comprising approximately 75% of the globe, this is the white portion seen when the horse rotates his eye. All muscles responsible for moving the eyeball within the socket attach to the sclera. It has a blood supply and sometimes a mottled pigmentation around the iris.
• Iris: The colored tissue surrounding the pupil that constricts or dilates it to accommodate light intensities. 
• Lens and Pupil: The “void” for light to penetrate. It appears clear, dark, or “mirrory” in normal light. The lens has tiny muscles to change its shape to alter focusing abilities at different distances.
• Nigra Bodies (or corpora nigra or granula iridica): Normal in horses, these small dark folds or bundles of tissue lay on the iris. They’re usually found on the upper part of the pupil, but sometimes on the lower part as well. They’re believed to be sunshades for the eye, guarding the lower portion of the retina from overhead sunlight while grazing.
• Extraocular muscles: Within the orbit are several muscles which attach to the sclera that move the eye within the socket, in all directions. These muscles are controlled by the cranial nerves, which come directly from the brain.

A wad of fat lies behind the eye (which shrinks in older horses to deepen the “salt cellar”) because there's no post–orbital wall behind the equine eye. Indeed if the fat was removed, we'd see the back of the eyeball. 

The equine pupil is an elongated oval when contracted and a softer oval when more dilated, but not round; it shouldn’t resemble a our eye, or a dog's eye. The pupil indicates eye rotation in concert with the sclera since it moves in accordance with the entire globe; the pupil cannot move or rotate within the iris independently. 

The eyelids help to protect the eye and sweep debris from the cornea. The upper eyelid tends to do most of the motion while the lower eyelid remains relatively stationary. The upper lid has long stiff eyelashes which cross each other like a trellis while the lower lid has only a few.

Equine eyes have a wide range of motion, helping to add expression to his face and amplify vision, and the sclera often indicates the position of the eye in the most obvious way. The eyes can move together forward or backward (sclera to the back of the iris or the front of the iris respectively), upwards or downwards (sclera under the iris or above the iris) or both can rotate around, especially when the head is turning or something is oscillating in from of him (one eye looking forward and the other backwards, like a cat clock). An interesting point to consider is that the horse can automatically retract the globe back into its socket if triggered by pain (or disease such as tetanus), stress, or fear, 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.

Now for the ears. They can be tricky to sculpt owing to their delicacy and complicated curved structure. That said, however, they’re an important point of equine expression and communication with a plethora of nuanced movements, so perfecting their qualities in sculpture is an important skill to learn. Indeed, we can deduce a lot about a sculptor's ability by the quality of their sculpted ears, and a lot about a painter's skill by how they paint them, too.

The ear's structure is as follows:

• Auricle or Pinea: Made of cartilage and delicately curved. The three cartilages associated with the pinea are the conchal cartilage, annual cartilage, and the scutiform cartilage. The conchal cartilage is trumpet shaped and responsible for the visible shape of the ear. The upper portion is thinner and flexible while the lower portion is thick and strong, serving as attachment for most of the ear muscles. The conchal cartilage covers the annual cartilage with a prolongation which is covered by muscles and the parotid gland. The annular cartilage is a small circle encasing the auditory process of the skull and helps to connect the conchal cartilage to the bone. The scuitform cartilage is a small plate on the temporal muscle and acts as a fixed location for muscle attachments of the ear.
• Middle ear: Contains the ear drum which transfers sound waves into the inner ear. 
• Inner ear: Fluid filled, it translates sound waves into impulses the brain can process. It’s also important for maintaining balance.
• Hair: Inside the pinea, the hair is fuzzy and long, protecting the inner structures (unless clipped, revealing the delicate ridges inside the ears).

Seated behind the zygomatic arches and the back of the mandible, and in front of the occipital crest and on the crown, the pinea aligns with the ear–eye–nostril line. This alignment of the ear really doesn’t vary, unlike the ear–eye–nostril alignment, because the seat of the ear is an anatomical feature rather than a conformational characteristic. Furthermore, note the delicate folds and the “V” formed at the base of the pinea, where the two rims meet. Also note how this “V” changes as the ear is rotated. Additionally, the medial rim is slightly rounded while the lateral rim is flatter. 

Only their cartilaginous attachment to the skull’s ear canal and the ear muscles themselves lash the ear onto the head so, surprisingly, the pinea floats on top of the underlying muscles like a hockey puck on ice. This lets the ear have a fluid range of motion, and can even be slightly drawn up or down. They're delicately fluted, mobile, and often busy, moving independently of each other. 

The ears are also an obvious tool for communication between horses, and are a good indicator of his emotional state, providing clues as to what he’s thinking or feeling, lending a tremendous amount of expression and realism to a piece if created skillfully. It’s also believed that where he directs his ears is an indication of what his eyes are focusing on. So to increase the authenticity of sculpted ears, these following additional details can be powerful tools:

• Wrinkles typical lay where the pinea meets the skull when rotation folds the skin. 
• Veins on the pinea, up the back of the flute and around the ear bulb. Veined ears really lend a sense of thin skin and fine hair or to imply athletic effort by reinforcing the visual of a pounding heart. 
• Fuzzy, unclipped ears can be a wonderful touch for drafters, ponies, feral horses, or for those horses depicted in a natural state. 
• Minor “faults” such as Lop Ears can add a bit of character to a piece meant to be eccentric. 
• Damaged ears such as nicks, cuts, missing tips, etc. can imply a narrative history which might prove effective for sculptures of feral horses, ranch horses, or rough stock. 
• Ear tags might also be accurate for certain pieces. 
• Twitchy ears can add a lot of life and character to a sculpture, imparting genuine equine character and behavior.

Ear size and shape are also a function of breed, gender, and even age. For example, some pony breeds require ears to be a certain length such as the Shetland whose ear should be no more than five inches long. The actual shape of the ears can be a point of breed type, too, such as the curly ears on a Kathwari or Marwari, or the delicately curved ears on an Arabian. Also, mares usually have longer ears than stallions while a foal’s ears are often proportionally larger than those of an adult. However, sometimes ears may appear to be placed higher on the crown on some breeds such as the ASB, Kathwari, Marwari, and Akhal–Teke, but this is more an illusion due to the often more narrow structure of the skull in this area with such breeds. 

Now for the nostrils. They can be a bit fiddly to sculpt, can't they? That's because of their mobility and fleshy nature. Yet it’s important to get them correct because they're a lovely visual line at the end of the muzzle that instill a sense of living vitality.

The nasal septum, the cartilage of the nasal cavity, provides the initial framework for the nostrils. It projects forward from the end of the nasal bone to attach above the incisors. Anatomically then, the nostrils fill in the large cavity on either side of the nasal bone, channeling air directly into the sinuses through the length of the head and into the lungs. The nostrils—and in particular the sinuses—serve to warm or cool, and moisten incoming air along with filtering out pollutants before it reaches the lungs.

The shape of the nostril is achieved by two rims encircling the nasal cavity. The anterior section is cartilage (the Alar cartilage, or sometimes called comma–cartilage) and is comma–shaped with a thick, curved, rounded upper head and a thinner, swooping lower portion ending in a tail bulb. The alar cartilage is easily palpated while its bulbous tailhead can often be seen as a slight bulge immediately behind the lower portion of the posterior rim with which it connects. Viewed from the front, the Alar cartilages form an “x” owing to their connected back–to–back portions that attach to the nasal septum and the lateral cartilage of the nasal bone, with which they articulate. 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.

As for the posterior rim, it's comprised of gooshy flesh and so is more swooping, elastic, and uniform in shape. It connects at the top of the Alar cartilage, where it forms the upper fold or “v," and the tail of the Alar cartilage, where it forms a noticeable depression at rest. However, during exertion, the nostril may be so flared that this depression is stretched to flatten out and widen.

A thin canal from the eye’s lacrimal gland descends under the mucus membranes of the nasal cavity (nasolacrimal duct) to a small opening in the nostril. In this way, excess fluid of from the eye is drained through the nostril. 

So altogether, the equine nostril has no bones or boney connection to the skull, being only cartilage and flesh attached loosely by fibrous connections. Consequently, the nostrils are very mobile and flexible, capable of many shapes and sizes to accommodate mood or circumstance. For example, flaring into a rather large oval shape during physical exertion or to a pinched shape to snort and blow. The shape and orientation of the nostrils can also be a function of breed type. For example, Oriental breeds tend to have a more horizontally placed nostril while stock breeds may have a more vertically oriented one.

Also, where the two rims meet at the top, or "v," is relatively fixed while the rest of the nostril can be independently moved and shaped by the facial muscles connected to it. However, nostril motion is also synchronized with muzzle motions, particularly the mouth and upper lip. That said, the nostrils and mouth can be moved independently on both sides, creating some interesting expressions. Along those lines, horses don’t use their nostrils just to breath, but to communicate (puffs, snorting and blowing), scent the air (watch how they delicately quiver and dilate), convey his emotion (note how they morph in sympathy to what the horse is feeling), and even to clear the nasal passages (with that all–too–familiar snot blow). So it’s really important to pay attention to them when sculpting. 

The nostril (or nare) itself has two compartments, the true nostril and the false nostril (or nasal diverticulum), a unique fleshy feature. The true nostril leads directly into the nasal cavity of the sinus whereas the false nostril is a pouch above it, running from the lateral dorsal aspect of the posterior rim and forming a dead–end at the junction between the premaxillary and the nasal bone. Several muscles activate the false nostril, which can therefore capture air or contort or bulge in interesting ways during certain airflow conditions or during communications such as snorting. 

During exertion, 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. Specifically, 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 is his breathing automatically becomes. And this is no easy feat! Indeed, during major exertion (like galloping), the wind streaming through the horse’s nasal passages rushes in at the astonishing speed of 400 mph. Now consider this…no wind that fast exists on the planet as current wind records are only 280 mph in Antartica and 318 mph in an F5 tornado. Furthermore, the intake of air is substantial. For example, a Thoroughbred at a full gallop requires 636–681 gallons of air per minute. That's a lot going on through the humble nostril! The horse truly is a marvel of bioengineering, isn't he?

Now as for the muzzle, it's blunt with a boxy, nearly prehensile lip. The skin of the lips is wrinkled, owning to their elasticity, and the skin is soft, warm, and velvety. There are more nerve endings in the skin of the muzzle than in the human finger, allowing him to explore and sort through things with great accuracy. Long whiskers protrude from the muzzle, growing from "whisker bumps" that serve as feelers. His lips are very expressive, indicating his emotional state. They may also twitch or the lower lip may droop or bob up and down if he's relaxed or dozing off. Also notice how the muzzle can show pleasure over a scratched itch, snarl when annoyed, become "pooky" when excited, droop pendulously when relaxed, or become tense when angry or stressed. Similarly, his chin can relax or tense up, lending more expression. Studying how a horse uses his muzzle to communicate his mood can reveal a lot of applications for sculpture.

He uses his lips to grasp and gather food, grinding it with his teeth and softening it with saliva. He also "mouths" objects to manipulate or explore them. Then when he drinks, he forms an "O" with his lips and sucks up water as through a straw, taking in gulps about one–half pint per swallow. The tongue acts like a suction pump to draw water in and the digastric sling works the hyoid apparatus for the "glug glug" motion that directs water into the pharynx and down the esophagus.

The muzzle can be a rather complicated area to sculpt owning to its subtle curves, texture, and flexibility. Things to note are the structural relationships between the lips. The anterior portion of the upper lip is often narrower than that of the lower lip, which tends to be broader. Likewise, at the corners of the mouth, the upper lip usually protrudes further out while that of the lower lip dip inwards. In short, the upper and lower lips are usually oriented in contrast to each other. Nonetheless, sometimes the upper and lower lips are of equal protrusion at the corners, depending on the individual nature of the horse's head. The one thing to really capture, however, is its fleshy, elastic, and soft quality in both sculpture and paintwork. 

The face also has a “Y” vein erupting from the end of the teardrop bone and nerves running from the front of the jaw to the back of the jaw. The "Y" vein is almost always seen though the nerves may or may not be, depending on circumstance. 

Lesson: The equine head is a study of economy. Every bit is there for a biological reason and so has very little fudge–factor for aesthetics or technical errors. Also, the head has only one joint, behind the eye, so the entire jaw must drop when the mouth is opened. 

Neck

The equine neck has seven cervical vertebrae forming an S–shaped curve deep inside the neck. It begins at the poll and terminates at the first thoracic vertebra, approximately mid-shoulder; it doesn't lie under the crest. This gives it an amoebic quality as the S–curve stretches to lengthen, compresses to tuck, and changes the shape of the neck depending on motion. 

The first two cervical vertebrae are of specific interest because their unique structure dictates equine neck articulation. The Atlas (first cervical vertebra) connects the head to the neck and is the shortest and broadest of the series (about the width of the brows). It's quickly identified by its prominent subcutaneous “wings” seen behind the ears. Its joint with the skull can only permit a “yes” motion. However, when the head is tucked, a little bit of side slippage is possible, referred to as "twirling of the head" in horsemanship. On the other hand, the Axis (second cervical vertebra), the longest vertebrae of the neck, articulates with the Atlas in a rotational screwdriver twisting "no" motion. Cervical vertebrae three through seven have a much greater range of motion and are the origin of much of the lateral neck motions we see. Working together, these mechanisms produce the neck movements so wonderful for sculpture. 

The neck also has an indentation before and after the wither; too many flawed necks have the indention only after the wither, especially when the neck is lowered.

Lesson: The horse cannot rotate or laterally turn his head at the poll (the Cranial-Atlas joint). That means  the “wings” of the Atlas must always be parallel to the back of the skull (use the back of the ears, jaw, and eye orbits as reference points). The only exception is when his head is tucked to allow that teensy bit of slide slippage. However, the horse also cannot extend or flex his neck at the Atlas-Axis joint, which results in the lovely break over, arch, or "mitbah." Therefore, not until the junction of the Axis and third vertebrae does true lateral flexion and rotation occur.

Torso

The torso consists of approximately eighteen thoracic vertebrae (back), five to six lumbar vertebrae (loins), five to six sacral vertebrae (croup), generally eighteen coccygeal vertebrae (tail), and then we have the pelvis lashed onto the sacrum.

The equine barrel isn't shaped like a tube or box, but like a canoe with a keel and the wide, well–sprung portion towards the flanks and groin. In motion, the sternum moves in synch with the spine, connected to it through the ribcage.

Contrary to belief, the equine back isn't a pliable rod; it cannot flex freely in any direction. In reality, the torso is quite rigid due to evolutionary requirements too lengthy to discuss here. Subsequently (and grossly oversimplified), an equine spine is:

• Capable of only approximately two and a quarter inches of dorsiflexion.
• Has the greatest potential for lateral bend before the thirteenth thoracic vertebra.
• Has a lumbar region inflexible for lateral bending and limited rotational movement being designed, instead, for “coiling” to tuck the hindquarter.
• Has a fused sacrum capable only of coiling at the lumbo-sacral joint (LS Joint).
• Is capable of a limited degree of rotation, which occurs mostly in the thoracic vertebrae. 
• The rotations of the spine determine the gaits, posture, and motions.

In shorthand this means:

• Unlike depicted in many horsemanship manuals, the horse cannot laterally bend in a smooth curve. Instead, he bends before the thirteen thoracic vertebra and banks a turn by pushing his ribcage outside the turn. In conjunction, he bends his neck into the turn thereby completing the illusion of a smooth curve.
• The greatest articulation occurs at the LS Joint, which can only tuck to curl the hindquarter under the body.
• The barrel swings over the supporting hindleg in reaction to the physics of motion, referred to as "schwung" in German.
• All movement begins in the spine (not the legs).

As for the tailbone, it's much like the lower five vertebrae of the neck—highly flexible. It should be noted that the tail is a reflection of what the spine is doing, too.

Now one may wonder why the pelvis is included in the spine section. Good question. And it's because the pelvis is a fused girdle of bone lashed onto the spine at the sacroiliac joint by a tight webbing of ligaments. Functioning as a solid unit then, the pelvis moves in synch with the spine. Whatever the spine does, so must the pelvis, making the pelvis essentially an extension of the spine.

The reference points of the pelvis are the ichium (point of buttock), tuber sacrale (point of croup) and the tuber coxae (point of hip). Wider than the forequarter, the pelvis can neither internally articulate within its points nor articulate with the spine independently. Consequently, these reference points are always parallel and aligned to each other to form a perfect box always bisected by the spine, regardless of movement.

Lesson: Horses have relatively rigid backs that cannot move according to what many mistaken diagrams insist; it's more flexible than a cow's back but less flexible than a dog's back. Boiled down then, the action of the spine is the first aspect to consider when designing a sculpture, being the origin of all postures and movements. What's more, the pelvis is obliged to do what the spine is doing since it's lashed onto the sacrum. So if the spine is rotated, the pelvis is thusly rotated. If the spine is hollowed, the pelvis is leveled. If the spine is rounded, the pelvis is tucked under the body. The pelvic girdle is also solid bone, therefore, asymmetrical reference points means the pelvis is broken. Similarly, if the spine doesn't bisect the pelvic girdle, the pelvis has broken away from the spine.  

Fascia

The body is covered in fascia, it's dense with it. It encases all the muscles and it sheaths tendons, ligaments, and organs, weaving a dense network throughout the body. In many ways, fascia is the forgotten component in equine anatomy, it being removed to reveal the "more important" aspects of anatomy. Yet fascia is critical! It gives the hide its texture, muscles their shape, and it interconnections every system and every portion together. It's the communicative common bond that meshes the entire body together. It also plays a part in biomechanics, helping to move sections, interconnecting regions, and morphing muscles as they contract and relax. Indeed, without fascia, the whole body would just fall apart and regions wouldn't synch properly. 

Lesson: Fascia often won't be found in anatomical references since it's what's removed to reveal the underlying anatomy. But don't forget it's there! It's an important component to sculpture and for understanding equine anatomy. Truly, we can deduce much about a artist's knowledge base by how they treat fascia in their sculpture work.

Shoulder and Forelegs

The scapula (shoulder blade) is not attached to the torso by bone, but by a sling of muscles and flesh. As such, its motion is a sliding one, forwards and backwards, upwards and downwards, and is remarkably supple, fluid, and dynamic, and acts as an effective shock absorber. 

The scapulo–humeral joint (between the scapula and the humerus) has a goodly potential range of movement. However, a tight network of ligaments and muscles restrict the full potential of this joint. Nevertheless, it can slightly flex and extend, and it's the location for foreleg rotation, abduction, and adduction, which is why the chest compresses and expands during lateral movements.

The humeral–radial joint (between the humerus and radius) is a hinge joint capable only of flexion and extension along the sagittal plane. It cannot rotate or laterally bend. Acting like a lever, the ulna (elbow) is fused to the radius (forearm), and so articulates with the radius like an extension of it.

The carpus (knee) is a hinge joint. However, it's capable of some twisting in flexion, clearly seen when a horse is laying down. It's a dual–hinge joint, having a 2–tier construction, or two layers of carpal bones. However, the carpus can only articulate at the joints between the radius and the 1st layer and between the 1st and the 2nd layer. The 2nd layer is so tightly lashed with ligaments to the metacarpal (cannon) that it cannot articulate, making it an extension of the metacarpal. This is why the first bend of the knee has a pointier angle and the lower bend is rounded. What's more, the equine foreleg is angled in a slightly knock–kneed structure, much like how our femurs are oriented to our tibias. That's to say there shouldn't be a straight line from the radius down through the metacarpal, a common fault in sculpture and often misrepresented in conformation text. To have such an orientation is to have a bowlegged forequarter.

The three points of articulation in the foot are the fetlock, pastern, and the coffin joint (or pedal joint). The sesamoids, though firmly attached by ligaments to the 1st phalanx, articulate slightly with the cannon during flexion. All three are hinge joints but, the pastern (which is bell–shaped from the front), and especially the coffin joint, can produce some shift and rotation. This can be easily seen on reclining horses, hoof "wobbling" during extreme movement, or cutting. (This is yet another reason why having healthy hooves in which the coronet is at or below the coffin joint is so imperative.)

As for motion, the foreleg is dependent on the shoulder through of the pulley system created both by the stay apparatus and mechanical evolutionary demands. In short, all foreleg movement is dictated by the shoulder through this pulley system that runs down the front and back of the foreleg. Indeed, muscles don't exist distal to the knee having turned into tendons that act on the lower leg through the contractions of the muscles higher up on the leg (the same is true for the hind leg and the hock). Therefore, lower leg motion is governed by the muscles of the forearm which are governed by the humerus which is governed by the scapula (which is ultimately governed by the spine). So whatever the hoof is doing is a reflection of what the knee, elbow, scapulo–humeral joint, and shoulder are doing. Thus, foreleg articulation is a measured even motion similar to a drafting lamp. Only if weighted, fatigued, uncommonly stressed, or injured do variations occur.

When it comes to the cannons (both foreleg and hind leg), we have the metacarpal in the front and the portion behind the splint bones is just tendons and ligaments. That means those areas shouldn't appear "filled in" or fleshy, but the bone and those tendons and ligaments need to be crisp and clean with good "hollows" to demonstrate a lack of injury or pathology that would manifest as puffiness.

Lesson: Don't recreate forelegs that are out of synch with the shoulders, or a pulley system that's out of whack. And refrain from creating lateral, medial, or rotational motion at the humeral–radial joint because, remember, this motion occurs primarily at the scapulo–humeral joint. Additionally, don't sculpt broken elbows (absent or curved ulnas) or broken knees (a carpus articulating between the 2nd layer and the cannon). And avoid "spaghetti legs" as typified by undulating, twisted, or curved parts of the legs which should be, instead, straight columns of bone. And always remember all foreleg motion is dictated according to the pulley–system; it always functions as a symbiotic system and not independently. Remember to think of a drafting lamp.

Hind Legs

The hind leg is also governed by its own pulley–system subject to the same mechanical concepts as previously discussed, but even more so. Therefore the hind leg also acts as a symbiotic system, too, connected together and behaving like a drafting lamp; no portion articulates independently. Everything is synched in close relationships, with activation of the lower limb occurring high up on the leg.

The pelvic–femoral joint (between the femur and pelvis) is a ball and socket joint capable of a wide spectrum of motion. Hind leg motion originates at this joint (though ultimately the spine), making it essentially the shoulder equivalent of the hind end. 

The stifle is actually comprised of two hinge joints: the femorotibial joint (between the femur and the tibia) and the femoropatellar joint (between femur and patella). And again, mirroring the foreleg (in this case the humeral–radial joint ), both are hinge joints, incapable of lateral or rotational motion. A detail to notice is that the patella, a locking mechanism in the stay apparatus, articulates with the femur in the femoral groove by sliding up and down in synch with the tibia. Therefore, the patella slides down when the hind leg is flexed and up when the hind leg is extended. Another important point to notice is that the stifle must pop out around the rib cage when flexed and drawn forwards, causing the entire leg to rotate outwards and slant inwards towards the median at the pelvic–femoral joint. 

The tarsus (hock) shares two similarities with the carpus (knee). First, it consists of a 2–tier arrangement. Next, the 2nd layer is tightly bound to the metatarsal (hind cannon), making this layer a mechanical extension of the metatarsal. But unlike the carpus, the bones of the tarsus are so lashed together by ligaments they are, essentially, fused with little shift or play. As a result, the only point of articulation in the hock is between the astragalus (or talus) and the tibia, the tibio–tarsal joint. Two important details to note here is that articulation is on top of the hock and the calcaneum (the point of hock) leverages with the metatarsal like the ulna with the radius. This means the hock articulates at the top and the calcaneum remains in line with the metatarsal. This also means that when the hock is flexed, the point of hock isn't pointy, a common error in sculpture, but more rounded, in keeping with the tip of the calcaneum. The hock is also built with a spiral construction* via the articular surface of the astragalus. So during flexion, this design makes the metatarsal rotate inwards and on an inward slant. Pair this with the popping of the stifle around the ribcage in flexion and we have the curious angulation of the hind leg in the flexed position, which becomes more pronounced the greater the flexion. (*Depicts the right hock from the front.)

And like the forelegs, the hind legs have their own unique angle when standing. Rather than angled forward from stifle to toe as we so often erroneously see in conformation books as "correct straight legs," the hind leg from stifle to toe should actually be angled moderately outward, away from the median, on an even plane. We see an extreme expression of this in the "close–hocked" stance of Clydesdales (which is not the same as being sickle–hocked). Indeed, if the hind leg is straight forwards (as we often see on the real thing because they've mistakenly parroted incorrect conformation tenets), that's actually bow–leggedness which is why such horses move with a hock–popping motion in extension.

Lesson: Avoid creating a broken pulley–system or inaccurate articulation in the hind leg; remember the drafting lamp. Also be sure the sculpture's hind leg movement is originating at the femoral joint and not at the stifle which is a common fault. Likewise, don't create a joint with a broken hock (bent calcaneum). And don't create hind legs moving on a forward–facing axis, which is perhaps the most common biomechanical error in sculpture. Instead, the dynamics of the popped out stifle and rotating joint of the astragalus create hind leg dynamics that move on a series of curious angles, not on a straight plane.

Foot Movement

Foot motion, specifically the fetlock, pastern, and coffin joints, can be tricky because the physical forces of the world influence their angulations despite the pulley–systems. For example, a foot may snap or wobble under the forces of thrust and inertia, often seen in racing or jumping. Or a slacken floppy foot (primarily in the forefoot) can be influenced greatly by thrust and gravity. Fatigue also may alter foot motion. So study foot movement in relation to motion, physics, and circumstance to gain better understanding of these peculiarities. And referring to quality reference photos is always a wise practice.

Nevertheless, there're a few concepts to keep in mind:

• Most flexion tends to occur at the fetlock joint, though the pastern and coffin joints do play a significant role, especially when articulation is extreme. Nevertheless, situations are variable. For example, primary flexion at the pastern and coffin joints may occur in the hind leg when it's well under the body, bracing for support, often seen in haute cole in the levade, or in the sliding stop. 
• When a standing leg is extended backwards with the hoof still on the ground, the pulley–system straightens out the fetlock and pastern joints leaving the pedal joint to keep the foot flat on the ground. In other words, the fetlock doesn't rotate, but straightens out, leaving the pastern joint and coffin joint to keep the hoof oriented towards the ground. Bending the fetlock forwards to set a hoof flat on the ground is another common mistake in sculpture.
• The emotions of the horse can influence foot movement. For instance, a lazy or relaxed horse may exhibit slackened foot articulation whereas an excited animal may produce tight snappy action.

We should also be well–versed on what constitutes a healthy foot if we're to avoid creating pathological ones in our sculptures. Problem hooves are a chronic source of errors in realistic equine sculpture, so we need to be proactive in our education to avoid creating them. The hooves also tell a story about the individual horse, which may prove important for a narrative or backstory.

Lesson: We should pay close attention to all the foot joints since their nature greatly affects the overall impression of our piece. In particular, we can't forget the pastern and coffin foot joints when articulating our feet in sculpture. Also many foot articulations are a function of natural coordination, so to get them wrong implies an injury or pathology. 

Proportion

Although it may seem odd to include proportion as part of anatomy, it's definitely an anatomical consideration as well as an artistic one. Everything about the animal has a proportional relationship to another, so it could be said that capturing these relationships accurately is the gist of sculpting anatomy realistically. To that end, knowing how to use anatomical landmarks to measure proportion is a critical skill to learn. A set of locking calipers can truly be a best friend! Yet there are some specific aspects of proportion that need special mention, as follows:

• Symmetry: Horses are bilaterally symmetrical, meaning that one side mirrors the other side from nose to tail. While there’s natural variation with each animal, of course, paired body areas should match more or less despite posture or motion, and that means careful measuring with locking calipers. This is especially true for legs, the head, ears, the Atlas bone, pelvic girdle, and hooves. 
• Harmony: Proportion as a whole should be given due attention, meaning that the entire sculpture should harmonize together realistically without one portion being way out of proportion to another. For instance, we don't want to see a head that's too big, hooves that are too small, a neck that's too long, or a hindquarter that's too short. There should be an overall balance to the piece.
• Standards: Some aspects of proportion are also about conformation and breed type, so we need to pay attention to those as well. For example, an Arabian should have a proportionally longer mitbah than a Shire, or a Quarter Horse have proportionally bulkier muscles than a Friesian.

Lesson: Proportion is a key component to realism as anatomy, so it needs our due attention just as much. Our sculpture should be symmetrical, harmonize, and have those proportional components consistent to conformation requirements or breed points of type to be convincing.

Placement

This pertains to the anatomical orientation of each body feature. Characteristic of the Equus blueprint, the equine has body parts placed in very specific areas, in very specific ways. While this seems painfully obvious, it bears repeating when we get to complicated areas like the head and legs when the issue of placement comes to the fore. Here if one component is off, it can either make the area look odd, or throw off the placement of surrounding structures altogether. When this happens, it's often hard for us to identify exactly where things went wrong since, by this point, the error is systemic.

The thing is, placement can't be fudged. Because when we start, we run into problems like asymmetries and disproportionate aspects as we try to force things. But putting together an accurate horse is like carpentry; things have to fit exactly, like a properly done dovetail joint, or jigsaw puzzle. An eye has to go exactly where an eye is supposed to go, for example; otherwise the jaw, zygomatic arches, ears, teardrop bone, and brow will be misplaced, too. In turn, that will misplace the nostril, mouth, chin, and buccinators, and therefore effectively fudging up the whole head. Indeed, just one misplacement can create a cascade effect that necessitates starting over from scratch.

Lesson: Where anatomical features are placed is critical for a realistic result. We can't have the eye slightly off, for example, and hope to create an accurate face. Misplacements tend to be really obvious mistakes, too, something that cannot be strategically hidden. 

Planes

Planes refers to the angle, slant, curve, or twist of each body component in relation to another. Similarly, characteristic of the anatomical blueprint, the horse's body has specific planes to its muscle bundles and bony projections that, together, give the animal the shapes and curves we'd recognize as "a horse." In other words, distill a horse down to pure form, realistically done (not stylized), and that's our set of planes we have to work with.

And like placement, these planes have to be spot on. Just as much, if even one of these planes is off, it'll throw off the planing of another portion, and so the error compounds. Nonetheless, planes are perhaps the most fudged aspect of realistic equine sculpture, which is why so many works can appear odd or misshapen, especially in the broad expanses of muscle mass. Yet correct planes are fundamentally important to equine sculpture, forming one of the baselines of realism. Indeed, so much so that we can sculpt an equine sculpture almost entirely from planes and still produce a realistic result. The work of Herbert Haseltine comes to mind.

Lesson: Correct planes form the basic strokes of realism, and an error in them is a fundamental error of realism. Incorrect planes also compound, taking whole sections away from accuracy.

Mass

Gravity and physics have important anatomical consequences for how we depict our subject in sculpture. For example, if our sculpture depicts motion but has “standing” pasterns, the piece won't be as believable as it could have been. But remember, the physical forces manifest themselves in all parts of the body simultaneously, not just in discreet areas. For instance, consider a galloping sculpture with one foreleg planted on the ground. Not only will that fetlock be flexed, but that entire foreleg will be "jammed" upwards into the torso under the forces of impact and weight–bearing, causing the foreleg to be set momentarily deeper into the body as the angles of the upper foreleg are compressed. That is to say, weight–bearing isn't just a factor of the pasterns, but also of the entire limb and into the body. The same can be said of all the forces visited onto the body such as centrifugal force, impact, sliding, spinning, etc. 

Lesson: We must account for physics in our sculpture; otherwise we create a piece depicting a "reality vacuum." This applies not only to the articulations of the sculpture, the nature of the flesh and flow of the hair, but even to the expression of the animal as he reacts to the physical forces he's experiencing.

Foals

When we sculpt foals, we have a lot more to consider than just what meets the eye. They have their own unique qualities that need attention because we certainly cannot sculpt them like "mini–me" versions of adults. Indeed, when we start applying adult concepts to foals, we run into trouble, so to avoid that, there are two ideas we should keep in mind as we sculpt them: "underdeveloped" and "infant." Foals are a "work in progress," and we need to capture these growing qualities about them along with their delightful personalities if we hope to capture all that is "foal."

Lesson: We have to sculpt and paint foals on their own terms, according to their own unique characteristics, and cannot apply adult structure or coloration to them.

Additional Points

An important point to understand is that anatomy and conformation aren't synonymous, but very different subjects. Speaking of straight shoulders, upright pasterns, short hips, goose rumps, calf–knees, and ewe necks is not the same as talking about the abduction of the femur, dorsiflexion of the thoracic vertebrae, vastus attachment to the femoral trochanter, contraction of the extensor pedis, extension of the humerus, and the internal malleolus of the radius. Anatomy is the blueprint of the genus whereas conformation is the blueprint of the breed; one is made by nature and the latter by people. For instance, a Brabant, POA, Teke, and Saddlebred may all be different breeds (conformation) but they're all Equus (anatomy). Even further, that means that while their knees may look different superficially because of their breed type, for example, they're all built and articulate like an equine knee because they're equine.

Similarly, when we sculpt, we have to keep viability and functionality in mind. Viability entails accurate anatomical structure that recreates an actual horse (anatomy) whereas functionality refers to structures that maintain soundness (conformation). For this reason, viability is mandatory for equine realism, by definition, but functionality represents a set of ideals that preserve the well–being of the animal. When we speak of quality equine realism then, we mean a piece that's exemplary in both these components.

We should also recognize that the horse doesn't move like an animated anatomy chart, but is imbued with "living flesh" that morphs with posture and movement. For example, muscles slacken, bunch, flatten, hollow, and change shape when contracted or relaxed. Therefore, an anatomy chart should only be used as a deciphering key and as a guide, not as a literal translation. In addition, horses have goo! Their flesh and hide morphs in motion into wrinkles, squishes, and stretches, all of which need to be captured accurately to recreate "living flesh."  This paired with fascia means that a horse's surface texture isn't smooth and polished all over, but has ripples, bumps, squiggles, and various textures reflecting what's going on beneath the coat.

Above all, we shouldn't regard the horse as sections that function independently of one another. While a regional approach helps to clarify specific systems, we also have to express the whole. That's because the horse operates as a whole system, meaning that whatever affects one aspect will affect others. This is how we recreate the fluid, graceful, athletic motion so characteristic of the equine. We should always ask ourselves, “How does moving this bit affect these bits?” Look for the relationships and synchronicity.

Finally, the sculpture and paintwork need to be free of imperfections that would mar or distract from a believable piece. It's not enough "to have everything, and everything in its place"—it has to be well done, too! Without a doubt, folded into the concept of "technical accuracy" is quality workmanship only because without it, our poor workmanship results in errors of realism just the same.

Conclusion

The equine skeleton is a marvel of biological design. Yet it's not obvious. Indeed, its mechanical systems can be better translated with education and observation. Life study is important, of course, but if access to horses is difficult, many other sources for study exist. Movies and videos can be useful as are horse shows and expos. Photos can be found online, and in books, calendars, magazines, and promotional materials. Also, attending workshops, clinics, and seminars are a great way to deepen our understanding and perhaps apply some hands–on techniques. The point is that opportunities are plentiful so don't rely on "booksmarts" alone. We not only need an understanding of how anatomy works in principle, but also how it works practically speaking—how it functions in real life. Knowing where the two diverge and where they overlap is critical for creating convincing work.

Deepening our knowledge about anatomy is beneficial for the community overall, too. Indeed, it's not just the artist who benefits by developing an ability to produce more believable sculptures. Buyers benefit, too, since they can better make purchases most likely to succeed in competition. What's more, judges benefit by improving their ability to reward those pieces that are most consistent to the goals of realism. Working symbiotically then, we can elevate the art form and advance our understanding and appreciation for this wondrous animal.

So ask questions, stay curious, and don't take anything for granted. Sure—anatomy charts and text seem intimidating, but with practice and discipline, the mechanical workings of the equine skeleton will become more apparent. In turn, this will inspire more proactive education on the subject, and deepen our commitment to this beautiful and complex animal. And learning about his physique is to learn about his natural history, too, and that's a fascinating story that deeply informs our creative choices. Truly, to fully understand our subject we not only need to know how he's structured, but why. With this under our belt, we can begin to tell a fuller story of Equus and amplify the authority of our work. The equine is truly a product of his evolutionary habitat just as equine realism is a product of discipline. Marry the two together, and we have real magic!

So until next time...It's only bio–logical, Captain.

"A great accomplishment shouldn't be the end of the road, just the starting point for the next leap forward."

~ Harvey Mackay

Terms: 

Median Plane: The plane intersecting the horse if the animal were sliced down the middle, nose to tail

Sagittal Plane: Any plane parallel to the median plane

Abduction: Motion away from the median plane

Adduction: Motion toward the median plane

Dorsiflexion: Flexion upward

Lateral: To the side, away from the median plane

Medial: To the inside, towards the median plane

Internal: To the inside, towards the median plane