Equine Anatomy and Biomechanics: A Primer of Equine Engineering for Advanced Students Part XIV, Details

Introduction

We've covered a lot of ground since Part I, haven't we? And all that's just the tip of the ice berg! Not only is there more information for the advanced student, but it's also a matter of application. It's not enough to know, we also have to do, right? And in that respect, a lot can go haywire. When we're dealing with the creation of fallible human hands, we can divert away from accuracy quite easily if we aren't careful. So translation is another matter entirely.

To that end, let's now discuss details, or those little touches we can add to our sculpture that infuse interest and believability. Details are neither easy to do nor should they be considered afterthoughts. Instead, they should be integrated into our sculpture with equal attention since they can make or break a piece. 

We should also understand that many details are fleshy or cornified and so their texture comes into play when we recreate them. We can't just approach them all with the same technique, but need to find those that reproduce how they would feel and look on the real animal. The flesh should appear gooshy and the cornified should appear rough or polished, depending on what they are. For instance, the difference between chestnuts and hoof horn, respectively.

So enough gab...let's go!

Veins

Veins are patterned and bilaterally symmetrical on either side of the horse, lengthwise. Smaller and finer capillaries obey certain patterns of their own, but are much more random and spontaneous. Both can be very prominent on thin-skinned horses, especially on hot days, or during exercise. Some such horses are the Arabian, Teke, and Thoroughbred.

Since arterial and vein structure is so complex, this section distills the veins into those most obvious on most horses. These primary subcutaneous veins are: 

• Facial vein (or submaxillary vein): The “Y” vein that comes from the front tip of the teardrop bone (facial crest) that splits and then branches towards the eye and nostril. It also goes under the jaw, to enter the jugular, and this portion is referred to as the Glosso facial vein; it can produce a subtle visible effect in the throatlatch area.
• Internal subcutaneous vein of the forearm: This major vein of the forelimb is a continuation of the internal metacarpal vein. It passes over the anterior top part of the subcutaneous shaft of the radius and travels to the back of the knee.
• Internal and External saphena vein: A major vein of the hindlimb, it erupts from the groin around the Gracilis and Sartiorus muscles and then branches. The larger anterior branch crosses over the top of the tibia and follows the groove between the front of the tibia and the Tibialis cranialis (Deep flexor metatarsi), over the front of the hock, crossing the top of the metarsal and becoming the internal metacarpal vein. The smaller posterior branch goes towards the Gastrocnemius muscle.
• Internal metacarpal vein: Major vein of the hindlimb and a continuation of the saphena vein.
• Digital veins of the limbs: Pass along the caudal aspects of the limbs into the foot and with two branches, reaches to the front of the limbs. They can be a visible aspect on the horse.
• Spur vein: A major vein of the torso, it often branches into two aspects, an inferior and superior branch. The parent branch lays along the top border of the Posterior deep pectoral muscle and goes into the armpit.
• Subcutaneous abdominal vein:  Passes along the posterior of the abdomen and into the posterior of the sternum. It's very distinct on a broodmare, often referred to as the "milk vein" when nursing. 
• Capillaries: A series of webbings or "chicken scratches" networking over a region of the body; they are subcutaneous and often very distinct during hot weather or exercise, or on thin-skinned horses. They most commonly manifest on the neck, shoulder, chest, lower haunch, gaskin, and forearm, groin area going up to the anus, with some appearing on front and rear portions of the barrel, or the barrel itself.

Landmarks and Reference Points for Veins

The teardrop bone is a good landmark for the facial vein, as is the front canthus of the eye. The internal malleolus of the radius is a handy reference for the internal vein of the forearm whereas the hock is a good landmark for the internal saphena vein. The top of the Posterior deep pectoral and the back of the elbow is a good start to trace the spur vein. The bag or sheath is a good reference for the abdominal vein.

Artistic Aspects to Consider about Veins

Veins should appear squishy and soft, with a rounded surface to blend with the surrounding flesh. They can also have bulbs where they're wider, as often seen with capillaries. The more veins and capillaries we apply, the more we imply heavy exertion or heat dissipation, so we need to make sure our sculpture warrants a heavy or light application of them. Certain breeds with thin skin and "dry" features also tend to exhibit more veins and capillaries such as Tekes, Thoroughbreds, and Arabians. The deft application of veins and capillaries can create a lovely contrast to firm muscle or hard bone, and really drive home the appearance of "living flesh."

Common Artistic Faults

Veins on sculptures are often structured improperly such as being randomly branching when they should be bilaterally symmetrical on either side of the body. This isn't to say they should mirror each other perfectly, but they should present the same general idea in terms of placement and intensity. They also tend to be misplaced, not following their anatomical patterns. For instance, a "Y" vein placed too low on the head or a spur vein placed too high or too low in the elbow area.

They can also be incorrect in structure being indicated as either simply grooves in the clay or flat. They may be clumsily executed and messy, too, due to a shaky hand or poor technique. Size is also a common problem as they can be too big in size or width. The veins and capillaries should also be smooth, and not pebbly, streaky, or rough in texture.

Genitalia

The vulva forms the external opening slit of the urinary and reproductive tracts of the mare. It's approximately four to five inches long and fleshy. The anus is at the top, immediately under the dock, and the vulva is a descending slender oval or hourglass shape. Between the hindlimbs, at the termination of the abdomen, the mare has a fleshy udder comprised of two dangling triangular portions of flesh with a teat on each.

Between the thighs, at the back of the abdomen, the stallion has a penis, sheath, and two testicles. The penis is enclosed in a double fold of skin called the prepuce that forms the visible sheath outside the body. A normal testicle is egg-shaped or round and about three to five inches long. The size of the testicles vary with individual stallions, but the left is sometimes larger than the right. The testicles are suspended in the scrotum and can be raised or lowered simultaneously or independently.

The vulva, anus, scrotum, and penile sheath are fleshy and relatively hairless.

Landmarks and Reference Points

In both the stallion and mare, the underside of the dock is a good point to gauge where to place the anus. Likewise, where the Gracilis muscles meet along the median seam of the body is a handy reference for placing either the udder or sheath and testicles.

In the mare, the vulva begins about where the points of buttock occur. It can be shaped like a soft oval, more like a rectangle, or have a slight hourglass shape, depending on the natural variation between mares. It should also be straight up and down, and not slanted outwards.

Artistic Aspects to Consider about Genitalia

All genitalia should appear soft and fleshy, not hard. They are passive aspects of flesh (when relaxed) and so are subject to the effects of physics. For example, the udders, sheath, and scrotum jiggle and bounce with motion, or are shifted from side to side. The anus can also pop in and out in synch with the gaits or movements.

Common Artistic Faults

Genitalia is commonly faulted by incorrect size, structure, and placement, even a lack of detail. Many times they're sculpted too harshly, blockish, or blobbish, too, obliterating their soft, fleshy nature. Sculpted vulvas are often slanted, which is a conformation fault. It's not enough to simply pop on blobs of roughly-shaped clay to indicate these features...they need careful shaping and detail just as any other part of the body.

Chestnuts

Chestnuts are small masses of cornified tissue (essentially, horn) on the internal aspect of the forearms and the hock. It's believed they're remnants of one of the digital foot pads lost during equine evolution. On the forearm they occur above the knee and on the hindlimb they occur on the back and bottom of the hock. They tend to be larger and more oval-shaped on the forearm and more slender and smaller on the hindlimb. Those of the forearm also tend to be placed on a forwards angle whereas those of the hindlimb tend to be more perpendicular to the ground. 

Each chestnut is distinctive and can sometimes be used to identify animals. They're rough in texture, often having ridges or a pebbly surface. However, they can be softened with oils or lotion, and peeled away to be flatter and smoother, a common practice in show grooming. Sometimes they're oiled for show, which darkens their coloration.

Landmarks and Reference Points

The internal "bump" of the knee is a good point to reference when placing the forearm chestnut whereas the top of the internal splint bone is a good landmark for placing the hindlimb chestnut.

Artistic Aspects to Consider about Chestnuts

Duplicating the cornified texture of a chestnut is important for believability just as much as getting then placed, oriented, and sized correctly. For painting, they can either be a grayish tan, tan, dark brown, dark grey, or nearly black (when oiled), depending on individual variation and the underlying coat color. Often, they're pink or light tan when in a white marking.

While they break off with growth, chestnuts can grow long under natural circumstances to stick out quite a ways from the surface of the arm or hock. This can be commonly seen on wild or feral horses.

Common Artistic Faults

Chestnuts are often misplaced, being placed too high or too low. Or they may be placed bilaterally asymmetrical. They can also be indicated simply by dollops of smooth clay, like smooth buttons, rather than as cornified tissue. They can be too big (often on the hindlimb), too, or not angled correctly (often on the forearm). Sometimes they aren't even present. Mules tend to only have chestnuts on the forearms and not the hind limbs.

Ergots

Like chestnuts, ergots are cornified flesh forming dime-sized peaks or "buttons" on the back of the fetlock joints. Usually hidden by hair, they're placed a bit lower on the fore fetlocks than on the hind fetlocks, and vary in size depending on individual variation. They're also though to be remnants of the ancient foot pads of eohippus.

Landmarks and Reference Points

The flatter aspect of the posterior fore fetlock makes for a useful landmark as does the pointier posterior aspect of the hind fetlock.

Artistic Aspects to Consider about Ergots

Ergots form a gentle, discreet point at the back of the fetlock joint, something especially obvious on a clipped lower leg. They tend to be smoother in texture than the chestnuts. However if allowed to grow, they can become quite pronounced, especially under feathers.

Common Artistic Faults

Ergots are often misplaced or too big.

Whisker Bumps

Whisker bumps or moles occur on the muzzle and around the eye. They vary in size, intensity, and number between individual horses. They can occur a bit randomly or in patterns, again, depending on individual variation and location.

Landmarks and Reference Points

The nostrils and mouth are good landmarks for placing moles on the muzzle. The lower lid is a handy reference for placing those around the eye.

Artistic Aspects to Consider about Moles

Moles are small raised buttons of flesh from which a whisker grows. They're fleshy, squishy, and not hard. They can vary in size on the same horse, or between horses. Those around the eye tend to be smaller than those on the muzzle.

Common Artistic Faults

Moles are most typically flawed by a clumsy rendition that does not make them appear fleshy and soft. They can also be too large and pronounced, lacking a discreet nature. They can also have sunken pits, like a collapsed souffle, or too pointy rather than being smoothly rounded. They can also lack the randomness or pattern indicative of an individual's variation.

Wrinkles

Wrinkles are soft folds of flesh caused by the compression or flexing of skin, or areas of skin that experience a great deal of stretching and movement such as the muzzle. When the animal is standing, wrinkles are most common between the ears, around the eyes, around the muzzle and nostril, in the throatlatch area, at the junction between the neck and the wither, and between the forelegs. In motion, however, wrinkles proliferate as movement dictates. 

Wrinkles can vary in size and intensity between individuals. Sometimes certain unusual regions of the body are predisposed to wrinkles, too, such as on the haunch during certain movements, the throatlatch area, or on a heavily crested neck along the crestline or span of the neck. Furthermore, if the animal is laterally bent, large wrinkles on the ribcage may appear on the barrel. Similarly, if the neck is cranked to one side, wrinkles will be apparent on the neck inside the turn, often fanning out onto the shoulder and sometimes the wither area.

Landmarks and Reference Points

Wrinkles occur wherever the flesh is compressed or an articulation has caused the skin to fold. So pay attention to the flesh around articulated joints, or on those areas that are subject to squishing and goo-ing.

Artistic Aspects to Consider about Wrinkles

We should sculpt our wrinkles so they appear as soft, folded flesh. We should also notice that they vary in width and intensity within a "bundle"; they aren't a clone of the one previous. Wrinkles can even wrinkle as clusters wrinkling on each other, such as we often seen in the throatlatch area of a tightly tucked head. Wrinkles also softly fade out into the surrounding skin and don't end abruptly with a definite border.

Wrinkles occur on the muzzle and around the eye, too, with great delicacy and softness. The amount of wrinkling here can vary between individuals, however, with some having few wrinkles while others having a dense proliferation of them.

Common Artistic Faults

Wrinkles are most typically faulted by a clumsy rendition that doesn't make them appear fleshy and soft, but hard and literal. They can also suffer from regimentation, like the folds of an accordion, rather than varying in depth, width, folding, and orientation. They can appear as gouged-out grooves, too, rather than rounded folds of flesh. Wrinkles can end too abruptly as well, rather than softly fading into the surrounding skin.

Secondary Sex Characteristics

Horses have a distinct difference between the genders so a stallion should look like a stallion and a mare look like a mare. Sure there are some exceptions, but on the general scale this works to our advantage in sculpture. 

For instance, stallions appear more compact and muscular, often sporting more of a crest and stronger jowls. On the other hands, mares appear "lower to the ground" and longer, with less pronounced jowls. Their necks also tend to be finder and their ears longer. 

Geldings, on the other hand, are a bit of a mixture of the two since the full expression of masculinity wasn't allowed to develop. They also often have some eccentricities that disqualified them from being a breeding stallion, and they're fun to inject into clay.

Common Artistic Faults

These characteristics can be overlooked in sculpture so we have a homogenized "horse" rather than a distinct gender. In other words, we can't just swap out "plumbing" and expect to pull off a convincing piece. 

Flesh and Hide, or "Goo"

The term "flesh" entails the muscles and fat beneath the skin. In turn, "hide" refers to the fascia and skin of the animal. Both express themselves, resonating and reacting to motion and articulation.

When it comes to flesh, the novel depressions and concavities of muscles during movement, the amebic absorption of boney parts during certain phases of motion, muscle resonance in response to movement, buckling caused by articulation or force, and the general jiggling and wriggling of flesh are all indicators of gooey flesh that need attention. In terms of hide, the sliding or rippling of skin, its stretched and compressed distortions during movement, and its wrinkles, bumps, stretches and pooches, and soft ridges are also good indicators of "living flesh."

So we can't simply sculpt correct anatomy...we also have to duplicate the nature of flesh as well as express the hide. Being so, it's a common oversight in equine sculpture in lieu of the underlying anatomy. Remember, the hide has been stripped away to reveal the muscular structures in an anatomy chart, but we have to reinstatel it for our sculpture to look real. Truly, a masterly artistic expression of flesh and hide and muscle...goo...is one of the most important ingredients for creating a convincing equine sculpture. What does that mean in practice? It means we should only use our anatomy charts as guides, not gospel. Life presents us with so much more to anatomy than a static chart.

Look how each pectoral is changed away from its resting state simply by walking. Unless we pay attention to muscle morphiing, our sculptures are going to appear artificial and formulaic. Horses don't move like articulated anatomy charts.

Artistic Aspects to Consider about Flesh and Hide

Flesh is soft and inviting, and it depresses and pooches with movement. That means it has a 3D quality, an in and out quality as it goos. Unless we capture this in our sculpture, our muscles will appear flat, contrived, and artificial, more like an articulated flat anatomy chart rather than "living flesh." Similarly, unless we understand and denote the nature of the hide, we're going to create a stylized piece of work that's too polished-looking and hyper-smooth to be truly believable. No horse is smooth like polished metal, but rich in the little imperfections of skin and fascia. Imbuing all this into our sculpture will go far in duplicating a life-like appearance.

Note all the hide and fleshy details to the skin. Horse's aren't "polished" smooth. The smooth finish on Maureen Love pieces is certainly pretty, for example, but it's not realistic. It's a manifestation of artistic style.

Common Artistic Faults

Sculptures are often faulted by a static, formulaic, or stylized rendition of flesh and hide. For example, we cannot apply standing anatomical configurations to a sculpture depicting motion just as much as we cannot apply the anatomy changed by motion onto our standing sculpture. Horses don't move like articulated anatomy charts, with the same configurations occurring over and over again despite anything else, like a "sculpt by numbers" formula. Instead, motion and physics can radically change the nature of the flesh and hide, and we need to capture these changes in our work.

Hooves

Know it or not, hooves are rich in detail from the bars to the clefts to the frog, yet they're often overlooked in sculpture. Detailing the hooves is important to fully express realism. For a more in-depth look at hooves, please refer to the blog post, Steppin' Out: Hooves From An Artistic Perspective.

Common Artistic Faults

Many sculpted hooves are pathological since the artist didn't understand what constitutes a healthy, adaptive, balanced foot. Many sculpted hooves also lack detailing on their palmar side, and usually we're lucky if merely the frog is indicated. But there's so much more to the underside of the foot and we need to factor all those structures into the sculpting of our feet.

Ears and Nostrils

These features are typically hollowed out to enhance realism. 

Artistic Aspects to Consider about Ears and Nostrils

Ears are complicated structures, being delicately fluted with a bulbous base. Where the rims meet at the bottom forms a "V" constructed of subtle curvaceous, bulbs, and lip-like structures rather than a literal, simple "V." What's more, these structures change as the ear is rotated, forming rather complex curves, pooches, angles, and overlapping flesh.

The nostril is similarly complicated being so fleshy and gooshy. Being so, the lateral rim stretches and misshapes depending on how its manipulated by the muscles, tendons, and fascia which can distort it well away from its resting aspect. The anterior "comma cartilage" rim can be distorted as well, expanding quite a bit from its resting state. There's also the false nostril that can distort, too, and even form pronounced raised flutes of flesh on either side of the nasal bone. On the side, the flared nostril typically has hollows, curves, and depressions confident to the muscles and tendons that activate them. 

Complicating things still further, each nostril can move up or down, or back and forth in relation to its pair, adding to expression, character, and effect.

Common Artistic Faults

Errors in the nostril are common. Of particular note is the error of overdoing it with the drill tool. Specifically, the more rounded, bulbous front rim of the nostril may be too thinned as the nostril is hollowed, and sometimes so that front rim can be reduced to a sharp edge. The rims of the nostril can also be sculpted in a clumsy way, creating an uneven, pebbly texture rather than a smooth, fleshy one. On the other hand, the nostrils may be sculpted incorrectly altogether, or with flutes (especially when flared) that don't possess the complex curves and indentions imposed by the overlaying musculature. Nostrils aren't simple things on the horse, and so shouldn't be so straight-forward on a sculpture.

Similarly, equine ears are complicated. Too often, however, they're "scooped out" when hollowed, forming more of a spoon-shape than a fluted one. The bottom "V" is often too simply rendered as well, as though it was simply two rims pinched together (a common sculptural flaw with pre-formed ears). Yet that "V" is characterized by a complicated folding of flesh with bulbs, twists, and overlapping features peculiar to equines alone, and unless our sculpted ears have similarly complex "Vs," those ears don't actually belong to Equus. Just as cat ears and dog ears aren't simple flat triangles (they have those curious folds on their lower, outside rim, where the outer rim meets the head), so a horse's ears aren't simple flutes with a simple "V" on the bottom either.

Mane and Tail

Sculpting the mane and tail (and feathers) is arguably one of the hardest aspects of sculpting realistic equines. It should never be treated as an afterthought. And there are many ways to expressive, but regardless, hair is passive to physics and moves in often complicated, unpredictable, chaotic ways. Its texture also presents a challenge, it being wispy, weighty, striated, and complex.

Artistic Aspects to Consider about the Mane and Tail

The manes and tails on certain breeds can be quite fine and soft, such as on the Arabian. On the other hand, it can be quite coarse such as on the Takhi or Fjord. Similarly, feathers can be silky and wispy on some breeds such as the Clydesdale or Shire, or wiry and dense such as on the Ardennes and Brabant. All this means that different types of hair flow differently, something we should pay attention to. In short, not all hair is created the same.

Hair also has layers; the mane and tail (and feathers) aren't all one equal length. These layers can operate independently or in synch with the other layers, depending on the situation. So unless we think in terms of layers, our manes and tails (and feathers) will appear odd and artificial.

And there are many ways to sculpt hair, given we meet the criteria of its flow and movement, and texture. Some artists merely indicate texture, opting for a more impressionistic approach while others go for a highly detailed rendition, with lots of little striations indicating each hair. It all depends on what we like for our sculptures.

We may also choose to impart waviness such as often seen when manes and tails are taken out of braids that preserve its length. This can be a really appealing touch on a sculpture, especially on Morgans, Andalusians, and Friesians, or other breeds known for a long mane and tail.

Common Artistic Faults

Missteps here are common. Often we see hair rendered as dreadlocks, tentacles, or "ropes," especially with flowing tendrils. Likewise, we may find the mane sculpted like soft-serve ice cream rather than naturally flowing hair (a flaw I used to do in the past as I was learning). Here's a handy trick: make the tips as thin as possible to make them appear wispy instead of ropey. 

Other times, layers haven't been accounted for so we have hair all the same length which gives it an artificial look rather than a natural, flowing one. We also often see texture gouged into the mane and tail (or feathers), often being pill-ed or with torn ridges, as though it was sculpted with a fork. Instead, the hair should be smooth and silky. 

Hair is also 3D, so it should have areas of hollows and areas of bunched-up thickness. If we sculpt it all the same, like a 2D rendition, it won't look natural and real.

More still, we find that the sculpted hair lacks the passive, unpredictable movement characteristic of its flow, instead taking on a regimented "safe" expression. Remember, hair is passive to motion, so we need to keep the movements of the body and physics in mind when we design it. And often times, hair can move counter-intuitively to the motion as a result, so pay attention to life study and reference photos.

Even so, we may find that the sculpting of the mane and tail (and feathers) is inconsistent to the motion such as with "standing" feathers on a cavorting piece. Or we can find it "stopping" the sense of motion by not flowing in reaction to motion such as a trotting horse with a static mane. 

Sometimes we'll see an amount of hair inconsistent to a breed's characteristics. For example, profuse, thick manes and tails on Arabians or Tekes, or thin, sparse manes and tails on an Andalusian or Vanner are errors.

When it comes to hair weaving and braids, we often see more problems. For example, a hair weave with hair "tassels" hanging down often don't flow perpendicular to the ground, obeying the laws of physics. Remember hair is passive and will hang always perpendicular to the ground unless impeded by motion or a breeze. Yet we see many of these hanging bits askew, as though a breeze was slightly blowing them, but without that breeze expressed in the tail or forelock. As for braids, we often see them too big or too sloppy, crudely sculpted on. Instead, braids do best when in-scale, correctly textured, and neatly done.

Conclusion to Part XIV

The "devil is in the details" and realistic equine sculpture definitely proves this to be undeniably true. They can really take our sculpture to the next level of realism, but only if done correctly and carefully; otherwise they can bust our illusion rather quickly. And learning to see details is a learned skill...it doesn't come so naturally when there's so much else to concentrate on it seems. We can be easily distracted. Yet once we do start to pay attention to them, the easier it becomes to pick them out, adding interest and believability to our work.

One last note about details though: the nail clenches with shoes shouldn't be located at the quarters of the hoof, but away from them, well towards the toe. The hoof does its primary contraction and expansion at the quarters which would be uninhibited by nails. Too many times, however, the clenches on sculptures flow well into the quarters, which is a serious fault in farriery, and should be considered a severe fault in detailing. Clenches that are too big, or uneven in size and shape should also be similarly penalized.

So anyway...in the next installment, we'll discuss physics and how that pertains to realistic equine sculpture. We have to instill a real world in our work, and we do this by infusing the effects of physics into our clay. It's fun and highly effective!

So until next time...bedevil those details!

"I admit I'm enthusiastically demanding."

~ Brad Bird

Equine Anatomy and Biomechanics: A Primer of Equine Engineering for Advanced Students Part XIV, Details

Equine Anatomy and Biomechanics: A Primer of Equine Engineering Part XIII, The Hindlimb

Introduction

Here we are again, back on track with the proper sequence in this 17-part series discussing equine anatomy and biomechanics in a bit more detail than we did in Anatomy 101. It's recommended to read that first, then dive into this series as that beginner level post provides a foundation for understanding this more advanced series.

So in this Part XIII we'll be exploring the hindlimb, which in this case is the femur down to the toe. In Part XII we discussed the pelvis, treating it separately since it's a component of both the spine (Part X) and the hindlimb and warranted its own post because of this interdependence. In previous posts, we've also touched on the head, neck, torso, forelimb plus evolution and some terminology. There's a lot to digest, but understanding our subject from the inside out is important not only for creating accurate work, but responsible work. The more we know about his biology, the more informed are our creative decisions. 

While the the hindlimb is a bit more complicated in function than the forelimb, it's also a snidge easier to sculpt from a sculptural point of view. The planing and angles are a tad simpler than the nuanced aspects of the forelimb, with the hock having simpler articulation. That being said, however, the hindlimb is nothing to sneeze at—it's tricky. Indeed both the forelimb and hindlimb are complex and fascinating, being finely-tuned, highly-developed, incredibly sophisticated running mechanisms. In particular, the stifle and hock flummox many artists in both structure and mechanics which is why we see so many problems in these areas. 

Nonetheless, wholly unique in the animal kingdom, his legs are a distinctive characteristic of Equus. So we have to get them as correct as possible which means we have to pay close attention to their structure, function, and topography when we sculpt since general approximations can lead to errors. Truly, we can tell a lot about an artist's simply skill by how they sculpt the legs.

So let's learn more...

Basic Structure of the Hindlimb

The hindlimb is constructed of alternating angles, starting at the LS-joint then at the femoral joint, then at the stifle joints, then at the hock, then at the fetlock joint, and on down into the foot. This causes the hindlimb to compress in flexion and lengthen in extension not because the bones themselves are shortening or lengthening, but because the angles of these joints are closing or opening. This structure also causes the hindlimb to flex and extend like a drafting lamp as a tensionally balanced system. 

It's also important to understand that muscles cease at the hock therefore all motion from the hock down is accomplished by a pulley system of ligaments acted upon by their muscle counterparts above the hock (like with the knee). Therefore, the femur and hindlimb should be regarded as a whole unit because of the mechanisms binding the structures together (as we learned in our discussion about the Stay Apparatus). 

We should also be aware that the shape of the hind hoof is different from that of the fore hoof due to the mechanics of propulsion and thrust, being more upright and pointed at the toe. (For more detailed information about the hind hoof, and hooves in general, please refer to my blog post, Steppin' Out: Hooves From An Artistic Perspective.) At birth, the hooves are the similar shape, but within three days, they begin to develop their diverging shapes due to these forces. This is an important detail for sculptures of newborns.

Skeletal Structure

The hindlimb consists of the femur, patella, tibia, tarsus (hock), metatarsal (hind cannon), sesamoids, first phalanx, second phalanx, third phalanx, and navicular bone. 

Joints are comprised of the femural joint (femur-pelvis), femoropatellar joint (patella-femur), femorotibial joint (femur-tibia), tarsal joint (hock), fetlock joint (cannon-first phalanx), pastern joint (first phalanx-second phalanx), coffin joint (second phalanx-third phalanx), both sesmoidean joints (with the cannon and first phalanx), and the navicular joint (navicular bone-third phalanx-second phalanx). This makes a total of ten joints in the hindlimb (eleven if we count the LS-joint). The degree of their motility is dependent on their individual structures and mechanisms. 

The femur is the largest long bone in the horse. It articulates on top with the pelvis deep within the hindquarter and below with the tibia and patella. The top part of the femur has a head, the middle is a shaft and the bottom part ends in a trochlea. The head articulates with the pelvis and has the trochanter major, which serves as the attachment of the Deep gluteus and the Middle gluteus muscles. The shaft has both the lesser trochanter on the inside, the attachment of the Psoa muscles, and the tertiary trochanter on the outside, the attachment of the Gluteus superficialis. The distal end has a front trochlea, comprised of two ridges forming an articular surface with the patella and the internal and lateral condyles that articulate with the condyles of the tibia. These two joints form the stifle joint. The whole thing is lashed together with ligaments, tendons, and muscle.

The patella is a solid little bone equivalent to the human kneecap. It's housed in a complex tight network of ligaments, tendon, and muscle. However, when standing, it forms the base of the depression of the overhanging muscles and when the hindlimb is flexed, becomes readily apparent as it protrudes outward.

The stifle joint corresponds to the human knee and consists of two joints, the femoropatellar joint and the femorotibial joint. A network of five important ligaments bind the femoropatellar joint and transfer communication to the femorotibial joint. Likewise, a network of four important ligaments bind and stabilize the femorotibial joint and transfer communication to the femoropatellar joint. 

The fibula of the horse is rudimentary, therefore the tibia (gaskin) bears the weight of the animal and is the most developed bone between the two. It's a long bone extending from the stifle to the hock. At the top, it articulates with the femur and at the bottom with the tarsus. The top part is large, three sided, and dominated by its medial and lateral condyles. The shaft is large and widens distally. The bottom part has an articular surface with two grooves angled about 12º-15º, slanting medially upwards to the dorsal plane, with the hock joint having a corresponding slant to mesh with this articular surface. The bottom of the tibia has a medial and lateral malleolus; the medial is more prominent. The fibula is a slender shaft with a top part that articulates with the lateral condyle of the tibia and a bottom part that's fused to the tibia, acting as an attachment for muscle and ligaments.

The spiral joint of the hock, left hock seen from the front.

Movement of the spiral joint, left hock seen from the front.

The tarsus (hock) is comprised of six bones formed into three layers, creating the distinct shape of the hock. The calcaneus is the largest hock bone, its proximal tip being the tuber calais or calcaneum (point of hock). The hock is composed of a number of articulations that can be generalized into three joints. The tarsocrural, intertarsal, and the tarsometatarsal. However, these joints are lashed together with ligaments, making the only working joint between the bottom of the tibia and the top of the metatarsals.

The three metatarsal bones (hind cannon), the cannon and its two splint bones, have the same basic structure as their metacarpal counterparts. However, the metatarsal is characteristically longer than the metacarpal. The external splint bone is also longer than the internal on the hindlimb, the reverse of the forelimb. 

The sesamoids on the hindlimb are a hair smaller than those on the forelimb. The first phalanx is also a bit shorter. The third phalanx, or coffin bone, is more narrow and pointed as well, mirroring the difference in hoof shape of the hindfoot. 

Basic Musculature of the Hindlimb

The principle stabilizing ligaments of the hindleg are the network of stifle ligaments, capsular ligament of the hip joint, lateral and medial patella ligaments, lateral collateral ligaments of the tarsal joint, plantar ligament, the suspensory (or interosseous) ligament, navicular ligament, and the sesamoidean ligaments. From the hock itself and down, nearly all the bones and ligamentous and tendinous structures are subcutaneous and are readily apparent on a clean-legged horse, morphing in and out of distinction dependent on motion and "dryness." 

Speaking of which, newborn and young foals typically have very "dry" legs with distinct ligamentary and tendinous definition on their knees, hocks, and lower legs. A great way to learn about such structures then is to study foals, especially those references depicting them in motion. Indeed, studying foals is a great way to learn about bony and fleshy landmarks, given we understand that their muscles and bones aren't fully developed yet.

Anyway, the muscles of the pelvis and hindleg are so interdependent because of its bridging muscles that there's some overlap in this inventory, so please reference back to the pelvis installment (Part XII) of this series. 

The basic muscles of the femur are:

(See Basic Musculature of the Pelvis in Part XII)

The basic muscles of the hindlimb are:

• Long digital extensor (or extensor pedis): Extends the phalanges, flexes the hock and fixes the stifle during motion.
• Lateral digital extensor (or peroneus tertius): Assists the long extensor in hock flexion and extension of the digits.
• Deep digital flexor (or perforans): Flexes the digits and extends the hock.
• Long digital flexor (or accesorius): Also referred to as the medial head of the deep digital flexor. Assists the action of the perforans muscle.
• Superficial digital flexor (or perforatus): Flexes the digits and extends the hock joint. Mostly tendinous.
• Tibialis cranialis (or deep flexor metatarsi): Flexes the hock.
• Popliteus:. Flexes the stifle and helps to rotate the leg inward.
• Soleus: Assists the gastrocnemius.
• Gastrocnemius: Extends the hock joint or flexes the stifle joint; these two motions cannot occur simultaneously.

Basic ligaments of the hindlimb are:

• Straight patellar ligaments: Stabilize the stifle joints.
• Internal and External ligaments of the stifle: Help to stabilize the stifle joints.
• Superfical digital flexor (or peroratus) : The tendon of the peroratus. Helps to flex the foreleg and support the forelimb.
• Deep digital flexor (perforans): The tendon of the perforans. Helps to flex the foreleg and support the forelimb.
• Suspensory ligament (or interosseous ligament): A strong brace for the fetlock joint that relieves the strain and effort required for the horse to stand and move. It is mostly inelastic, but does retain some elasticity. Its branches travel over the first phalanx, clearly visible on a clean-legged horse.
• Metacarpal ligaments: Helps to stabilize the hock. 
• Check ligament: Helps to relieve the strain of standing and force generated by motion; sometimes absent in the hindlimb.
• Long digital extensor tendon (or extensor pedis tendon): Helps to extend the foreleg and stabilize the forelimb.

Biomechanics of the Hindlimb

The hindquarter and, therefore, the hindlimb, is responsible for propulsion and impulsion; it's the engine of the whole system. This is why the hind foot is shaped differently from the fore foot, and why the hindlimb is built as a series of angles to add "spring" for forward motion.

Notice how the system articulates like a drafting lamp?

Hindleg motion begins with the femur (though technically it begins in the spine) in a ball-and-socket joint. The femural joint is very stable due to the depth of the "cup" of the pelvic aspect. It's the femur that initiates the drafting lamp-like motion, the pulley system of the tensionally-adjusted arrangement of tendons and ligaments that automatically flex or extend the entire hindlimb. This means the femoral joint, stifle, hock, fetlock, and foot joints are mechanically moved by this tensionally-balanced, interdependent system and cannot move independently. So when the femoral joint flexes, so must the stifle and the hock, and when it extends, so must the stifle and hock, all at similar angles. If there's any out-of-synch articulation in this system it means there's a catastrophic rupture, a serious injury. 

So any sculpture depicting motion exhibiting such a rupture is fundamentally nonviable since no horse would be able to adequately move with such an injury. It also means that if there's slack in the system, which can naturally happen during motion, the foot can wobble or flip upwards or backwards as we sometimes see during performance or fatigue. Sometimes when the hind limb is extended backwards and the hoof flexed, we'll again see slack in the system as a moderate bump caused the by the crumbling of the ligaments and tendons on the back of the cannon.

Sometimes we'll see the ligaments and tendons "bunch up" as a function of the slack created by a flipped and flexed foot.

The stifle joint corresponds to the human knee. It's a bicondylar joint, consisting of two joints, the femoropatellar joint and the femorotibial joint. Being the largest and most complex in the horse, it's primarily only capable of extension and flexion (like a hinge joint) with an articular angle of about 150º. However, there's some minor rotational play when the hind leg is strongly extended, such as the extension phase of the gallop, slightly spinning the entire limb down to the hind toe towards the median; seen from the front, the spin is slightly clockwise in the left and slightly counterclockwise in the right. The patella slides in synch with the tibia due to the thick casing of ligaments and tendons; down when flexed and up when extended, creating the distinct profiles of this area with the different hindleg articulations. This also means that the distance between the patella and the upper rim of the tibia remains about the same despite extension or flexion. There can be no lateral or medial play at the stifle. All lateral and medial motion can only come from the femoral joint, so the entire hindquarter (and therefore the spine) is engaged during such movements.

Abduction and adduction of the hindlimb. Note how it happens at the femoral joint and not the stifle?

The tarsocrural joint is composed of astragalus and tibial articulation. The astragalus (or talus) is the upper portion of the tarsus, right next to the calcaneus, and is distinctive since its trochlea consist of two oblique ridges that articulate and correspond to those of the bottom part of the tibia. This creates a spiral hinge-joint directed at an angle of about 12º-15º, slanting medially upwards to the dorsal plane. Therefore, the hock cannot articulate straight forwards and backwards like a classic hinge joint (like the elbow), but on an oblique inward angle that spins the metatarsal (counterclockwise for the left hock and clockwise for the right hock when seen from the front), especially when articulation is extreme. This allows the hindcannon to be positioned relatively straight forwards, readying the hind hoof for straight-forward impact and take-off despite the inwards angulation of the gaskin produced by the popped-out stifle. This mechanism increases efficiency and speed while reducing injury and interference. In short, hindlegs cannot move on a straight vertical plane from stifle to toe.

The intertarsal (between the bones of the hock) and the tarsometatarsal (between the tarsal and the metacarpal, or hind cannon) articulations are planar joints, but a tight binding of ligaments and tendons bind the bones of the tarsus into one unit and also lash the tarsals to the metatarsal, forming a long lever activated by muscle action on the calcaneus and the front of the hindcannon. It's the astragalus that articulates with the distal end of the tibia then to create hock flexion, with the remaining tarsus bones and the metatarsal acting as a single structure. In other words, the calcaneum is an extension of the hind cannon and cannot articulate independently. Therefore, the hock is distilled it into a strict hinge joint, capable only of extension and flexion on its unique oblique angle.

 The calcaneum functions as an extension of the metatarsal. Notice the rounded tip of the calcaneum? Flexed hocks aren't pointy.

The articulations of the hind foot are similar to those of the fore leg, so please refer back to that part of the series in Part XI: The Forelimb.

When the horse swings his hindleg forward, the stifle must clear the widely sprung posterior portion of the barrel and, therefore, the stifle pops out around it, spinning and forcing the hindlimb inwards at a slant with an outward rotation. Then the spiral joint of the hock "straightens" and spins the metatarsal to orient it more forwards for effective planting and push-off. Now when bearing or especially when pulling great weight, the hocks tend to come together, increasing leverage and tork. Furthermore, in motion, the horse naturally angles his entire hindlimb inwards at the toe, towards the median, as a function of coordination, speed, and physics. Indeed, the faster the gait, the more towards the median the foot is placed.

All of these stifles are "popping out" to get around the wide barrel. Note how it angles and rotates the hindlimb.

Landmarks and Reference Points

Boney Points of Reference

The great trochanter and third trochanter of the femur can be found under the flesh as well as the bottom ridges of the medial and lateral femoral condyles. The patella can be palpated along with the upper rim of the tibia. In the tibia, the external malleolus, nearly the entire internal aspect of the bone and the internal malleolus are all superficial and important landmarks.

The hock is devoid of muscle tissue and is therefore easily palpable under the skin, tendons, and ligamentous bindings. These bindings and bones are prominent landmarks on the surface of the hock, especially the point of hock. The ligaments of the hock and hindcannon are sometimes readily visible, especially during movement and on clean, "dry" legs.

The three bones of the hindcannon are all subcutaneous and easily palpable, as are the first phalanx and the sesamoids. While many of the ligaments of the pastern can be pin-pointed, the suspensory ligament, is particularly noticeable. The upper portion of the second phalanx is also palpable.

A healthy hock, cannon, fetlock, pastern and pastern joint will not be smooth and uniform, but possess the crisp definitions and distinct geography of the internal surfaces that lay beneath the skin. In other words, they have specific lumps n' bumps characteristic of its design.

Like the knee, the hock is also constructed in a trapezoidal design, when seen from the front:

Left hind leg, seen from the front.

Fleshy Points of Reference

The Biceps femoris group is often readily apparent, especially in extreme motion as is the Semitendinosis, which is subcutaneous. The Tensor fascia latea can often be seen in certain movements along with the Gracilis on the inside of the leg. On the gaskin, the Extensor digitorum longus and the Extensor digitorum pedis lateralis are superficial and their definition can sometimes be seen during articulation or under forces. 

On the inside, the Tibialis anterior, Flexor digitorum pedis longus and its tendon can often be seen as can the Tibialis posterior and the Flexor hallucis longus. The tendons on the hock also contribute to its shape and are often clearly visible, especially in motion. The hamstring is subcutaneously composed of Achilles tendon and the Superficial flexor tendon. The hind cannon has definition similar to the forelimb's cannon.

Artistic Aspects to Consider about the Hindlimb

The hock's only joint exists on the end of the tibia and the top of the hock grouping so the calcaneum (point of hock) moves in synch with the metatarsal (cannon). When the cannon is bend, the point of hock lowers, away from the tibia; when the cannon is straightened, the point of hock rises, towards the tibia. The point of hock also has a distinct shape so when it becomes apparent in articulation, it's peculiar softly-rounded, squared off tip becomes more pronounced. From the back, the point of hock is also broader than the Achilles tendon and the Superficial flexor tendon, producing a "bulb" at its point. This means that the point of hock is neither pointy, as we so often see in sculpture, nor is it narrow to match the back of the hind cannon.

The correct plumb-line from the point of buttock down the back of the metatarsal. Notice the flexion of the fetlock between the two stances.

Hindlimb muscles markedly morph, stretch, goosh, and pooch in motion such as during flexion and extension because the femur is so thickly encased in flesh. So we need to not only know the skeletal structure underneath all that flesh, but also to pay attention to fleshy changes and planes at the different phases of a gait or movement. This is why the hindlimb may appear to "shorten" in extreme flexion (such as with Hackney ponies or Saddlebreds) and "lengthen" with extension (such as with a halter stretch) as the tibia is alternately squished into the hindlimb musculature or stretched out from it. 

The hindlimb is also capable of a great range of motion, thanks to the femoral joint, making it a powerful and dynamic component of movement. This also means that when the hindlimb is bearing weight or pushing off with impulsion, many of these muscles become more readily apparent under the thrust but relax and soften when not weight bearing. Sometimes muscle striations can be seen, often on the Vastus (long muscle of the biceps femoris group) under exertion. Sometimes in extension, wrinkles can be seen between the tail and the femoral joint, in the biceps area, or on the "semis." 

Different breeds or lineages within a breed sometimes have different hindquarter builds, some being rather angular (such as the Teke), square-ish like the Friesian, or round and bulbous (such as the drafter). Certain breeds even have extra development around the dock such as often seen on the Iberian, or an "apple butt." This is noticeably so with asses and sometimes mules.

We also need to know the proper angulation of the hindlimb; that would be a plumb line up the back of the metatarsal to the point of buttock. When standing, this plumb line holds true no matter how the hindlimb is positioned, whether postured behind the body or underneath.

Different breeds have different degrees of limb "dryness." For example, thin-skinned breeds such as the Arabian, Teke and other "hot" breeds will have sharp, clear definition of their leg details. However on heavier breeds such as drafters and draft ponies, or "cold" breeds, there will be less definition.

Common Artistic Faults with the Hindlimb

The hindlimb is commonly flawed in the mechanisms dictated by the Stay Apparatus and Reciprocal Apparatus (both discussed in Part IV: Systems) with joints articulating independently rather than together as a system. Typical faults are a stifle and hock out of synch, or the fetlock and hock out of synch. More subtle, because it's housed in flesh, is a femur out of synch with the hock. Subtler still is a patella out of synch with hindlimb articulation, as it's often forgotten or misplaced in sculpture. This problem typically leads to the oddly shaped formations between the stifle and the tip of the tibia especially in flexion, manifested as too many or too few profile "bumps" along the front of the area, or misplaced bumps. But it's also seen in extension and standing as well. We have to remember that the patella slides in synch with the flexion or extension of the tibia which means that the orientation of and distance between each "point" in relation to the other stays about the same, and is important to correctly render in sculpture. We also often see a femur not long enough and sometimes with a missing patella, producing a roundish profile to the front of the stifle area in flexion. This is usually paired with a standing femur with a flexed hock and a fetlock joint not flexed enough, indicating a ruptured Stay Apparatus and Reciprocal System.

Like the foreleg, the hindlimb even more so moves like a drafting lamp. Note the orientations of the femurs and metatarsals along with the scapulae and the radii, and the humeri and the metacarpals.

Another common error is a straight plane during hindlimb flexion that misinterprets the anatomical structure and biomechanical function of the stifle and hock. Seen from the front, the equine hindlimb cannot flex on a straight plane due to the "popping out" of the stifle and the spiral construction of the hock. Only in extension does the hindlimb straighten out, more or less. Likewise, when the sculpture depicts a standing horse, the hindlimb is usually facing forward on a straight plane (as so often erroneously illustrated in conformation texts) when, in fact, it should be slightly oriented outward as a whole unit, from stifle to toe, away from the median. This is the correct, natural angulation of the hind leg and shouldn't be confused with sickle-hocks. When the hind leg is oriented straight forwards from stifle to toe, the hind leg is actually bow-legged, a conformational fault.

Other common faults are found in the misinterpretation of the stifle, hock, fetlock and foot joints in function or structure, creating mistaken biomechanics, misshapen forms, or incorrect topography. A lack of symmetry from the hock down can also be found, most notably in the fetlock joint and the structure and articulation of the foot structures and joints. The hock is also typically flawed in topography, having "lumps and bumps" in the wrong locations, either seen from the side, or from the front and back. More often, the hock is indistinct and puffy, lacking the necessary lumps n' bumps altogether. Additionally, the hocks's calcaneum is also often too pointy or not moving in synch with the metatarsal, being curved either upwards or downwards away from its proper alignment. Sometimes a flexed hock will be compressed in the front aspect, as through the bones themselves were squished. However, the hock bends by leveraging, not by compression. 

These photos demonstrate the natural and normal outward angulation of the hindlimb. The hindlimb should never be oriented on a straight forwards plane when standing.

Seen from the back, the calcaneum and associated tendons and ligaments down the back of the leg are also often mistakenly the same width instead of the subtle curve and width of the calcaneum in relation to its surrounding structures. In other words, there's no "bulb" to indicate the tip of the calcaneum. Hindlimb's can also be pathological, having puffiness, swellings, depressions, or asymmetries where none should be found. Also found is a misunderstanding of the structure of the tibia, cannon, and pastern bones as seen in a curvature of these bones, often referred to as "spaghetti legs," which we see in the forelimb as well, at times.

The hindleg is set towards the outside of the hindquarter, not in the middle.

Sometimes we also see hind legs oriented in the middle of it's haunch, as though the bulk of the Gracilis didn't exist. This causes a narrow hind end and stance. Instead, the hind legs are oriented towards to outside of the haunch, away form the median.

Furthermore, a lack of adequate physics in motion is typical, with the hindlimb not sufficiently demonstrating the power of thrust, weight-bearing, or stopping necessary to best mimic the effort we see in life. We can sometimes see this in the fetlock joint not being depressed downward, but instead the foot bones are "perched" like a bird on a branch in relation to the ground.

We also find a confusion in the topographical planes of the hindlimb, most notably those planes falling away from the femur and stifle as well as those of the gaskin and especially the hock. Musculature can be confused, especially in how the hindquarter musculature merges with that of the gaskin. Gaskin muscles are also often too bulbous or engaged when they should be relaxed, which can be seen in some extended hindlimbs. Muscles can be carved in for delineation rather than indicated by soft curves and the 3D quality of muscle masses. In other words, the muscles of the hindquarter are defined by lines, or gouged tracing rather than fleshy indications; the musculature expresses more as soft troughs and bulges rather than harsh lines. Think of bulk rather than defined lines indicating muscle. The muscles of the hindquarter also undergo a great deal of morphing in motion, so we shouldn't see "standing" musculature on a piece depiction motion, and visa versa. Likewise, standing muscle definition and delineation is too often seen on hindlimbs that depict motion. Remember, the horse's muscles don't manifest like an articulated anatomy chart...they morph and contort, merge, goo, and pooch in response to articulation and physics. 

Biological Aspects to Consider about the Hindlimb

The equine hindlimb was designed by nature for thrust and propulsion while the forelimbs are designed more as rods, pole-vaulting the forequarter forwards in motion, especially in the gallop. This gives the animal a springy, powerful stride and contributes to that look of hovering flight. And the faster the animal goes, the more important that coordination of the pole-vault motion of the forelimbs becomes. This is why horses trip in the forelimbs whereas they fall out from under themselves in the hindlimbs. It's also why horses forced to move too slowly or out of synch with the coordination of a normal gait can plod, trip, or kick up dirt with their fore hooves because the pole-vaulting effect of the forelimbs cannot be adequately exploited by the hindlimbs. We can sometimes see this with "peanut-rollers" or Western Pleasure horses made to move with impure gaits.

Furthermore, motion that's consistent to natural equine coordination and movement will preserve the integrity of the hind leg joints, even under extreme performance demands. Injuries and pathologies of the hindlimb occur when the animal is forced into unnatural postures that alter natural coordination and function, which can lead to puffiness, bumps, and swellings. We often see this in dressage as so many horses are forced into false collection due to rollkur and "push-pull" or "frame" riding. Indeed, false collection can lead to many pathologies in the hindlimb from String Halt to being "strung out" behind, to even ruptured stifle ligaments.

The hindlimb also moves fast. Those legs positions and push off occur in the blink of an eye and the forelimbs "catch" the forward motion to support the forequarter. Capturing this kind of nimble movement is important for an equine sculpture since a "clunky" feel can really "stop" the motion visually.

Conclusion to Part XIII

Phew! That was a lot to chew on, wasn't it? The hindlimb maybe be a bit easier than the forelimb, but it's complicated nonetheless. So many details to factor into our equation! But how we sculpt the hindlimb can mean the difference between a convincing sculpture and one that's not since the physics of impulsion influence them so greatly. If done right, they can really capture the idea of mass, thrust, and force, as well as fleeting agility so characteristic of this animal's motion.

So now that we have this last aspect under our belt, let's delve into details in the next installment. The equine has lots of them that deserve our careful attention, plus they're fun to infuse into our work because they really bring a piece alive.

So until next time...propel forwards in your understanding equine biology!

"The better an artist can mimic how the eye sees, the more effective and natural the paintings will be."

~ Kenn Backhaus

Equine Anatomy and Biomechanics: A Primer of Equine Engineering Part XIII, The Hindlimb