Tuesday, October 11, 2016

Equine Anatomy and Biomechanics: A Primer of Equine Engineering Part IX, The Neck


Hello again and welcome back to this 17-part series about equine anatomy and biomechanics, from an Intermediate perspective. We started with Equine Anatomy 101, a primer for beginners, and now we're moving onto additional insights. For this reason, it's highly recommended to read "Equine Anatomy 101" first before delving into this series simply because it has details this series doesn't; they're complementary.

So moving onwards from the head in Part VIII, this post will be about the equine neck, that confounding element that can give us so much trouble, especially in movement. The equine neck is a sublime thing...elegant, muscular, beautiful...and tricky! But once we begin to understand its anatomical structure, perhaps we'll get a better handle on sculpting this puppy in clay.

So with that in mind...let's go!...

Basic Structure of the Neck

The equine neck is an elegant, graceful, supple part of the body, capable of great flexibility, stretches and tucks. It's musculature is pretty consistent across breeds, with riding playing a big part in which muscles are developed; poor riding develops tell-tale musculature (such as an overdeveloped Rhomboideus muscle) just as easily as quality riding does (which has a developed Complexus muscle). Some necks may be thin (seen from the side), some may be triangular in shape, some may be arched, some may have a pronounced crest, some may be "swan nicks" and "snakey," some may be "ewe-necked," and some may be anywhere in between. The point is, the equine neck is as unique as the individual possessing it, so don't be afraid to give it some character.

On that note, when the cervical column erupting straight forwards from the shoulders, that creates a straight neck, like what we see on speed-based breeds such as the Quarter Horse and Thoroughbred. When it erupts high from the shoulder, in a more upright position, we have the arched neck of the classic riding horses, the Arabian, Saddlebred, and Morgan. 

Neck musculature can be "normal" to produce the typically muscled neck, or the musculature can hug the cervical column as a "swan neck." In other words, the only structural difference between an ideal arched neck and a swan neck is how the musculature manifests; they're essentially the same thing when it comes to internal orientation.

In addition, when it comes to riding, the more important quality of the neck isn't length, but shape and set. In other words, length doesn't compromise a horse's ability to achieve bascule, but shape and set certainly will. Straighter necks have a harder time while ewe-necks or hammer-heads have a decidedly difficult time achieving bascule. So what we tend to look for in a "riding neck" is a high set with an arch and "mitbah," even if the neck is a bit short. Indeed, a neck that's too long leads to cervical dislocations and that translates into nerve damage, pain, and comprised performance.

Skeletal Structure

The neck consists of seven cervical vertebrae that form an S-shaped curve. The cervical column doesn't circumscribe the crest, but begins deep within the body at the first thoracic vertebra, approximately mid-shoulder, and terminates at the back of the skull, at the poll.

The Atlas bone attaches the head to the neck and has a prominent “wing” on either side, both protruding almost as wide as the brow when viewed from above. The Axis bone attaches the Atlas and head to the rest of the cervical column. The rest of the cervical vertebrae are built somewhat similarly.

When the attachment between the Atlas and the Axis bone is long, that produces the long "mitbah," that "hooked" neck, or long breakover, so prized in Arabian breeding.

Basic Musculature of the Neck

The nuchal ligament runs down the crest from the poll (the funicular portion), over the withers and blends with the dorsal ligament of the back. It also runs down into the neck to connect to the cervical vertebrae C2-C5 (the laminar portion), though the connection to C5 is sometimes thinner. It acts as a strong brace between the thorax, skull, and cervical vertebrae. It also supports much of the burden of the head without fatiguing the muscles and allows it to be raised and lowered easily. It plays a role in bascule, too, by helping to raise the base of the neck where it meets the thoracic vertebrae.

As for the muscles, the basic ones are:
  • Longissiumus capitis: Helps to elevate the neck and torso.
  • Semispinalis colli: A continuation of the semispinalis dorsi. Assists the longissiumus dorsi and longissimus capitis to elevate the neck and spine.
  • Intertransveralis colli: Aids to flex and extend the neck.
  • Longus colli (the cervical portion is also called the multifidus cervicis): Helps to telescope the neck and helps to produce neck flexion.
  • Rhomboideus (cervical portion): Helps to pull the scapula forward and upward, but when the horse is at rest, the cervical portion aids to lift the neck.
  • Splenius: Aids the lifting of the head or to bend of the neck.
  • Trapezius (cervical portion): Help pull the shoulder forward and the back laterally or up.
  • Serratus ventralis (cervical part): Part of the shoulder sling that can bend the neck laterally and lift the ribcage. Works with or antagonistically with its thoracic counterpart. 
  • Brachiocephalicus: Helps to pull the foreleg forward, extend the shoulder and flex the head and neck.
  • Omotransversarius: Helps to elevate the scapula and extend the forelimb.
  • Sternothyroideus and sternohyoideus: Helps to depress the jaw.
  • Omohyoideus (or subscapula-hyoidens): Aids swallowing.
  • Sternocephalicus: Helps to laterally bend or flex the head and neck.
  • Scalenus: Helps to telescope the neck, lifting the base of the neck, in bascule.
  • Cervicalis ascendens: Aids in neck flexion.
  • Rectus capitis lateralis (or small oblique of the head):: Aids in the extension of the head.
  • Rectus capitis ventralis major (or great Anterior Straight Muscle): Aids in neck flexion and flexes the head to help produce the beautiful curve to the neck and throatlatch area.
  • Rectus capitis ventralis minor: Extends the head.
  • Obliquus capitis anterior and posterior (or large oblique muscle of the head): Extends the head.
  • Obliquus capitis dorsalis major and minor (or posterior straight muscle of the head): Extends the head.
  • Anterior large oblique: Extends the head and rotates or corkscrews the axis joint.
  • Complexus (semispinalis capitis): Extends the head and can laterally flex it and the neck. Helps for form the beautiful curve to the neck and throatlatch area.
  • Longissimus capitus and atlantis (or trachelo-mastoid or complexus minor): Works to bend the head laterally or extend the head. The two strip shows clearly on a dry or well-muscled neck, especially when arched.
  • Cutaneus colli (or cervical panniculus): Helps to flex the neck and also acts as a “fly shaker."
Biomechanics of the Neck

The joints between Atlas and the skull, all the cervical vertebrae and the seventh cervical vertebra and the first thoracic vertebra all function, creating eight neck joints. Evolution commandeered T1 for the cervical column to get the head down to the grass, effectively turning it into a cervical joint.

The Atlas and Axis bones are of particular interest because their special features uniquely dictate neck articulation in this area. The Atlas, the first cervical vertebra, with its prominent wings on either side, is the broadest of the neck vertebrae. The structure on the back of the skull acts upon the Atlas wings in articulation, making the only possible motion of this joint an up-and-down “yes” movement, (extension and flexion), with negligible lateral flexion. Therefore, a horse cannot laterally turn or corkskrew his head behind the ears. A handy rule of thumb is that the Atlas wings should be parallel to the back of the skull, mirroring the orientation of the brow when viewed from above, regardless of neck motion. However, there's one exception: when the head is tucked, a marginal side-slippage can occur, referred to as "twirling of the head" in horsemanship. But this twirl can only happen when the head is dropped at the poll and tucked.

The equine neck cannot bend behind the ears (except for "twirling of the head") or at the Atlanto-Axial joint.

In contrast, the Axis, the second cervical vertebra, is the longest and largest in the neck and has a peculiar crest like a fin. Due to its structure and connection with the Atlas, the only motion permitted at the Atlas-Axis joint is a rotational, screw-driver “no” type of motion, with no lateral, or flexion or extension movement.

In accordance with the unique qualities of these bones, the Atlas and Axis bones are automatically engaged when the head is moved, so only at the third cervical vertebrae does true lateral flexion begin. In fact, if lateral flexion occurs significantly between the skull and Atlas, and between the Atlas and the Axis, the neck is broken. 

Cervical vertebra three to seven, on the other hand, have greater mobility, allowing for the spectrum of neck motion from extension, flexion, lateral bends, arches, twisting, etc. Yet whenever the cervical vertebrae are rotated, flexed, tucked, or extended, the whole neck is compressed or stretched in amebic fashion. In this way, the S-shape of the cervical column allows the neck to actually "lengthen" when stretched and "shorten" when tucked.

When the head is cranked to either left or right, as to look backward or scratch an itch, most of the bend occurs at the base of the neck, not in the middle. So if the neck is bent at the base, the inside curve will be shorter than the outside curve as the flesh morphs to accommodate, in a series of kinks rather than a smooth curve. In turn, the neck muscles running along the bottom of the neck don't curve in this posture, like a pipe-cleaner or ribbon, but find the "shortest length between two points" when the neck is curved thusly. In other words, those underline muscles will track a straighter path from the chest to the throat and not curve around, like a semi-circle.

Contrary to some belief, a "thick" cresty neck doesn't inhibit neck motion given the neck set and shape are ideally set high and arched. The only things that inhibit neck motion are its set, shape, riding, and riding gadgets such as martingales and tie-downs. A cresty neck can also wobble in motion, which can be a charming feature to instill in sculpture when appropriate. Likewise, the underline muscles of the neck can also wobble in motion, when they're relaxed. An excessive crest, however, indicates obesity, so it's a careful balance.

Landmarks and Reference Points

Boney Points of Reference

The wings of the Atlas bone are easily palatable under the surface, being obvious an landmark. Make sure they're parallel to the brows (when seen from above), unless there's "twirling of the head" going on in the sculpture. And while the rest of the tuberosities of the cervical vertebrae can be palpated with experience, they're essentially buried deep within muscle. That said, the transverse processes of the last three cervical vertebrae can be (gently) felt under the muscles.

Fleshy Points of Reference

The neck is a series of complex muscular layers that are highly mobile that makes finding consistent fleshy landmarks a bit tricky.

The most obvious neck muscle, despite motion, is the Sternomandibularis (or Sternocephalicus) and its resultant jugular groove it has with the Brachiocephalicus (or Omotransversarius). The other one would be the mass of the Complexus, that familiar curved teardrop-shaped muscle from behind the ear to the upper half of the shoulder. Another one would be the bulk of the Brachiocephalicus (or Omotransversarius). From the front, there's the Cervical panniculus, that "V" muscle above the sternum. Of course there's the crest as well. 

Artistic Aspects to Consider about the Neck

The neck is no easy part to sculpt. Its amebic nature, thanks to the S-curve paired with its uneven nature of articulation, causes some curious distortions whenever the neck is moved. But we can break it down to simple shapes to get our bearing. For example, we can see a curved "M," and certainly the jugular, and the outline made by the underlying musculature and the crest is simple enough.

The "M" configuration of the neck muscles. Foals make terrific anatomical studies since their lack of fat and robust muscle development doesn't obscure the simple structures.

From above, we should see that the Atlas wings are about as broad as the brows, yet the rest of the neck is relatively flat, making the neck rather narrow, but with a subtle hour-glass shape. It's a mistake then to make our necks broad and beefy unless we're sculpting one of those hulking European draft horses or squat, stocky types. On the other hand, it's also a flaw to sculpt our necks too thin and weedy.

We should also see that the neck is highly expressive of the animal's emotions. It can be held stiffly and rigidly if the animal is anxious, in pain, or stressed, or it can be highly arched when the animals is excited or has a piqued interest in something. When the horse shakes his head, such as dusting himself off with a good shake, the slack neck muscles will also wiggle and wobble in unison with the motion, creating some fun touches for sculpture.

The hide of the neck can also exhibit a lot of eccentricities and wrinkles, so look for them during observation or referencing photos. 

Common Artistic Faults with the Neck

The most common flaw in the neck are incorrect articulations between the skull and the Atlas, and between the Atlas and the Axis cervical vertebrae. It's an understandable mistake, however, since the unique functioning of those joints isn't so obvious in reference photos. Many sculpted necks also have asymmetrical, missing, or too-narrow wings of the Atlas bone.

Another typical flaw is "Crane Neck" in which the cervical column seems to be underneath the crest and hoisted from some pivot point at the wither. There also tends to no dip where the neck meets the wither, creating a straight drop from the top of the wither to the line of the back. Similarly, "Rainbow Neck" is the same idea, only with a highly arched neck, which also happens to be made oddly long (otherwise known as a "Giraffe Neck").

Many artists also misinterpret a bend that orients the head backwards laterally, creating a "Ribbon Neck" with too much length from "point A to point B," especially with the underline. Too often the crest and the underline musculature are smooth curves, too, along with the smooth curve of the sides of the neck, like it was a piece of licorice rope simply bending around. In reality, the equine neck turns as a series of kinks consistent to its anatomy, and  the underline muscles find the path of shortest distance, bending more on a straighter path than a curved one unlike the crest which does tend to curve. 

From above, misinterpreted necks also often lack the slight hour-glass shape, being like a straight block, or like a 2 x 4. Necks can also have overdeveloped or underdeveloped Atlas "wings" rather than them being somewhat broad as the brows, more or less. Many times when the head is tucked, we'll find a bulk of neck musculature appearing at the poll right behind the ears as well (when seen from the side), as if the head tucked between the Atlas and Axis bones, which indicates a broken neck. When the neck is turned, sometimes we'll see asymmetrical Atlas wings, too.

"Broken Neck Syndrome" is common in equine art, largely because an artist simply parrots what they see without a horsemanship perspective. Broken Neck Syndrome is when "push-pull" riding has caused a horse brace against the bit, to bend his neck not by arching the base, arching the column to then drop his head at the poll. Instead as he resists, he drops the base of his neck, clenches his jaw, and adopts a kink, or bend between the Axis bone and the 3rd cervical vertebra. This produces a distinct outline to the crest (and false collection throughout the body), often with a tuft of mane sticking up at the point of the bend. It's ubiquitous in modern dressage, but it's not the same roller, or hyperflexion, which is another sore misgiving. However, it's also common in racehorses (we can often see it as the jockey holds the horse back during the ride-up parade) and showing in general, even in Western Pleasure classes. It's also referred to as "Double-Hinged Neck" in Morgan circles as though it was a positive trait. 

This is an adopted posture, and being totally unnatural to the horse, it's very difficult to unlearn once it has been, and it has bad consequences for the soundness of the rest of his motion. (By the way, Sustainable Dressage is a very good resource to visit.)

All of these horses are suffering from Broken-Neck Syndrome. It's a mistake to look at the legs to judge collection when the neck (and the LS-Joint) is the surest, quickest indicator between false collection and self-carriage.

Art has even created a particularly odd form of Broken Neck Syndrome by which a curve or bump is added directly behind the ears on a tucked head, as though the horse's neck articulated between the skull and the Atlas bone. But since it cannot, this represents a broken neck.

This has been Photoshopped to mimic that bulge behind the ears so often mistakenly seen in equine sculpture. Perhaps the artist wanted to make the poll the only bending part by cheating this way, but that's not how the equine neck moves. This actually indicates a broken neck by exhibiting a bend between the Atlas and Axis bones.

Many necks have misinterpreted musculature as well, often with Complexus muscles misplaced or jugular groves incorrect or the orientation of the muscles around the throatlatch are incorrect. On that note, many sculptures inadvertently display poor horsemanship through the musculature of the neck. For example, an overdeveloped Rhomboideus means that there's too much dependence on riding gadgets or too much resistance to a tight rein which is causing the horse to brace his neck against the bit. 

Biological Aspects to Consider about the Neck

The neck isn't simply a means to connect the head to the body. In the horse, it's a critical mechanism for motion. It acts as an cantilever to the body and is an important component to  natural coordination and balance. Indeed, he tries to keep his neck (and therefore his head) in front of his pelvis as a function of normal coordination. It also plays a pivotal role in bascule, and that means it directly affects the rider, too.

We've seen necks get longer over the decades, as if that was a preferable quality. It may have been a reaction to necks that were thought too short, and not understanding that it was set and shape that were the more important attributes. Perhaps it was aesthetic. Whatever the reason, it's gotten to the point now that necks are generally too long, and that can lead to nerve damage and incoordination. Long-necked horses also take longer for their skeletons to mature, taking up to nine to ten years rather than the normal seven. As a general rule then, there should be no more than one head length between the back of the jaw and about where the wing of the Atlas occurs. However, there's some leeway for the Saddlebred since his neck is set so high that the "stacking" of the cervical bones protects against nerve damage a bit better than the "hanging" of the cervical bones, in say, the straight neck of a Quarter Horse. That said, long-necked horses are much more difficult to ride since they can become "rubber-necked" rather quickly and need a savvy, delicate hand.

As for length of neck, foals should have short necks. The spine is the last portion of the equine skeleton to mature, so that means a foal isn't long-legged, he's actually short-backed! So if we have a young foal with a really long neck, that's going to translate into an adult with a really long neck.

On another note, there's a tendency for the neck to be "dry" if the facial features are "dry." The skin is relatively thin on the neck as well, and lots of details can pop up. This is especially evident on Arabians, Tekes, and Thoroughbreds.

[Note: There's a congenital malformation of C6 and C7 in approximately 38% of Thoroughbreds and Thoroughbred derivatives. The malformation forces certain muscles to find other locations for insertion, causing abnormalities in musculature and movement. Being so, this malformation affects behavior, function, and posture, and can prevent the horse from attaining straightness and bascule. Symptoms include an inability to move straight despite intense schooling, a club foot on one foot and a long toe and low heels (often with crushed heels) on the other, uneven hips even when standing square, asymmetries in the neck musculature, crookedness of the neck, and sometimes asymmetrical chest musculature.]

Conclusion to Part IX

The equine neck is a beautiful thing, isn't it? Capable of so many lovely postures and so nicely muscled—it's like the proverbial "cherry on top" when it comes to sculpting equines! But it's also easy to get wrong, so practice and observation makes perfect.

So now that we've got neck under our belt, it's time to move onto the rest of the body, or rather the rest of the spine. The neck is part of the spine as well, but warranted its own section regardless. There's a lot to digest! And this is just the beginning. In Part X then, we'll get to the torso, the biggest portion we'll be discussing.

So until next time...go ahead and stick your neck out!

"Hasten slowly, and without losing heart, put your work twenty times upon the anvil."
~ Nicolas Boileau

Related Posts with Thumbnails