Absolute and Relative Beauties of the Equine Vertebral Column

I am starting this post with a very brief synopsis of Goubaux and Barrier’s principles of ‘zootechnics’ to make sure my readers have context. You can find a longer introduction here. Even though The Exterior of the Horse was published in 1891, it still offers a very different approach from either veterinary pathologists who look for what is wrong after an issue arises or breeders and trainers of the competitive horse industry, who produce horses that sell young rather than horses that stay sound over the long term. Zootechnics is defined through the reciprocal fitness of the all parts of a horse for the work that they are required to do.

In order to answer those questions, Goubaux and Barrier divided the horse into three functional regions, the head and neck, the body, and the limbs. However, I have to admit that when it came to defining the absolute and relative beauties of the horse’s neck I had to take a few deep breaths and recover from a few false starts. First, I had to come to terms with the fact that horse breeders and trainers have denied and distorted the functioning of the horse’s central nervous system and vertebral column for so long that we have produced a whole array of pathologies of the equine neck and back in domestic horses. However distressing I find the mindset that can justify producing horses with predictable inheritable pathologies, that is a different subject for a different post.

I decided to start with the realization that most of the absolute beauties of the equine vertebral column are not easily discerned in the living horse. Paleontologists and equine anatomists are just now studying what exactly makes the vertebral column of grazing animals uniquely suited to eating with their heads down while being able to rapidly transition to a head up speedy get away. What has been learned so far is that the horse’s vertebral column is designed to become more rigid and so more efficient at redirecting the forces of gravity and thrust into forward motion as the horse speeds up.

The vertebral column is generally named in sections by type, cervical, thoracic, lumbar, sacral and caudal. Researchers have found that the number of the highly specialized cervical vertebra is consistently seven in all equids, including donkeys, zebra and quaggas. The number of caudal vertebra in the tail has the greatest range of variation. While the total number of thoracic, lumbar and sacral vertebrae in all equids is generally consistent at 30 bones, the number of each type varies between individuals, regardless of species. Equids of all kinds may have 5-7 lumbar or sacral vertebra and 17-19 thoracic vertebrae.

Anatomically, the horse’s vertebral column functions as one complete whole, from the tip of their tail to the complexity of their skull, jaw and hyoid bone. A Dutch researcher named E. J. Slijpner first described the dynamic functioning of the equine vertebral column in 1946. He illustrated the complex of muscle fascicules, each connecting three to five vertebrae that control the rotation of individual vertebra through their action on the spinal processes.

  • Above the horse’s spine, the latissimus dorsii and the spinus dorsii act as overlapping antagonists when they contract, increasing the rigidity of the horse’s spine as their speed increases
  • Below the horse’s spine, the muscles and connective tissue that stabilize the horse’s back and support their organs create such a complex individualized web of support that the whole construct is simply known as the multifidus complex.

Each and every equine vertebra is uniquely shaped. The faces of each vertebra are faceted so that muscle tension causes them to interlock tightly with their neighbors. The degree of rotation does vary between vertebrae. Like all bones, the vertebral processes respond to stress by increasing density. More details on how the height of the vertical processes of withers and croup reflect the direction and stresses on the vertebrae can be found in the posts on the fore and hindquarters.

  • The caudal vertebrae of the tail, much like the cervical vertebrae. have a fair degree of flexibility and rotation
  • The sacral vertebrae of the croup are fused and do not rotate at all
  • The lumbo-sacral joint has the greatest degree of rotation at 20+ degrees.
  • The lumbar vertebrae have minimal rotation and processes of the lumbar vertebra may be faceted so the whole surface locks together when the horse accelerates
  • The vertebrae connected with floating ribs have the greatest number of facets and degree of rotation of the thoracic vertebra.
  • The fused first eight thoracic vertebrae have a minimal amount of rotation. They connect to the fixed ribs that are in turn stabilized by the sternum.

Protection of the spinal cord is an absolute beauty of the spinal column. That protection is specialized in the horse as the neural reflex chains that control the patterns of movement of the front and hind legs are regulated by enlargements of the spinal cord. The cervical enlargement regulating the front legs is protected by the rigidity of the thoracic vertebra connected to the fixed ribs. The lumbar enlargement regulating the hind legs is protected by the rigidity of the fused sacral vertebrae. Each of these enlargements is sufficiently complex to be regarded as a small brain in their own right.

However, the horse’s balance and proprioception are directed by the structures of the horse’s head and neck. The activity of the hyoid apparatus and the movements of the skull, jaw, along with the movements of atlas and axis bones trigger specific neural reflexes. Horses can learn to consciously over ride their neural reflexes when humans demand it of them. This list of reflexes is their natural state when the horse is not in pain or interfered with by humans:

Reflex Chains Triggered by Pressure on the Back, Croup and Tail

  • Pressure along the horse’s back, from the withers to the lumbo-sacral joint will cause a horse to drop or dorsiflex their back.
  • Pressure along the horse’s croup, from the lumbo-sacral joint to the base of the tail will cause a horse to arch or ventroflex their back
  • Raising the horse’s tail will cause the horse’s hind legs to extend and stiffen. Lowering the tail initiates flexion in the hind legs. Research suggests that caudal neural reflexes area also active during normal locomotion. The influence of the tail is most evident at the gallop and over jumps.

Since the single boney joint connecting the horse’s vertebrae to any of their limbs is the sacro-iliac joint, the horse’s vertebral column actually functions much more like a cantilever or diving board than a bridge or a bow. Instead of a collarbone, the horse has a thoracic girdle composed of thick fibrous tendinous connective and muscle tissue that suspend the weight of the horse’s body between the shoulder blades. The muscles and tendons of the thoracic girdle extend along the underside of the neck to the hyoid process situated between the lower jawbones. The neck itself is the extreme end of the cantilever and has both an active muscular support and a passive tendinous support of connective tissue.

The absolute beauty of the superior border of the horse’s neck is defined by the passive support system of the nuchal ligament, which runs from the poll to the withers supporting the cervical vertebrae and the head. The top of the horse’s neck should be slightly convex, thin and erect, supporting the mane without collapsing.

Reflex Chains Triggered by Extending and Flexing the Neck include:

  • Dorsiflexion or elevation of the neck triggers the extension of the forelegs and flexion of thee hind legs
  • Ventroflexion or lowering of the neck triggers the flexion of the forelegs and extension of the hind legs.

The absolute beauty of the inferior border of the neck is thick and rounded, its shape defined by the trachea or windpipe and the esophagus. The jugular vein and carotid arteries also travel the length of the inferior side of the neck. That means there are also relative beauties of the equine neck and head.

  • Cold-blooded Trotters are often criticized for having thick coarse throatlatches. In actuality, this is a relative beauty as horses adapted to survive in cold damp climates prevent devastating heat loss by protecting the inner structures of the neck, the blood vessels, windpipe and esophagus, and warming both air and food before it reaches their core with a layer of fat, muscle, skin and hair
  • Hot-blooded Rectangular Gallopers have well- defined throatlatches with the blood vessels especially close to the surface. This is a relative beauty that allows horses adapted to hot humid climates to quickly dissipate heat.
  • The Square Horse has a unique throatlatch that is a relative beauty for horses adapted to arid, high altitude and rugged terrain to maximize agility while warming and moistening the air they breathe and perhaps even the food they eat.

In spite of all research and empirical evidence to the contrary, we humans still define and treat the head and neck of the horse as though they were discrete parts we could pop off and reposition on the horse’s body like parts of a plastic doll. However, there are vital neural reflexes triggered by shifts in the position of the horse’s head, neck, jaw and hyoid apparatus.

Vestibular Reflex Chains include:

  • Extension of the head (including the jaw) causes the forelegs to flex and the hind legs to extend as when grazing.
  • Flexion of the head (including the jaw) causes the forelegs to extend and the hind legs to flex, as when a grazing horse is startled
  • Tilting the head and/or bending the neck triggers neural reflexes that straighten the head and extend the legs on one side.
  • Pandiculation, also referred to as SYS, the Stretch-Yawn-Syndrome, is a reset of both the tension of the myofascial field and the central nervous system. Since horses are strictly nostril breathers, they do not exactly yawn. However, small movements of the complex joint of the lower jaw, hyoid apparatus via the tongue and the poll, the junction of the occiput of the skull and the atlas bone accomplish the same result. The ability to reset the tension of the myofascial field is especially important in an animal whose self-carriage depends on a sling of soft tissue to support their forehand. The ability to reset the degree of reactivity of the central nervous system, CNS, is especially important in an animal whose survival depends on their startle reflex.

Those who have questions about how the information in these posts can be pragmatically applied to schooling their horses can find detailed descriptions and instructions in my horse training series, Light in the Saddle. Those want to discuss the implication of these posts on work under saddle are welcome to join my group aimed at trainers and serious riders, The Isometric Rider.


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