The spine of a horse generally contains 54 vertebrae, divided into cervical, thoracic, lumbar, recommended and caudal vertebrae. Horses have 7 cervical vertebrae, mostly 18 thoracic vertebrae, a very few 19, lumbar vertebrae 5-6, recommended vertebrae 5, tail vertebrae 15-25, an average of 18.
Differences in the number of vertebrae are particularly relevant to horse breeds, for example some Arabian horses may have 17 thoracic vertebrae instead of the common 18; there may be 5 lumbar vertebrae instead of the common 6; and there may be 16 or 17 caudal vertebrae instead of the common 18.
The main functions of the spine:
Protects the spinal cord, which is the main pathway for nerve information transmission, transmitting sensory and motor signals.
For muscle and ligament attachment
Protects the aortic artoa located under the vertebral bodies
Protects internal organs, such as the kidney located just below the spine
Provides strength to the suspension of the torso
Cervical vertebrae
The horse's cervical spine is the most flexible part of the spine. Viewed from the side, the horse's cervical vertebrae are arranged in an S-shape, lower position than we thought. The cervical spine protects the esophagus (oesophagus), the trachea, and the main cervical nerves and blood vessels.
Thoracic vertebrae
The thoracic segment is relatively stiff and not as flexible as the cervical segment. This is because the shape of the articular process, which fits the thoracic segments, and the strong fibrochondrite limit the activity of the thoracic spine. It is precisely because of the strength and hardness of the thoracic segment that the horse is able to carry the rider on its back. Spinous processes of the thoracic spine are the longest in the maned nails (withers). These spinous processes provide bone attachment sites and levers for muscles, as well as nuchal ligaments and supraspinous ligaments that affect the position of the head and neck.
18 pairs of ribs are connected to 18 cervical vertebrae through joints. The ribs protect the heart, lungs, liver, stomach, and part of the intestine. On inhale and exhale, the mobility of the ribs allows the thoracic cage to contract and expand. The ribs allow the torso to bend. When the trunk is bent, the spacing between the medial ribs decreases and the spacing between the lateral ribs increases.
Lumbar vertebrae
The lumbar segment consists of 6 vertebrae, which are flatter, wider and heavier than the thoracic vertebrae. This is the part of the horse's back that has the least flexibility. The large and horizontal transverse process of the lumbar spine provides a wide space for muscle attachment, increasing strength and stability. Any force generated by the horse's hind limbs is transmitted to the side of the skull through these 6 lumbar vertebrae. Lateral movement is gradually limited in the tail direction. The lumbar spine has neither pelvic support nor rib support, so this part of the spine is more tolerant of muscle soreness and strain.
背（the back）
The thoracic and lumbar vertebrae of a horse together make up the dorsal, or the dorsal refers to the thoracic-lumbar segment of the spine. The interlocking vertebrae on the spine, along with cartilage, tendons, ligaments, and muscle tissue, are equivalent to the structure of a suspension bridge. If any one of these parts is damaged, the rest of the back is subjected to excess pulling force, which creates dysfunction.
Sacral vertebrae
The 5 healed vertebrae form sacrum, which functions as a bone. The first vertebrae has a pair of prominent transverse processes called sacral wing.
The lumbar-sacral junction
The connection between the lumbar spine and the bone is called the lumbar joint. The last lumbar spinous process is tilted forward, the bony spinous process is tilted backwards, and the gap in the middle is very obvious and can be touched.
The lumbar joint allows the pelvis to tilt and has a very large effect on a horse's hind legs when it runs, jumps or performs high-end dance steps. Although the extension and flexion of the lumbar joint can reach 20 degrees, it is difficult to make this level of movement. Like people, our bodies can be forked, but only a few people can make forks.
The lumbar joint is the most flexible part of the spine after the neck and tail. The powerful forces produced by the muscles and bones of the hind limbs are absorbed at this location and transferred to the cranial side. When the horse moves or jumps rapidly on the hard ground, the moment the hooves touch the ground, the impact of the hindquarter meets the shock wave from the forelimbs at the lumbar joint.
Maintaining stability and flexibility in the lumbar joints is the key to collection, high-order dance moves, and successful jumps, and will also make the horse stand out in any sport. Any restriction on lumbar joint movement is not conducive to the way the horse moves, and it will also affect the horse's use of the hind limbs.
This joint is fragile, with saddle injuries, poorly balanced riders, unbalanced pressure, and muscle stretching all contributing to stiff muscles in the horse's back, reduced flexibility, limited movement, and impaired function. This is equivalent to us enduring back pain and doing exercise. Regular massage treatments, active and passive stretching, and a careful combination of gradual warm-up activities are very helpful in maintaining flexibility in the back muscles and lumbar joints.
Tail vertebrae
Most of the spinal cord and major spinous ligaments gradually disappear after the bone, and only a few muscles and ligaments remain in the caudal vertebrae, allowing the horse to swing its tail. The tail does little to the horse's athletic ability, but the posture of the tail is related to the health of the spine.
Movement and position of the tail
Horses communicate with their tails and express emotions. For example, a manic flicking of the tail indicates a horse's psychological or physical discomfort. An excited horse will raise its head, and its tail will also along with it. When a frustrated horse stands, it will bow its head and tail. A horse clamping its tail is considered an uncomfortable sign. Horses their tails during estrus season, and if there are stallions nearby, they will their tails to one side. A stallion will also its tail to indicate that it is interested in the mare. Breeders can quickly their tails and hang their tails to determine that the stallion's bodily fluids have been ejaculated.
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Resources:
Higgins G. 2012. Horse Anatomy for Performance: A Practical Guide to Training, Riding and Horse Care. David&Charles
CHEN Yaoxing. Anatomy of livestock and poultry. 3rd Ed. China Agricultural University Press
Wikipedia -Skeletal system of the horse