Back (horse)

The back describes the area of horse anatomy where the saddle goes, and in popular usage extends to include the loin or lumbar region behind the thoracic vertebrae that also is crucial to a horse's weight-carrying ability. These two sections of the vertebral column beginning at the withers, the start of the thoracic vertebrae, and extend to the last lumbar vertebra. Because horses are ridden by humans, the strength and structure of the horse's back is critical to the animal's usefulness.

The thoracic vertebrae are the true "back" vertebral structures of the skeleton, providing the underlying support of the saddle, and the lumbar vertebrae of the loin provide the coupling that joins the back to the hindquarters. Integral to the back structure is the rib cage, which also provides support to the horse and rider. A complex design of bone, muscle, tendons and ligaments all work together to allow a horse to support the weight of a rider.

The structure of the back varies from horse to horse and varies a great deal by breed, age and condition of the animal.

A horse has an average total of 18 thoracic vertebrae, with 5 located in the withers. Each thoracic vertebra is also associated with a rib. A horse also has, on average, six lumbar vertebrae. Some breeds, such as the Arabian, will sometimes, but not always, have five lumbar vertebrae and 17 thoracic vertebrae. There appears to be little correlation between back length and number of vertebrae, as many horses with short backs do have the typical number of vertebrae. The length of each vertebra in the lumbar region seems to have the greater influence on the strength of the horse's back.

The horse has no collarbone. Hence the entire torso is attached to the shoulders by powerful muscles, tendons, and ligaments. The spine of a horse's back is supported by muscles, three ligaments, and abdominal muscles. The Spinalis Dorsi originates on the fourth cervical vertebra and inserts beneath the thoracic section of the Trapezius. The Longissimus dorsi originates from the last four cervical vertebrae, and courses along the spine, inserting eventually into the ilium and sacrum. This muscle contracts the spine and also raises and supports the head and neck, and is the main muscle used for rearing, kicking, jumping, and turning. It is the longest and strongest muscle in the body, and is the muscle the rider sits on. The Intercostal muscles begin at the spaces between the ribs and aid in breathing. The external and internal abdominal obliques are attached to the ribs and pelvic bones, and support the internal organs. The Supraspinous ligament begins at the and ends at the croup (sacral vertebrae). It supports the head and neck, and its traction force aids in supporting the weaker thoracic and lumbar areas. It spreads out and attaches to the spines of the cervical vertebrae. In the wither and neck area, it is called the nuchal ligament.

...
Wikipedia