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Similar to many other animals, the horse has a spine that is made up of a number of vertebrae, individual bones that fit together in a way that allows the back to flex both vertically and horizontally. Each of these bones is designated by a letter and a number based on where it is in the body and within each group. Horses have an average of 54 vertebrae, though the number can vary between 51 and 58.

Cervical vertebrae form the horse’s neck; thoracic vertebrae sit above the rib cage; lumbar vertebrae are generally under the back of a saddle and in front of the highest point of the hindquarters; sacral vertebrae slope down to the top of the horse’s tail; and caudal vertebrae lie within the fleshy part of the tail.

Beginning just back of the skull, seven cervical vertebrae make up the neck area. The first two, the atlas and axis, are known as C1 and C2. These bones help to support the skull and allow the horse to nod its head. Rather than lying just below the mane, the rest of the cervical vertebrae dive almost vertically from the skull, meeting the more horizontally oriented part of the spine at the horse’s shoulder.

Horses have 18 thoracic vertebrae designated T1 to T18. These form the part of the spine that includes the withers and part of the back. In the withers area, the vertebrae have prominent vertical flanges called dorsal spinal processes. These flanges are the reason the withers are higher than the rest of the back. Their purpose is to form a point of attachment for the heavy muscles supporting the head, neck, and shoulders. Each thoracic vertebra also anchors a set of ribs. An occasional horse has an extra vertebra in the thoracic region, though this does not make the back appreciably longer than in other horses.

The six lumbar vertebrae, L1 to L6, make up the spine between the thoracic vertebrae and the pelvis. Their dorsal spinal processes are heavier and taller than the ones in the middle of the horse’s back, but not as high as those in the withers. Some horses have only five lumbar vertebrae. This characteristic is found in quite a few Spanish Mustangs and Arabian horses. The strength of a horse’s back and how much weight he can comfortably carry are dependent on the size of the vertebrae in this area.

The five sacral vertebrae are fused together to form the sacrum which runs from the pelvis to the upper end of the tail. Because they act as one solid bone, the sacral vertebrae do not flex as the rest of the spine does. The sacrum forms a point of attachment for the ligaments and muscles of the pelvis, and also helps to anchor the hind legs.

The smallest vertebrae are found within the fleshy part of the horse’s tail.  Horses have an average of 18 of these caudal vertebrae, with some horses having as many as 25 and others only 15.

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