Thoracic Region (Back)
The thoracic vertebrae are relatively slender but have tall dorsal spinous processes. Despite this, the region is very stable due to the rib cage, which forms a strong structural enclosure. The shoulder blades stabilize the front part of the body, covering approximately the 3rd to 7th or 8th thoracic vertebra.
The angle of the spinous processes tilts backward in the first thoracic vertebrae. Toward the end of the withers, the spines become more upright through the saddle region. The 16th thoracic vertebra stands vertically, after which the last two vertebrae tilt progressively forward, transitioning into the lumbar region.
Lumbar Region (Loins)
The lumbar vertebrae have relatively short, forward-leaning spinous processes and lack ribs. Instead, they feature flat transverse processes that project laterally in an almost horizontal plane.
As an anatomical variation, Arabian horses typically have only five lumbar vertebrae, compared to the usual six in most breeds.
Sacrum
The sacrum has a triangular shape and forms when the individual sacral vertebrae fuse between the ages of 3 and 4 years. Typically, there are five, but this number may vary between four and six. The sacrum continues from the last lumbar vertebra with backward-pointing spinous processes.
At the lumbosacral junction, these processes form a large "V" shape, allowing significant up-and-down mobility, which enables the horse to bring its hindquarters underneath its body.
On each side, the sacrum articulates with the pelvis via the sacroiliac joints — structures of great importance for transferring power from the hindquarters forward through the spine.