Cervical Vertebrae

The horse has seven vertebrae in his neck. These are referred to as cervical vertebrae. 

The first vertebra attached to the skull is called the atlas, and the second down is called the axis. 

The cervical vertebrae do not follow the topline like often imagined. Instead they follow closer to where the jugular groove is located.

Thoracic Vertebrae and Ribs

The horse has 18 vertebrae which hold his ribs. These are referred to as the thoracic vertebrae. 

This section of the horse's back is very rigid, with only one or two degrees of movement between each joint. Therefore, the horse cannot physically "bend through his ribs" despite this being commonly referred to in training. 

Horses typically have 18 pairs of ribs which attach with synovial joints between the thoracic vertebrae. The first eight pairs are called the true ribs. They join directly to the sternum and protect the heart and lungs. 

The remaining ten pairs of ribs are referred to as floating or false ribs as they do not attach to the sternum. They attach to the neighboring ribs with ligaments and cartilage. 

Lumbar Spine

The horse's loin is behind the thoracic spine and consists of six vertebrae, referred to as the lumbar vertebrae. You can easily find the first lumbar vertebrae by feeling for the horse’s last rib and sliding your hand straight up to his spine. The spinal processes are very broad and provide an anchor for large ligaments and muscles.

Sacrum

The sacrum consists of five vertebrae which are fused together by the time the horse is five years old. The joint between the last lumbar vertebrae and the sacrum is called the lumbo-sacral junction. 

The first sacral vertebra has a large transverse process which is called the sacral wing, which forms the sacroiliac joint together with the iliac wing of the pelvis. 

The lumbo-sacral junction is the most flexible part of the spine after the neck and tail, having a range of motion of approximately 20 degrees. The horse uses this mobility to round his back in the canter and be able to tilt his pelvis. 

Caudal Vertebrae

The horse's tail is the end of his spine, typically consisting of 18 caudal vertebrae (although this can vary between breeds). The tail is strong and mobile. The horse uses it to communicate, to swish at biting insects, and to aid engaging the powerful hindquarter muscles.