Overview
The horse has eighteen thoracic vertebrae (vertebral formula C7, T18, L6, S5, Cd15–21), more than cattle (13) or the dog (13). Together with the ribs and the sternum they form the rib cage. Their spinous processes are long in the cranial part and form the withers, the bony base of the interscapular region; from there they shorten progressively towards the lumbar region, where the saddle sits.
Key features
- Body (corpus vertebrae): short, with cranial and caudal costal foveae; the head of each rib articulates with two consecutive vertebrae.
- Spinous processes: the longest are around T3–T5 and form the highest point of the withers. Their tips are cartilaginous in the foal and ossify slowly. They slope caudally as far as the anticlinal vertebra (T16), whose process is vertical; the following ones slope cranially, like the lumbar ones.
- Transverse processes: short, with a transverse costal fovea for the tubercle of the rib of the same number. In the last thoracic vertebrae they carry mamillary processes.
- Supraspinous ligament: joins the tips of the spinous processes; at the withers the supraspinous bursa separates it from the bone.
- Mobility: the thoracic back of the horse is not very flexible; most dorsoventral flexion takes place at the lumbosacral joint.
Clinical relevance
- Overriding spinous processes (kissing spines): the spinous processes of the vertebrae under the saddle, mainly between T13 and T18, lie too close, touch or overlap. It is a recognised cause of back pain, although the radiographic changes are also found in horses without signs.
- Fistulous withers: inflammation and infection of the supraspinous bursa, with draining tracts at the withers; historically associated with Brucella abortus.
- Fractures of the withers: of the spinous processes of the first thoracic vertebrae, in horses that rear and fall over backwards.
- Saddle fit: a saddle that presses on the withers or on the area of the last thoracic vertebrae causes pain, skin lesions and gait changes.