Excuse my ignorance but are these vertebrae nearer the withers? Vet will have told me but I was re-reading some notes about my horse yesterday from vet and can't remember!
C6 and C7 are the last cervical vertebrae at the 'withers end' of the neck though, if you look at a diagram of the horse's skeleton, there's actually quite a distance between these vertebrae and the withers themselves which are formed by the long spinous processes of the thoracic vertebrae.