Complex sentence: They are made up by the dorsal spinal processes of the first 5 to 9 thoracic vertebrae (every horse has 18 thoracic vertebrae), which are unusually long in this area and are less than 6 in height on the withers of the average horse.
Split into simpler sentences: They are made up by the dorsal spinal processes of the first 5 to 9 thoracic vertebrae (every horse has 18 thoracic vertebrae), which are unusually long in this area. The processes of the withers are less than 6 in height on the average horse.