<P> In birds, there is a variable number of cervical vertebrae, which often form the only truly flexible part of the spine . The thoracic vertebrae are partially fused, providing a solid brace for the wings during flight . The sacral vertebrae are fused with the lumbar vertebrae, and some thoracic and caudal vertebrae, to form a single structure, the synsacrum, which is thus of greater relative length than the sacrum of mammals . In living birds, the remaining caudal vertebrae are fused into a further bone, the pygostyle, for attachment of the tail feathers . </P> <P> Aside from the tail, the number of vertebrae in mammals is generally fairly constant . There are almost always seven cervical vertebrae (sloths and manatees are among the few exceptions), followed by around twenty or so further vertebrae, divided between the thoracic and lumbar forms, depending on the number of ribs . There are generally three to five vertebrae with the sacrum, and anything up to fifty caudal vertebrae . </P> <P> The vertebral column in dinosaurs consists of the cervical (neck), dorsal (back), sacral (hips), and caudal (tail) vertebrae . Saurischian dinosaur vertebrae sometimes possess features known as pleurocoels, which are hollow depressions on the lateral portions of the vertebrae, perforated to create an entrance into the air chambers within the vertebrae, which served to decrease the weight of these bones without sacrificing strength . These pleurocoels were filled with air sacs, which would have further decreased weight . In sauropod dinosaurs, the largest known land vertebrates, pleurocoels and air sacs may have reduced the animal's weight by over a ton in some instances, a handy evolutionary adaption in animals that grew to over 30 metres in length . In many hadrosaur and theropod dinosaurs, the caudal vertebrae were reinforced by ossified tendons . The presence of three or more sacral vertebrae, in association with the hip bones, is one of the defining characteristics of dinosaurs . The occipital condyle is a structure on the posterior part of a dinosaur's skull which articulates with the first cervical vertebra . </P>

Which curve of the backbone is the last to form