Five different regions of the backbone may be distinguished, in each of which the vertebræ are modified in a characteristic way. There is (1) the _cervical_ region, or neck, the vertebræ of which, among mammals (with only one or two exceptions) are always seven in number, however long or short the neck may be; the immoderately long neck of the Giraffe has no more and the almost invisible neck of the Whale has no less, and thus the elongation of the neck is accomplished by lengthening the individual vertebræ and not by increasing their number. (2) Those vertebræ to which ribs are attached are named _dorsal_ or _thoracic_ and can always be recognized by the pits or articular facets which receive the heads of the ribs. (3) Behind the dorsal is the _lumbar_ region, or that of the loins, made up of a number of vertebræ which carry no ribs. The dorso-lumbars are known collectively as the _trunk-vertebræ_ and are generally quite constant in number for a given group of mammals, though often differently divided between the two regions in different members of the same group. In the Artiodactyla, for example, there are very constantly 19 trunk-vertebræ, but the Hippopotamus has 15 dorsals and 4 lumbars, the Reindeer (_Rangifer_) 14 D., 5 L., the Ox (_Bos taurus_) 13 D., 6 L., the Camel (_Camelus dromedarius_) 12 D. and 7 L. (4) Next follows the _sacrum_, which consists of a varying number of coalesced vertebræ. The number of sacral vertebræ varies from 2 to 13, but is usually from 3 to 5. (5) Finally, there are the _caudal_ vertebræ, or those of the tail, which are extremely variable in number and size, depending upon the length and thickness of the tail. We must next consider briefly some of the structural features which characterize the vertebræ of the different regions. (1) The length of the neck varies greatly in different mammals and, up to a certain point, flexibility increases with length, but, as the number of 7 cervicals is almost universally constant among mammals and the lengthening of the neck is accomplished by an elongation of the individual vertebræ, a point is eventually reached, where greater length is accompanied by a diminution of mobility. For instance, in the Giraffe the movements of the neck are rather stiff and awkward, in striking contrast to the graceful flexibility of the Swan’s neck, which has 23 vertebræ, more than three times as many. [Illustration: FIG. 11.—Atlas of Wolf, anterior end and left side. _cot._, anterior cotyles. _n.c._, neural canal. _n.a._, neural arch. _tr._, transverse process. _v.a._, posterior opening of the canal for the vertebral artery.]