The vertebral column is divided for convenience of description into five regions—the cervical, thoracic or dorsal, lumbar, sacral, and caudal. This division is useful, especially as it is not entirely arbitrary, and in most cases is capable of ready definition; but at the contiguous extremities of the regions the characters of the vertebræ of one are apt to blend into those of the next region, either normally or as peculiarities of individual skeletons. [Illustration: FIG. 11.—Anterior surface of sixth cervical vertebra of Dog. _s_, Spinous process; _az_, anterior zygapophysis; _v_, vertebrarterial canal; _t_, transverse process; _t′_, its inferior lamella.] _Cervical Vertebræ._—The _cervical_ region constitutes the most anterior portion of the column, or that which joins the cranium. The vertebræ which belong to it are either entirely destitute of movable ribs, or if they have any these are small, and do not join the sternum. As a general rule they have a considerable perforation through the base of the transverse process (the vertebrarterial canal, Fig. 11, _v_); or, as it is sometimes described, they have two transverse processes, superior and inferior, which meet at their extremities to enclose a canal. This, however, rarely applies to the last vertebra of the region, in which only the upper transverse process is usually developed. The transverse process, moreover, very often sends down near its extremity a more or less compressed plate (inferior lamella), which, being considered serially homologous with the ribs of the thoracic vertebræ (though not developed autogenously), is often called the “costal” or “pleurapophysial” plate. This is usually largest on the sixth, and altogether wanting on the seventh vertebra. The first and second cervical vertebræ, called respectively “atlas” and “axis,” are specially modified for the function of supporting and permitting the free movements of the head. They are not united together by the intervertebral substance, but connected only by ordinary ligaments and synovial joints. The cervical region in mammals presents the remarkable peculiarity that, whatever the length or flexibility of the neck, the number of vertebræ is the same, viz. seven, with the exception of the Manatee and Hoffman’s Two-toed Sloth (_Cholœpus hoffmanni_), which both have but six, and the Three-toed Sloth (_Bradypus tridactylus_), which has nine, though in this case the last two usually support movable ribs, which are not sufficiently developed to reach the sternum. According to Parker there may occasionally be eight cervicals in the Pangolins (_Manis_). _Dorsal Vertebræ._—The _dorsal_ (or, as it would be more correctly termed, _thoracic_) region consists of the vertebræ succeeding those of the neck, which have ribs movably articulated to them. These ribs arch round the thorax—the anterior one, and usually the greater number of those that follow, being attached below to the sternum.