_Vertebral Column._—The _vertebral column_ consists of a series of distinct bones called vertebræ, arranged in close connection with each other along the dorsal side of the neck and trunk, and in the median line.[10] It is generally prolonged posteriorly beyond the trunk, to form the axial support of the appendage called the tail. Anteriorly it is articulated with the occipital region of the skull. The number of distinct bones composing the vertebral column varies greatly among the Mammalia, the main variation being due to the degree of elongation of the tail. Apart from this, in most mammals the number is not far from thirty, though it may fall as low as twenty-six (as in some Bats), or rise as high as forty (_Hyrax_ and _Cholœpus_). The different vertebræ, with some exceptions, remain through life quite distinct from each other, though closely connected by means of fibrous structures which allow of a certain, but limited, amount of motion between them. The exceptions are the following:—(1) near the posterior part of the trunk, in nearly all mammals which possess completely developed hinder limbs, two or more vertebræ become ankylosed together to form the “sacrum,” or portion of the vertebral column to which the pelvic girdle is attached; (2) in some species of Whales and Armadillos there are constant ossific unions of certain vertebræ of the cervical region. [Illustration: FIG. 10.—Side view of the first lumbar vertebra of a Dog (_Canis familiaris_). _s_, Spinous process; _az_, anterior zygapophysis; _pz_, posterior zygapophysis; _m_, metapophysis; _a_, anapophysis; _t_, transverse process.]