Great mobility is secured by the particular arrangements observable in the region where the two upper cervical vertebrae articulate with each other and with the skull, and in the region of the lower dorsal and upper lumbar vertebrae. On the other hand, the transverse processes of the lower cervical vertebrae and the imbricated neural spines of the upper dorsal vertebrae prevent the possibility of any great range of movement between any two of the constituent segments of those portions of the spinal column.
The cervical vertebrae are seven in number, as almost invariably in the Mammalian class; the numbers of the dorsal and lumbar series are variable, but twelve and seven, the numbers of the dorsal and lumbar vertebrae respectively in the Rabbit, are very common numbers for those series throughout the class. The number of the caudal vertebrae is the most variable, that of the lumbar next, that of the dorsal less than that of the lumbar, that of the cervical the least variable of these four sets of vertebrae. As the number of the cervical vertebrae is all but invariable, the variability of the length of the cervical region depends upon variations in the length of the bodies of the seven vertebrae. The first cervical vertebra or 'atlas' is the widest from side to side of all the neck vertebrae; it has a low but broad neural arch, and superadded to it in front a smaller arch which is in the perfect condition of the parts made into a ring for the reception of the 'odontoid process' of the next vertebra by a transverse ligament. Its neural arch is overhung by the spine of that vertebra, and it does not give any point of attachment to the ligamentum nuchae.
It contains two more or less separated canals for segments of the vertebral artery; one of them pierces the base of its broad 'transverse process' from behind forwards, the other turns more or less horizontally from without, inwards, behind and below the articular processes. This latter canal may be represented merely by a groove in the Rat, and ordinarily has this imperfect character in the human subject. The former has generally a short horizontal canal leading forward from it and opening on the anterior surface of the transverse process; it is however absent in the Leporidae, though present in the Rat and many or most other Rodents. The second cervical or 'axis' vertebra has its spine greatly developed, both anteroposteriorly and vertically, giving attachment by it both to the muscles which move, and the elastic ligamentum nuchae which supports, the head. It has no anterior articulating processes upon its neural arch in mammals, but it comes into articular relation with the atlas by means of two oblique zygapophysial surfaces developed on either side of the base and a third on the front of its odontoid process, which is the backwardly displaced and anchylosed centrum of that vertebra.
It is the deepest from above downwards, and the longest from before backwards, but also the narrowest from side to side of the cervical series. The first two cervical vertebrae articulate with each other and with the occiput by means of synovial joints as the neurapophysial processes are articulated to each other throughout the rest of the trunk, where however the centra are connected by interarticular fibrocartilaginous discs containing in their central pulp remnants of the primitive chorda dorsalis. The neural spines of the third and fourth cervical vertebrae are low but long, corresponding with the long neural roof which these two vertebrae possess; the spines of the shorter neural arches of the fifth, sixth, and seventh vertebrae have more of the shape which their name implies. The lateral processes or 'cervical ribs' of these vertebrae are greatly developed; those of the atlas more or less obliquely outwards, those of the axis backwards; those of the third, fourth, fifth, and sixth, both anteriorly and posteriorly, and those of the seventh outwardly.
The fourth, fifth, sixth, and seventh have prominent upgrowths developed on this process or rib which are homologous apparently with the prominent tubercles of the ribs of these creatures, or, possibly, with the metapophyses of the trunk vertebrae. This process makes up by itself almost the whole of the transverse process of the seventh cervical vertebra, the inferior, antero-posteriorly-produced, process, which is much larger in the preceding vertebrae and largest of all in the one immediately preceding, being lost in this, the last of the series. These inferior elements of the transverse processes, by bending inwards form with the vertebral bodies furrows, in which the long anterior neck muscles are lodged, a central slightly-raised line marking the line of separation of these muscles and representing the homologously-placed hypapophyses of certain lower verte-brata. The segment of bone which completes the ring of the atlas anteriorly is homologous with these hypapophysial downgrowths. The last cervical vertebra in the Rabbit has not, as it has in the Rat, any connection with the tubercle of the first dorsal rib.
Eight of the dorsal vertebrae, from the second to the ninth inclusively, have, each, two half facets on their centra, the first has one whole facet anteriorly and a half facet posteriorly, and the tenth, eleventh, and twelfth have, each of them, single whole facets placed on the anterior superior angle of the lateral aspect of their centra, for articulation with the heads of the ribs. The neural spines of the dorsal vertebrae are largely developed, their apices from the second to the ninth showing a tendency to become bifid antero-posteriorly. The tenth is the anticlinal vertebra (for which, see p. 8, supra), and upon it and upon each succeeding vertebra down to the sacrum a large, as upon the ninth and eighth dorsal vertebra a small, metapophysis is developed. A small anapophysis is also seen to take origin from the base of its neural arch, and to be possessed by each succeeding vertebra up to the antepenultimate lumbar. Several of the anterior, as also of the posterior dorsal vertebrae, have low hypapophysial ridges developed subcen-trally; and longer ones possessing the character of spines are developed on the three anterior lumbar vertebrae, in relation in the living animal with the crura of the diaphragm.
The Hedgehog, Erinaceus europaeus, and the Mole, Talpa europea, have paired unanchylosed ossicles developed intervertebral^ in the same region, like caudal chevron-bones. The lumbar vertebrae as wholes, and also most of their processes, increase in size from before backwards as far as the penultimate one; the transverse processes point obliquely forward, but form a more open angle with the long axis of the column than they do in the Rat. Behind the lumbar vertebrae we have, though not invariably, four vertebrae united to each other by anchylosis of their centra, their transverse, and their articular processes; and united to a fifth vertebra by anchylosis of the lateral processes. These five vertebrae may be taken as corresponding to the OS sacrum of anthropotomy. The two most anteriorly placed of these five vertebrae form by their transverse processes a pouched-shaped or auricular articular surface for the ilium, the posteriorly placed convex end of which is constituted by the transverse process of the second and the two sides by that of the first. In the Beaver the second vertebra contributes a relatively much smaller proportion to this articular surface, and in the Rat and many other Rodents it scarcely contributes anything.
The third and fourth of these post-lumbar vertebrae do not in any Rodent furnish any articular surface to the ilium.
The four or five anterior caudal vertebrae have largely developed sub-quadrate transverse processes, with their free angles, both anterior and posterior, somewhat produced. From eight to ten more rudimentary vertebrae follow upon these, the most posteriorly placed being merely bars of bone, with dilated ends corresponding to the articular aspects of the centra of other vertebrae. The caudal vertebrae of the Rabbit have no chevron-bones as have those of the the long-tailed Rodents, and of many other such animals from the Ichthyosauri to the Primates, with the exception of the Ungulata and the Proboscidea, which are allied in so many other points to Rodents.