The Cervical vertebrae.
These are seven in number, as in almost all mammals. They are characterised by the fact that they have small ribs fused with them, forming transverse processes perforated by canals through which the vertebral arteries run.
The first, or atlas vertebra (fig. 69, A), differs much from all the others; it is drawn out into a pair of wide wing-like transverse processes (fig. 69, A, 1), and forms a ring surrounding a large cavity. This cavity is during life divided into two parts by a transverse ligament; the upper cavity is the true neural canal, while the lower lodges the odontoid process of the second vertebra, which is the detached centrum of the atlas. The neural arch is broad and regular; it has no spinous process, and is perforated in front by a pair of foramina for the passage of the first spinal nerves. The mid-ventral portion of the atlas is rather thick, and bears a minute backwardly-projecting hypapophysis. The bases of the broad transverse processes are perforated by the vertebrarterial canals (fig. 69, A, 2). The atlas bears at each end a pair of large articulating surfaces; those at the anterior end articulate with the condyles of the skull, and are very deeply concave; those at the posterior end for articulation with the axis, are nearly as large, but are flattened. The atlas ossifies from three centres, one forming the mid-ventral portion, the others the two halves of the remainder.
The second, or axis vertebra (fig. 69, B), also differs much from the other cervicals. The long and broad centrum has a very flat dorsal surface, and is produced in front into the conical odontoid process (fig. 69, B, 5), and bears a pair of very large convex outwardly-directed surfaces for articulation with the atlas. At its posterior end it is drawn out into a pair of small backwardly-directed spines, the transverse processes; these are perforated at their bases by the vertebrarterial canals. The neural arch is deeply notched in front and behind for the passage of the spinal nerves, and is drawn out above into a very long compressed neural spine (fig. 69, B, 4), which projects a long way forwards, and behind becomes bifid and thickened, bearing a pair of flat downwardly directed postzygapophyses. In the young animal the odontoid process is readily seen to ossify from a centre anterior to that forming the anterior epiphysis of the axis.
The remaining five cervical vertebrae, the third to the seventh inclusive, have rather flattened wide centra, obliquely truncated at either end. The neural spine progressively increases in size as the vertebrae are followed back. The transverse processes vary considerably; those of the third are divided into a thicker backwardly-, and a more slender forwardly-projecting portion; those of the fourth and fifth mainly extend downwards, and that of the sixth is divided into a horizontal portion and a downwardly-projecting inferior lamella. All the cervical vertebrae except the seventh have the bases of the transverse processes perforated by the vertebrarterial canals. The prezygapophyses in each case look upwards and slightly inwards, while the postzygapophyses look downwards and slightly outwards.
The Thoracic vertebrae.
The thoracic vertebrae are twelve or thirteen in number, and all bear movably articulated ribs. As a group they are characterised by their comparative shortness, and in the case of the first eight or nine by the great length of the backwardly-sloping neural spine. The posterior thoracic vertebrae approach in character the succeeding lumbar vertebrae.
As type of the anterior thoracic vertebrae, take any one between the second and sixth inclusive. The centrum is short, and has its terminations vertically truncated. At the top of the centrum, at both anterior and posterior ends on each side, is a demi-facet (fig. 70, A, 4), which, together with that on the adjacent vertebra, forms an articulating surface for the capitulum of the rib. The neural arch is small and deeply notched behind for the passage of the spinal nerve. It is drawn out above into a very long neural spine (fig. 70, A, 1), whose base extends back over the succeeding vertebra and bears the downwardly-directed postzygapophyses (fig. 70, A, 6). The summit of the neural arch is deeply notched in front, and on each side of the notch are the prezygapophyses, which look almost vertically upwards. The transverse processes are short and blunt, and are flattened below (fig. 70, A, 3) for the articulation of the tubercula of the ribs.
Fig. 70. A, second thoracic, and B, second lumbar vertebra of a Dog (Canis familiaris) SEEN FROM THE RIGHT SIDE (after von Zittel).