Section 77. In Section 3 (which the student should refer to) we have a division of the vertebrae into four varieties. Of these most representative is the thoracic. A thoracic vertebra (Figure 4, Sheet 5, T.V.) consists of a central bony mass, the body or centrum (b.), from which there arises dorsally an arch, the neural arch (n.a.), completed by a keystone, the neural spine (n.s.); and coming off laterally from the arch is the transverse process (tr.p.). Looking at the vertebra sideways, we see that the arch is notched, for the exit of nerves. Jointed to the thoracic vertebrae on either side are the ribs (r.). Each rib has a process, the tuberculum, going up to articulate with the transverse process, and one, the capitulum articulating between the bodies of two contiguous vertebrae. The facets for the articulation of the capitulum are indicated in the side view by shading. At either end of the body of a vertebra of a young rabbit are bony caps, the epiphyses (ep.), separated from the body by a plane of unossified cartilage (indicated, by the dots). These epiphyses to the vertebral bodies occur only among mammals, and are even absent in some cases within the class. In the adult rabbit they have ossified continuously with the rest of the body. Section 78. A cervical vertebra (C.V.) seems, upon cursory inspection, to have no rib. The transverse processes differ from those of thoracic series in having a perforation, the vertebrarterial canal, through which the vertebral artery runs up the neck. A study of the development of these bones shows that the part marked f.r. ossifies separately from the rest of the transverse process; and the form of the equivalent structures in certain peculiar lower mammals and in reptiles leaves no doubt that f.r. is really an abbreviated rib; fused up with the transverse process and body. The two anterior cervical vertebrae are peculiar. The first (at.) is called the Atlas-- the figure shows the anterior view-- and has great articular faces for the condyles (Section 86) of the skull, and a deficient centrum. The next is the axis, and it is distinguished by an odontoid peg (od.p.), which fits into the space where the body of the atlas is deficient. In development the centrum of the axis ossifies from one centre, and the odontoid, peg from another, which at that time occupies the position of centrum of the atlas. So that it would seem that the atlas is a vertebra minus a centrum, and the axis is a vertebra plus a centrum, added at the expense of the atlas.