| 1. pro-atlas. | 7. tubercular portion of fourth |
| 2. lateral portion of atlas. | cervical rib. |
| 3. odontoid process. | 8. first cervical rib. |
| 4. ventral portion of atlas. | 9. second cervical rib. |
| 5. neural spine of axis. | 10. convex posterior surface of |
| 6. postzygapophysis of | centrum of fourth vertebra. |
| fourth vertebra. |
The Cervical vertebrae.
Counting as cervical all those vertebrae which are anterior to the first one whose ribs meet the sternum, there are nine cervical vertebrae, all of which bear ribs.
As a type of the cervical vertebrae the fifth may be taken. It has a short cylindrical centrum deeply concave in front and convex behind. From the anterior part of the ventral surface of the centrum arises a short hypapophysis, and on each side is a facet with which the lower limb (capitulum) of the cervical rib articulates. The neural arch is strongly developed and drawn out dorsally into a long neural spine, in front of which are a pair of upstanding processes bearing the prominent upwardly and inwardly directed prezygapophyses. At the sides and slightly behind the neural spine are a corresponding pair of processes bearing the postzygapophyses, which look downwards and outwards. At the point where it joins the centrum the neural arch is drawn out into a short blunt transverse process with which the upper limb (tuberculum) of the cervical rib articulates. The sides of the neural arch are slightly notched behind for the exit of the spinal nerves.
The first or atlas vertebra differs much from any of the others, and consists of four quite detached portions, a ventral arch, with two lateral portions and one dorsal. The ventral arch (fig. 41, 4) is flat below and slightly concave in front, forming together with two flattened surfaces on the lateral portions a large articulating surface for the occipital condyle of the skull. Its posterior face is bevelled off and forms with a second pair of facets on the lateral portions a surface with which the odontoid process of the second vertebra articulates. The postero-lateral surfaces of the ventral arch also bear a pair of little facets with which the cervical ribs articulate. The lateral portions are somewhat flattened and expanded, and bear in addition to those previously mentioned a pair of small downwardly directed facets, the postzygapophyses, which articulate with the prezygapophyses of the second vertebra. The dorsal portion (fig. 41, 1) is somewhat triangular in shape, and overhangs the occipital condyle. It is often regarded as the neural arch of a vertebra in front of the atlas and is called the pro-atlas; but as it is a membrane bone it is not properly a vertebral element.
The second or axis vertebra also differs a good deal from the other cervicals. The centrum is massive, and is terminated in front by a very large slightly concave articulating surface formed by the odontoid process (fig. 41, 3) which is united with the centrum by suture only, and is really the detached centrum of the first vertebra. The cervical rib (fig. 41, 9) articulates with two little irregularities on the odontoid process. The posterior surface of the centrum is convex. The neural arch is strongly developed and terminated dorsally by a long neural spine (fig. 41, 5), its sides are notched, slightly in front and more prominently behind for the exit of the spinal nerves. It is drawn out in front into two little processes bearing a pair of upwardly and outwardly directed prezygapophyses, while the postzygapophyses are similar to those of the other cervical vertebrae.
The last two cervical vertebrae resemble the succeeding thoracic vertebrae, in the increased length of the transverse processes, and the shifting dorsalwards of the facet with which the capitulum of the rib articulates.
The Thoracic vertebrae.
The thoracic vertebrae commence with the first of those that bears ribs reaching the sternum. They are ten in number, and the first eight are directly connected with the sternum by ribs.
The third of them may be taken as a type. It has a thick cylindrical centrum, concave in front and convex behind, there is a slight hypapophysis, and the centrum is suturally united with a strong neural arch enclosing a narrow neural canal. The neural arch is drawn out dorsally into a wide truncated neural spine, and laterally into two prominent transverse processes, with the ends of which the tubercula of the ribs articulate, while the capitulum articulates in each case with a step-like facet (fig. 42, A, 3) on the anterior face of the transverse process. The prezygapophyses (fig. 42, A, 2) are borne on outgrowths from the bases of the transverse processes, and the postzygapophyses on outgrowths at the base of the neural spine.