All the vertebrae anterior to the first one that bears a rib meeting the sternum are regarded as cervical vertebrae. There are therefore sixteen cervical vertebrae, the last two of which bear well developed ribs. All are freely movable on one another.
As a typical cervical vertebrae, any one from the fifth to the ninth may be taken. The vertebra is rather elongated, and is very lightly and strongly made, its most characteristic feature being the shape of the articulating surfaces of the centra, which are generally described as saddle-shaped. The anterior surface is convex from above downward, and concave from side to side, while the posterior and more prominent surface is concave from above downwards and convex from side to side. The neural arch is low, and is drawn out into a slight blade-like neural spine. Its base is deeply notched on both sides posteriorly for the exit of the spinal nerves. Above these notches it is drawn out into two rather prominent diverging processes, which bear the postzygapophyses,—two flattened surfaces which look downwards and outwards. The transverse processes form irregular outgrowths from the anterior two-thirds of the sides of the vertebra; each projects for a short distance downwards and outwards, and is terminated posteriorly by a short backwardly-projecting spine. The transverse processes are shown by development to ossify from separate centres, and are therefore to be regarded as cervical ribs, and each is perforated at its base by a canal for the passage of the vertebral artery. Above the anterior end of the vertebrarterial canal are a pair of thickened outgrowths, which bear upwardly and inwardly directed prezygapophyses. Each transverse process is perforated near its middle by a prominent foramen through which passes a vein which is connected with the jugular vein.
The third and fourth cervical vertebrae resemble the succeeding ones in most respects, but have small hypapophyses, and the neural spines are less blade-like. The posterior cervical vertebrae (tenth to sixteenth) differ somewhat from the middle ones. They are shorter and more massive, the neural arch is much shorter, being deeply notched in the middle line in front and behind. The transverse processes arise from the anterior half of the vertebra only, and in the eleventh vertebra each is drawn out below into a pair of rather prominent downwardly and inwardly directed processes. In the twelfth vertebra these processes have almost coalesced, and in the thirteenth vertebra they have coalesced completely, forming a prominent hypapophysis. In the succeeding vertebrae this hypapophysis rapidly decreases in size.
The fifteenth and sixteenth cervical vertebrae resemble the succeeding thoracic vertebrae, having short thick centra and prominent squarely truncated neural spines; the sides of the neural arches are very deeply notched. The fifteenth vertebra has a short transverse process, perforated by a wide vertebrarterial foramen, but this foramen is absent in the sixteenth. The transverse processes of the fifteenth vertebra bear two facets for the articulation of the capitulum and tuberculum of the rib. The sixteenth vertebra has its tubercular facet on the transverse process, but the capitular facet is borne on the centrum.
The second or axis vertebra is small, and has the centrum drawn out into a comparatively very large hypapophysis. The posterior articulating surface of the centrum is saddle-shaped, the anterior nearly flat: above it the centrum is prolonged into the prominent odontoid process, which is shown by development to be the detached centrum of the atlas. The neural arch is deeply notched in the middle line in front, and at the sides behind. It is drawn out posteriorly into a wide massive outgrowth, which overhangs the third vertebra and bears the downwardly-directed postzygapophyses. The prezygapophyses are situated at the sides of the anterior end of the neural arch, and look directly outwards. The transverse processes are very slightly developed, and are pierced by the vertebrarterial canals.
The atlas vertebra is a very slight ring-like structure, thickened ventrally and bearing in front a prominent concave cavity for articulation with the occipital condyle of the skull. Posteriorly it bears a more or less flattened surface for articulation with the centrum of the axis. It surrounds a large cavity partially divided into a larger dorsal portion, which is the neural canal, and a smaller ventral portion which lodges the odontoid process. The sides of the atlas are pierced by the vertebrarterial canals, above which there are two slight backwardly-projecting outgrowths bearing the postzygapophyses on their inner faces.
The Thoracic Vertebrae.
The thoracic region includes all the vertebrae bearing free ribs, except the first two, viz. those whose ribs do not reach the sternum. There are seven thoracic vertebrae. The first four have centra with saddle-shaped articulating surfaces, but are more or less firmly united together by their neural spines; the last two are completely ankylosed by their centra to the lumbar vertebrae.
Each of the first five vertebrae has a prominent, vertical, abruptly terminated neural spine, and straight transverse processes. The zygapophyses and articulating surfaces at the ends of the centra are well developed. The third, fourth, fifth, and sixth vertebrae have very prominent hypapophyses. The articular facets for the ribs are well marked, those for the tubercula lying at the free ends of the transverse processes, and those for the capitula at the sides of the anterior ends of the centra. The sixth and seventh thoracic vertebrae are firmly fused by their centra and neural arches to one another and to the lumbar vertebrae behind, and by their transverse processes to the ilia. The sixth has its centrum terminated in front by a saddle-shaped articulating surface, and bears a pair of prominent prezygapophyses. Its transverse processes and centrum bear facets for the tubercula and capitula of the ribs respectively. In the seventh vertebra the tubercular facet is wanting.