The cervical vertebrae (figs. 267, 273), seven in number, are the smallest of the movable vertebrae and can be identified easily owing to the peculiarity of their transverse processes, each of which is perforated by a foramen. The first, second and seventh cervical vertebrae present special distinguishing features, but the remaining four conform to a common type and are termed the typical cervical vertebrae.
A typical cervical vertebra.--General features.-The body is small and is broader from side to side than from before backwards. The vertebral foramen is large in proportion to the size of the body and it is triangular in outline. These two features are accounted for, in part, by "the direction of the pedicles, which project laterally as well as backwards (fig. 267). The superior and inferior vertebral notches are almost equal, for the pedicle is attached to the body nearly midway between its upper and lower borders. The laminae are relatively long and narrow, and are thinner above than below. The spine (spinous process) is short and bifid and its terminal tubercles are often unequal in size. The superior and inferior articular processes pro s form an articular pillar, which projects laterally at the junction of the pedicle and lamina. The transverse process is pierced by the foramen. Transversarium (fig. 267). It consists of an anterior and a posterior root, connected to each other on the lateral side of the foramen transversarium by a bar of bone, often termed the costotransverse bar. The anterior and lateral roots are homologous with the rib in the thoracic region; the posterior root is the homologue of the thoracic transverse process.
Particular features.-The anterior surface of the body is convex from side to side, and its upper and lower borders, give attachment to the fibers of the anterior longitrndinal ligament. On each side of the ligament a slight. depression rives attachment to fibers of the vertical portion of the longus cervicis (longus colli) muscle. The posterior surface is flattened and pierced near its centre by two or more vascular foramina, which transmit the basivertebral veins. Its upper and lower borders give attachment to the posterior longitudinal ligament. The superior surface is concave transversely and bas an upwardly projecting lip on each side (fig. 273); its anterior border may be slightly bevelled. The inferior surface is saddle shaped, being convex from side to side and concave from before backwards. A small synovial joint is placed between the bevelled lateral border and the projecting lip of the upper surface of the vertebra, below. The anterior border of the lower surface projects downwards and covers the intervertebral disc. The upper borders of the laminae and the lower part of their anterior aspects give attachment to the ligamenta flava. The spines give attachment to the ligamentum nuchum and to a number of the deep muscles of the back of the neck, viz. semispinalis thoracis, multifidus, interspinales, spinalis cervicis and semispinalis cervicis.
The articular pillars of the third and fourth cervical vertebra are grooved on their lateral aspects by the posterior primary rami of the third and fourth cervical nerves, which pass backwards across them. The superior articular facets are flat and oval in outline; they are directed backwards and upwards. The inferior articular facets, similar in shape, are directed forwards and downwards. The foramens transversarium transmits the vertebral artery and veins and a plexus of oblique sympathetic nerve fibers. The anterior root of the transverse process ends in a rough tubercle, which gives attachment to the scalenus anterior, the longus capitis and the oblique portions of the longus cervicis (longus colli) muscles. The anterior tubercle of the transverse process of the sixth cervical vertebra is enlarged and lies posterior to the common carotid artery, which can be compressed against it. It is therefore termed the carotid tubercle. The costotransverse bar is the third cervical vertebra, passing downwards, backwards and laterally; in. the fourth also it is oblique, but it is slightly grooved by the emerging anterior primary ramus of the fourth cervical nerve. In the fifth the groove is deeper, and in the sixth it is conspicuously wide and shallow. The posterior root of the transverse process terminates laterally in a rounded tubercle, which lies lateral to the anterior tubercle and at a lower level in all except the sixth, in which the two tubercles lie approximately on the same plane The posterior tubercles give attachment to a number of muscles, including levator scapula, scalenus medius, scalenus posterior, splenius cervicis, longissimus cervicis and costocervicalis (iliocostalis cervicis).
The first cervical vertebra.---General features: The first cervical vertebra (fig. 269) is named the Atlas, because it supports the globe of the head. It differs from all the other vertebra in having no body, and this is due to the fact that the centrum of the atlas has fused with the centrum of the second cervical vertebra. In addition, it has no spine. The atlas consists of two bulky lateral masses, connected to each other in front by a short anterior arch, and behind by a long, curved, posterior arch. It therefore forms a ring of bone. When the atlas and the second cervical vertebra are articulated together, an upward projection from the latter, which is termed the odontoid process and is the centrum of the atlas, comes into position behind the anterior arch, and the resemblance to a vertebra is restored (fig.. 269).
The anterior arch is slightly curved from side to side, with a forward convexity which is accentuated by the presence of a roughened anterior tubercle. Its posterior aspect is marked by a median, oval or circular facet, which articulates with the front of the odontoid process (dens). The lateral mass is set obliquely, and its long axis runs forwards and medially. Its upper surface forms an elongated, concave facet for articulation with the prominent condyle of the occipital bone of the skull. The inferior surface is covered with a nearly circular facet, flattened or gently concave for articulation with the superior facets of the second cervical vertebra. The posterior arch forms for about two-fifths of the ring. Its upper surface shows a wide groove immediately behind the lateral mass, and its spine is replaced by a small, rough posterior tubercle. The transverse process is unusually long (fig. 273) and, as a result, the width of the atlas, measured from tip to tip of its transverse processes, greatly exceeds that of any other cervical vertebra, except the seventh. The length and strength of these transverse processes enables them to function as adequate levers for the rotatory movements of the head, which are effected by the rotation of the skull and the atlas around the pivot provided by the odontoid process of the second cervical vertebra.
Particular features..---The anterior tubercle gives attachment to the anterior longitudinal ligament in the median plane : its lateral aspect provides insertion for the upper oblique portion of the longus cervicis (longus colli) muscle. The upper and lower borders of the anterior arch give attachment respectively to the anterior atlanto-occipital membrane and the lateral fibers of the anterior longitudinal ligament (anterior atlanto-axial membrane). The superior articular facet of the lateral mass is directed upwards and medially and is admirably adapted to the nodding movements of the head, which occur at the atlanto-occipital joints. It is usually constricted near its middle and may be subdivided into two separate areas. Its margins give attachment to the articular capsule of the, atlanto-occipital joint. The inferior articular facet is directed downwards, medially- and slightly backwards ; its margins give attachment to the articular capsule of the atlanto-axial joint. The medial aspect of the lateral mass presents a small roughened tubercle for the attachment of the transverse ligament of the atlas (fig, 516), which passes behind the odontoid process (dens) and helps to retain it in place. This ligament divides the ring of the atlas into an anterior smaller part, which contains the odontoid process (fig. 269); and a posterior larger part, which transmits the spinal cord and its membrane: the anterior aspect of the lateral mass gives origin to the rectus capitis anterior muscle.
The posterior tubercle gives attachment in the median plane to the ligamentum nuchae and on each side, to the rectus capitis posterior minor. Its small size prevents it from interfering with the nodding movements of the head. The upper surface of the posterior arch is grooved anteriorly, where it is overhung on each side by the lateral mass. The groove transmits the vertebral artery, which winds backwards and medially round the posterior aspect of the lateral mass (.fib. 518). Occasionally the groove For the vertebral artery is converted into a. foramen by a bony spicule which arches ba:ektt-ards from the posterior part of the upper surface of the lateral mass. The first cervical nerve. As it emerges from the vertebral canal. intervenes between the vertebreal artery and its groove (fig. 518). Behind the groove on each side. the upper border of the posterior arch gives attachment to the posterior atlanto-occipital membrane ; its lower border gives attachment to the highest pair of ligamenta flava.
The length of the transverse process and its functional significance have already received mention. Its down-turned extremity, which. shows no differentiation into anterior and posterior tubercles, may be felt through the shin between the tip of the mastoid process and the angle of the mandible. Its recognition is not easy for it lies deeply under cover of the parotid gland. Numerous muscles are attached to the process. The rectus capitis lateralis, anteriorly, and the superior oblique muscle of the head, posteriorly, arise from its upper surface : the levator scapula takes origin from its lateral margin and lover border, concealing the uppermost slip of insertion of the splenius cervicis and itself often hidden in the origin of the scalenus medius. A small, often imperceptible, tubercle may be present on the anterior aspect of the lateral mass; it represents the anterior tubercle of the typical transverse process, so that the transverse process of the atlas corresponds to the posterior root and the costotransverse bar. The anterior primary ramus of the first cervical nerve runs forwards on the lateral aspect of the lateral mass and then turns downwards across the anterior aspect of the transverse process. As it does so it is covered by the internal jugular vein, which is crossed in this situation by the accessory nerve and the occipital artery.
The second cervical vertebra. General features : The second cervical vertebra, which is named the Axis (figs. 270, 271), provides the pivot upon which the atlas, and with it the skull, rotate. It can easily be distinguished font the other vertebrae by means of the strong tooth-like process, named the odontoid process (dens), which projects upwards from the body.
The process bears on its anterior surface a small oval facet for articulation with the facet on the posterior surface of the anterior arch of the atlas, and the lower part of its posterior surface is grooved slightly by a ligament which helps to retain it in position (fig. 270). On each side a large, oval or circular facet is borne on the lateral part of the upper surface of the body and the adjoining part of the pedicle for articulation with the inferior facet of the lateral mass of the atlas.
The laminae are thick and strong and the vertebral foramen is large and roomy. The spine is large and very strong, for it has to provide attachments not only for muscles which extend the neck but also for muscles which retract the head and rotate it from side to side. The transverse process, on the other hand, is very small and its blunt tip shows no differentiation into anterior and posterior tubercles. The foramen transversarium is directed upwards and laterally. The inferior articular facets look downwards and forwards, as they do in the typical cervical vertebra.
Particular features.-The odontoid process is conical in shape and about 1-5 cm. long; it is constricted below, where it lies in contact posteriorly with the transverse ligament of the atlas. A bursa is usually interposed between the bone and the ligament. The apex is pointed and gives attachment to the apical ligament. Below the apex the sides of the process are flattened. where the alar ligaments are attached (fig. 270). In structure the odontoid process is more compact than the body. The upper surface of the body is obscured by the odontoid process and the superior articular facets. Its anterior surface presents a hollowed out impression on each side of the median plane for the insertion of vertical fibers of the longus cervicis (longus colli) muscle. The lower border gives attachment to the anterior longitudinal ligament, and its downward projection is a characteristic feature of the axis. Posteriorly the lower border of the body gives attachment to the posterior longitudinal ligament and to the membrana tectoria, which represents its upward continuation to the occipital bone. The pedicles are stout and the inferior vertebral notches are deep, in contrast with the superior notches, which are scarcely discernible.
The laminae are thicker and stronger than the laminae of any of the other cervical vertebrae ; they provide attachment for the ligaments. flava. The coarse, strong spine gives origin to the inferior oblique muscle; which arises from a rough impression on its lateral aspect, and also to the rectus capitis posterior major, which arises from its posterior border. The wide gap at its extremity gives attachment to the ligamentum nuchae. In addition it receives the insertions of portions of the semispinalis cervicis, spinalis cervicis, interspinalis and multifidus. The foramen transversarium is directed upwards and laterally as, owing to the difference in length between the transverse process of the axis and that of the atlas, the vertebral artery must deviate in a lateral direction after leaving the axis. The small transverse process resembles the large process of the atlas in the position of its anterior tubercle, which is situated at or near the point where the anterior part of the process reaches the body. Its extremity gives origin to the levator scapulae and, more anteriorly, to the scalenus medius; more posteriorly, it gives insertion to the splenius cervicis. Intertransverse muscles also gain attachment to its upper and lower aspects.
The seventh cervical vertebra.--General features: The seventh cervical vertebra (fig. 272) is named the vertebra prominens because of its long spine, the tip of which can be felt through the skin at the lower end of the furrow. This process is thick and nearly horizontal in direction; it is not bifurcated but ends in a tubercle. The transverse processes are of considerable size and their posterior parts are large and prominent. .The anterior parts are usually slender, but they may form separate bones, which are then known as cervical ribs. The foramen transversarium is relatively small; it is sometimes double, or it may be entirely absent.
Particular features - The tip of the spine gives attachment to the lower end of the ligamentum nuchae , as well as to a number of muscles. These include the trapezius, rhomboideus minor, serratus posterior superior, splenius cervicis, semispinalis thoracis, spinalis cervicis, interspinales and multifidus. The foramen transversarium is traversed by an accessory vertebral vein or veins : it is traversed by the vertebral artery only on very rare occasions. The costotransverse bar of the transverse process, which shows a shallow groove for the anterior primary ramus of C7, is often partly deficient. The prominent posterior tubercle gives origin to the scalenus minimus, when it is present, and to the aponeurotic layer of fascia which covers the cervical dome of the pleura [suprapleural membrane]. Its lower border gives origin to the highest of the levatores costarum muscles.