The centrum, or body, is elongated transversely, and of oval form : it is thick anteriorly, where a process, or spine (haemal), dips downwards in front of the vertebra which succeeds it; the upper surface is slightly concave from side to side ; the lower surface is convex; the posterior surface, bounding anteriorly the vertebral or neural canal, is smooth and flat.
The transverse process includes a parapophysis, a diapophysis, and a small pleurapophysis, or rib. The parapophysis is a small pedicle which arises from the lateral surface of the centrum; the diapophysis arises between and a little in front of the zygapophyses, or articulating processes. The interval between the two is completed into a foramen (vertebral) by the pleurapophysis, which is coalesced with the extremities of the dia and parapophyses, and is hollowed out into a groove, to receive the cervical nerve as it emerges from the intervertebral foramen : the extremity of this groove gives to the transverse process a bifid appearance. If so much of the transverse process of a cervical vertebra be sawn off as will lay open the vertebral foramen, the pleurapophysial element will be separated from the dia and parapophyses, with which it is united. The haemal arch is incomplete.
The neurapophyses commence by a constricted portion, the pedicle, which inclines outwards, and then becoming broader, bends inwards, to unite with its fellow at the neural spine, which, in the cervical region, is directed obliquely backwards and downwards, and retains in the bifid extremity the mark of its formation from two lateral halves. At the junction of the pedicle with the flatter portion of the neurapophysis are situated the zygapophyses and the root of the diapophysis. This spot is of interest, as being one of the primitive points of ossification of a vertebra. The deposit of bone extends in three directions, forwards and inwards, towards the centrum, where it unites with a central nucleus of bone ; backwards and inwards, along the cartilaginous neurapophysis to the neural spine ; and outwards, towards the cartilaginous pleurapophysis, to form the diapophysis. The parapophysis is a growth from the centrum.
The neural canal is triangular, and large in proportion to the segments of the nervous centre which it contains. The zygapophyses, or articulating processes, are oblique, the upper pair looking upwards and backwards, the lower pair downwards and forwards.
The neural canal contains the myelon (spinal chord) and its membranes, and conveys the accessory nerves to the interior of the cranium.
The lateral canals contain the vertebral artery, which enters the skull by the foramen magnum, or the neural canal of the occipital bone; the vertebral vein, which commences on the outer and back part of the head; and some branches of the sympathetic nerve.
The first, second, and seventh cervical vertebrae possess characters which require particular description.
The first, named the atlas, from its supporting the expanded globe-like cranium, is a ring of bone placed horizontally so as to rotate freely upon the second cervical vertebra, or the axis. The lateral masses, which terminate above and below in the zygapophyses, seem to divide the ring of the atlas into an anterior and a posterior arch.
The anterior arch, part of the centrum, is hollowed out posteriorly, where there is a smooth articulating surface, to receive the odontoid process of the axis. Upon the front of the thin arch-shaped portion of the centrum is a tubercle, the haemal spine, here rudimentary, and coalesced with the centrum. Were the haemal spine removed from the centrum, and the haemal arch completed by its proper bones, then the clavicles would leave their connection with the coraco-scapular arch, and extend as haemapo-physes from the centrum of the atlas to the haemal spine, or anterior tubercle.
The centrum is constricted vertically in the mesial line, to allow of the free downward movement of the head, in nodding, bowing, etc.
The transverse processes are long, and are composed of a parapophysis, directed obliquely downwards and outwards, which arises from the thick lateral masses of the centrum between the zygapo-physes; of a diapophysis more horizontal, which arises from the neural arch; and of a rudimentary pleurapophysis, coalesced with the distal extremities of the preceding, and completing the foramen for the vertebral artery. This pleurapophysis is not grooved to support the first pair of cervical nerves, because they escape from the neural canal behind the zygapophyses, and behind the whole transverse process ; but it is a smooth flattened tubercle, the anterior surface of which is directed obliquely forwards and upwards.
The transverse processes of the atlas are long, to afford leverage to the superior and inferior oblique muscles of the head.
The neurapophyses are thin and flat, and arch backwards, to terminate in a small posterior tubercle, the neural spine.
The upper pair of zygapophyses, hollowed out to receive the condyles of the occipital bone, are directed upwards and inwards, and converge somewhat anteriorly. At their junction with the root of the neurapophyses is a groove, which receives the vertebral artery previous to its entrance into the cranium, and which serves to transmit the first pair of cervical nerves. The lower pair of zygapophyses are nearly horizontal, and are flat, to allow of the rotatory movement of the atlas upon the axis. Upon the inner surface of the thick lateral portions of the atlas, just above the lower pair of zygapophyses, is a rough tubercle, which gives attachment to the transverse ligament. The ring of the atlas is divided by this ligament into an anterior and a posterior part; the anterior, the smaller, contains the odontoid process; the posterior, and larger, gives passage to the myelon, or spinal chord.
The second cervical vertebra, the axis, so called because the atlas rotates upon it as upon a pivot, is thicker and stronger than the atlas, but the transverse processes are not nearly so long and prominent. The under surface of the centrum, which articulates with the third cervical vertebra, is flat from side to side, but has a concave appearance from before backwards, in consequence of the prolongation downwards of a tongue of bone, as in the other cervical vertebras. The front surface presents a prominent ridge, the haemal spine, which separates two concavities from whence arise the oblique portions of the longus colli muscle. The posterior surface is rough and flat; it affords attachment to ligaments, and presents several large foramina, which contain, in the fresh state, veins from the interior of the bone.
The upper surface of the centrum seems prolonged into that prominent bony cylinder, the odontoid process, which fits into the atlas, so as to occupy the position of the centrum. In front there is a smooth oval surface, in the fresh state crusted by cartilage, which articulates with the smooth surface upon the concavity of the centrum of the atlas. Behind there is a narrow constricted portion, also smooth, and covered by cartilage, which seems to divide the odontoid process into a head and neck, and which lodges the transverse ligament of the atlas. The apex of the odontoid process is flattened, and rough, for the attachment of the oblique ligaments.
The transverse processes support the upper pair of zygapophyses, which, of large size, round, and nearly horizontal, lie on either side of the odontoid process, in front of the inferior pair of zygapophyses. The parapophysis is directed obliquely downwards and outwards ; the diapophysis, reduced to a thin line of bone, inclines forwards ; they both terminate in a small rounded tubercle, the rudimentary pleu-rapophysis, which extends but a few lines beyond the outer border of the corresponding inferior zyga-pophysis. The foramen for the vertebral artery is formed by the coalescence of the dia and parapophyses, with the rudimentary pleurapophysis, or rib, and is directed upwards and outwards.
The inferior zygapophyses, smaller than the superior, and of oval form, are directed obliquely downwards and forwards. The neurapophyses, 4}road and strong, incline backwards and inwards, to unite and form a prominent neural spine, which terminates below in a bifid extremity, the trace of its early separation into two lateral halves. On either side of its base is a depression, to which is attached part of the semispinalis colli muscJe. The neural canal is smaller than that of the atlas, and is of oval form. The long neural spine of the atlas gives origin to the obliqui capitis inferiores muscles, which, inclining upwards and outwards, are inserted into the equally prominent transverse processes of the atlas. From the latter points of bone arise the obliqui capitis superiores muscles.
The neural spine of the atlas and the transverse processes of the axis, which do not give attachment to muscles of great power and importance, are arrested in growth as tubercles of bone. The small oblique muscles of the head, with the levers to which they are attached, undergo their highest degree of development in those animals which, supplied with fore-limbs fashioned for seizing and holding their prey, employ the mouth, armed with strong canine and incisor teeth, for tearing it to pieces.
The seventh cervical vertebra approaches in some of its characters those of the dorsal region. The direction of the inferior pair of zygapophyses is more vertical; the parapophysis is thin, and sometimes imperfect, but it combines with the diapophysis to support a pleurapophysis, which, often of considerable length, bears a resemblance, not to be mistaken, to the movable ribs of the thorax. The groove for the spinal nerve is very shallow. The neural spine is long, and surmounted by a tubercle. It can be felt under the integument, and gives origin to the ligamentum nuchas, which covers the neural spines of the cervical vertebrae which precede it.