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frequently becomes developed so as to form a bridge of bone across it, converting the groove into a canal through which the vertebral artery and the posterior ramus of the suboccipital nerve pass-a condition normally met with in many animals. It is noteworthy that the grooves traversed by the highest two spinal nerves lie behind the articular processes, in place of in front, as in other parts of the column.
The ring formed by the lateral masses and the anterior and posterior arches is of irregular outline. The anterior part, cut off from the rest by the transverse ligament, serves for the lodgment of the dens of the epistropheus; the larger part behind corresponds to the upper part of the vertebral canal.
Epistropheus or Second Cervical Vertebra.-This is characterised by the presence of the tooth-like dens (O.T. odontoid process) which projects upwards from the superior surface of the body. Slightly constricted where it joins the body, the dens tapers to a blunt point superiorly, on the sides of which there are surfaces for the attachment of the alar ligaments. When the atlas and epistropheus are articulated this process lies behind the anterior arch of the atlas, and displays on its anterior surface an oval or circular facet which rests on that on the posterior surface of the anterior arch of the atlas. On the posterior aspect of the neck of the dens there is a shallow groove in which lies the transverse ligament of the atlas, which holds the dens in position.
FIG. 109.-EPISTROPHEUS (O.T. AXIS), (A) from behind and above, (B) from the left side.
The anterior surface of the body has a raised triangular surface, which ends superiorly in a ridge passing upwards to the neck of the dens. The roots of the vertebral arches are concealed above by the superior articular processes; inferiorly, they are deeply grooved. The lamina-prismatic on section-are thick and strong, ending in a stout, broad, and bifid spinous process, the under surface of which is deeply grooved, whilst its sides meet superiorly in a ridge. Placed over the roots of the vertebral arches and the anterior root of the transverse processes are the superior articular surfaces. These are more or less circular in shape, slightly convex from before backwards, flat from side to side, and are directed upwards and a little laterally. They are channelled inferiorly by the foramina transversaria, which turn laterally beneath them. The grooves by which the second cervical nerves leave the vertebral canal cross the laminæ immediately behind the superior articular processes. The inferior articular processes agree in form and position with those of the remaining members of the series, and are placed behind the inferior intervertebral notches. The transverse process is markedly down-turned, and its lateral extremity is not bifid.
The sixth cervical vertebra often displays an enlargement of the anterior tubercle on the transverse process, called the carotid tubercle from the circumstance that the carotid artery may be conveniently compressed against it. It is necessary to add, however, that the tubercle is not always well developed.
The seventh cervical vertebra (vertebra prominens) receives the latter name from the outstanding nature of its spinous process, which ends in a single broad tubercle. This forms a well-marked surface projection at the back of the root of the neck. The transverse processes are broad, being flattened from above downwards; they project considerably beyond those of the sixth. The maximum width between
their extremities agrees with that between the transverse processes of the atlas, these two constituting the widest members of the cervical series. The anterior tubercle is very small and is placed near the body. The foramen transversarium is small and does not as a rule transmit the vertebral artery. Usually a small vein passes through it. Not infrequently the costal element is separate from the true transverse process, thus constituting a cervical rib.
The thoracic vertebræ, twelve in number, are distinguished by having facets. on the sides of their bodies for the heads of the ribs, and in most instances also articular surfaces on their transverse processes for the tubercles of the ribs (Fig. 124, p. 111).
The body is described as characteristically heart-shaped, though in the upper and lower members of the series it undergoes transition to the typical forms of the cervical and lumbar vertebræ, respectively. Its antero-posterior and transverse measurements are nearly equal; the latter is greatest in line with the facets for the heads of the ribs. The bodies are slightly thicker behind than in front, thus adapting themselves to the anterior concavity which the column displays in this region. The bodies of the second to the ninth thoracic vertebræ inclusive, each possess four fovea costales or costal facets, a superior and larger pair placed on the superior margin of the body, close to the junction of the root of the vertebral arch with the body, and an inferior and smaller pair situated on the inferior edge, close to and in front of the inferior intervertebral grooves.
When contiguous vertebræ are articulated, the upper pair of facets of the lower vertebra coincide with the lower facets of the higher vertebra, and, together with the intervening intervertebral fibro-cartilage, form an articular cup for the reception of the head of a rib. Of these facets on the body the upper pair are the primary articular surfaces for the head of the rib; the lower are only acquired secondarily. Moreover, these facets, though apparently placed on the body, are in reality developed on the sides of the roots of the vertebral arches behind the line of union of the roots with the body (neurocentral synchondrosis), as will be explained hereafter.
The roots of the vertebral arches (O.T. pedicles) are short and thick, and directed posteriorly and slightly upwards. The superior vertebral notch is faintly marked; the inferior is deep. The lamina are broad, flat, and sloping, having sharp superior and inferior margins. When the vertebræ are superposed the latter overlap the former in an imbricated manner. The vertebral foramen is smaller than in the cervical and lumbar regions, and nearly circular in shape.
The spinous processes vary in length and direction, being shorter and more horizontal in the upper and lower members of the series, longest and most oblique in direction towards the middle of this part of the column. Nearly all have a downward inclination, and are so arranged that they overlap one another. Triangular in section where they spring from the vertebral arch, they become compressed from side to side towards their extremities, which are capped by more or less distinct tubercles. The transverse processes are directed backwards and laterally, and a little upwards. They gradually decrease in size and length from above downwards. Each has a somewhat expanded extremity, the anterior surface of which, in the case of the upper ten vertebræ, is hollowed out in the form of a circular facet for articulation with the tubercle of the rib which rests in the upper facet of the vertebra to which the transverse process belongs. The superior articular processes are vertical, and have their surfaces directed backwards, slightly upwards, and a little laterally; the inferior, correspondingly forwards, downwards, and medially.
Certain of the thoracic vertebræ display characters by which they can readily
be recognised. These are the first, tenth, eleventh, and twelfth, and sometimes
FIG. 110. FIRST, NINTH, TENTH, ELEVENTH, AND TWELFTH
1. Inferior articular process, with
2. Single facet for head of XIIth rib; no facet on transverse process.
3. Single facet for head of Xith rib;
no facet on transverse process. 4. Single facet for head of Xth rib. 5. Occasional facet for head of Xth rib.
6. Facet for head of IXth rib.
Facet on transverse process for
The first thoracic vertebra resembles the seventh cervical in the shape of its body, and the length and direction of its spine. There is a circular facet on either side of the body for the head of the first rib, and one facet on each side at the inferior border of its body, to complete the socket for the head of the second rib. Its trans
verse processes are long, and the superior intervertebral notch is better marked than in other members of the thoracic series. The superior articular surfaces are directed backwards and upwards, not laterally as in the lower members of the series.
The ninth thoracic vertebra occasionally has only the upper pair of facets on its body; at other times it conforms to the usual type.
The tenth thoracic vertebra may have only one complete costal facet on each side for the tenth rib, though sometimes the articular socket may be completed by the ninth thoracic vertebra. The facet on the transverse process is generally small, and sometimes absent.
The eleventh thoracic vertebra has a complete circular facet on the lateral side of each root of the vertebral arch for articulation with the eleventh rib. Its transverse processes are short and stunted, and have no facets.
The twelfth thoracic Facet on transverse process for vertebra has a single facet tuberosity of Xth rib, in this on each root of the vertebral particular instance
S. Superior Tubercles (Mamillary.
arch for the twelfth rib. Its transverse processes, short
Accessory. and stunted, have no facets,
and are broken up into smaller tubercles, called the lateral, superior, and inferior tubercles. These are homologous with the transverse, mamillary, and accessory processes of the lumbar vertebræ. Indications of these processes may also be met with in the tenth and eleventh thoracic vertebræ. The twelfth thoracic vertebra may usually be distinguished
from the eleventh by the arrangement of its inferior articular processes, which resemble those of the lumbar series in being turned laterally; but the eleventh occasionally displays the same arrangement, in which case it is not always easy to distinguish between them.
The lumbar vertebræ, five in number, are the largest of the movable vertebræ. They have no costal articular facets, nor are their transverse processes pierced by a foramen. In this way they can be readily distinguished from the members of the cervical and thoracic
greater than the antero
posterior. The anterior vertical thickness is slightly greater than the posterior, being thus adapted to the anterior convex curve of the column in this region.
The roots of vertebral arches (O.T. pedicles), directed horizontally backwards, are short and stout; the superior notches are shallow, but deeper than in the thoracic region; the inferior grooves are deep. The lamina are broad and
nearly vertical, sloping but 3 3x1
little. They support on
their inferior margins the inferiorarticular processes. The vertebral foramen is large and triangular.
The spinous processes, spatula shaped, with a thickened posterior margin, project backwards and slightly downwards. The transverse processes, more slender than in the thoracic region, pass horizontally laterally, with a
Superior articular process
Inferior articular process
and (B) from the left side.
slight backward inclination and usually with an upward tilt. Arising from the junction of the roots of the vertebral arches with the lamina in the higher members of the series, they tend to advance so as to become fused with the lateral side of the root and posterior aspect of the body in the lower two lumbar vertebræ. In these latter vertebræ the superior intervertebral grooves are carried obliquely across the superior surfaces of the bases of the transverse processes. The transverse processes lie in line with the lateral tubercles of the lower thoracic vertebræ, with
which they are serially homologous, and are to be regarded as representing the costal elements. Placed on their bases posteriorly, and just lateral to and inferior to the superior articular processes, are the small accessory processes, which are in series with the inferior tubercles of the lower thoracic vertebræ. The superior articular processes are stout, oval, curved plates of bone, fused in front with the roots and laminæ, and having their concave articular surfaces vertical and turned medially. Laterally, and on their posterior edge, the bone rises in the form of an elongated oval tubercle, the processus mamillaris (mamillary process); these are in correspondence with the superior tubercles of the lower thoracic transverse processes.
The inferior articular processes lie on either side of the root of the spinous process, supported on the inferior margin of the laminæ. Their articular surfaces, oval in outline, convex from side to side, and plane from above downwards, are turned laterally. The inferior articular processes are much closer together than the superior; so that when the vertebræ are articulated the superior articular processes of the lower vertebra embrace the inferior articular processes of the higher vertebra. The fifth lumbar vertebra is characterised by the size of its body, which is the largest of all the vertebræ. Further, the inferior surface of the body is cut away at the expense of its posterior part: hence the thickness of the body in front much exceeds the vertical diameter behind. By its articulation with the first sacral segment the inferior border of the body of this bone assists in the formation of the sacro-vertebral angle. The transverse process is pyramidal in form, and stouter than those of the other lumbar vertebræ. It arises by a broad base from the side of the back of the body, as well as from the pedicle, and is directed laterally and a little backwards and upwards. Its upper surface is slightly grooved by the superior intervertebral notch. A deep notch separates it posteriorly from the superior articular processes, which are less in-turned than in the other members of the series, their articular surfaces being directed more backwards than inwards, and displaying less concavity. The inferior articular processes are further apart than is the case with the other members of the series; they lie in line with the superior. The spinous process is shorter and narrower than the other lumbar spines, particularly so in the female. vertebral canal is somewhat compressed at its lateral angles.
THE FALSE OR FIXED VERTEBRÆ.
The sacrum, of roughly triangular shape, is formed normally by the fusion of five vertebræ. The anterior surface of the bone is slightly hollow from side to side and concave from above downwards, the curve being usually most pronounced opposite the third sacral segment. The central part corresponds to the bodies of the sacral vertebrae, the lines of fusion of which are indicated by a series of four parallel ridges which cross the median part of the bone at gradually diminishing intervals from above downwards; on each side these ridges disappear on the medial walls of the four anterior sacral foramina. The size of these holes decreases from above downwards. The upper and lower border of each foramen is formed by a stout bar of bone, of which there are five on each side, corresponding in number with the vertebræ present. These unite laterally so as to form the pars lateralis (O.T. lateral mass), thus enclosing the foramina to the lateral side, though there the edge is not abrupt, but sloped so as to pass gradually into the canal. The large anterior rami of the sacral nerves pass through these foramina and occupy the shallow grooves. The bone is broadest across the first sacral vertebra, tends to narrow opposite the second, and again usually increases in width opposite the third. When this condition is well marked, the edge has a notched appearance (sacral notch) which assists in the interlocking of the sacro