The spine (vertebral column) consists of thirty-three superimposed bones termed vertebrae. Of these the upper twenty-four remain separate throughout life and form three groups. The first seven are called cervical, the succeeding twelve thoracic (dorsal), and the last five lumbar. In adult life the last nine vertebras ankylose to form two composite bones named the sacrum and the coccyx. The sacrum is formed by the fusion of five vertebrae from the twenty-fifth to the twenty-ninth inclusive; the four terminal are vestigial, and form the coccyx. In order to gain a general notion of the characters of a vertebra, it is desirable to select a bone from the middle of the thoracic series.
A vertebra presents the following parts: The body or centrum is a solid disc of bone slightly concave on its superior and inferior aspects, and wider transversely than antero-posteriorly. The upper and lower surfaces are rough for intervertebral discs, and the margins are slightly lipped. The circumference of the body is, in front, concave vertically, but convex from side to side; posteriorly it is excavated, and presents foramina for the escape of veins from the cancellous tissue. On the sides of the body, at the upper and lower angles, there are four demi-facets; when two vertebra are superimposed, the adjacent demi-facets form a complete articular facet for the head of a rib.
A thoracic vertebra (side view.)
The pedicles are two constricted short piers of bone projecting horizontally backwards from the upper angles of the posterior surface. The lower border of each pedicle is deeply notched; hence, when two vertebrae are in position the notches are converted into intervertebral foramina for the transmission of spinal nerves and vessels.
The laminae are broad plates of bone continuous with the pedicles; each lamina meets its fellow dorsally to complete the neural arch, and conjointly form the spinous process. The superior borders of the lamina are rough for the insertion of ligamenta subflava. The anterior surface, in its upper part, is smooth where it bounds the neural canal. The lower part is rough for the origin of the ligamenta subflava. This rough surface is continuous with the inferior border of the spinous process.
The spinous process projects backwards and downwards from the confluent laminaB. To its upper and lower borders the interspinous ligaments are attached; its tip is rounded for the supraspinous ligament. It is mainly a muscular process.
A thoracic Vertebra.
The articular processes are four in number: two are superior and have the articular facets directed backwards with a slight outward tendency; their anterior surfaces complete the intervertebral foramina; posteriorly their margins give attachment to capsular ligaments. The inferior articular processes are slightly concave oval facets on the lower and outer angles of the anterior surface of the laminae. They are directed forwards and slightly inwards.
The transverse processes are two in number, and jut outwards from the lamina between the superior and inferior articular processes. The tip presents an oval facet for articulation with the tubercle of the rib. When the rib is in situ its neck forms with the process a costo-transverse foramen. The transverse processes, in addition to supporting the ribs, afford powerful leverage to muscles.
The cervical vertebrae
A typical cervical vertebra (from the third to sixth inclusive) presents the following characters. The centrum is smaller than in other regions of the column, and is of oval shape, the major axis being transverse. The upper surface has its lateral margins raised into prominent lips, whilst the lower surface is somewhat concave, its anterior margin being lipped so as to slightly overlap the anterior surface of the vertebra below. The inferior lateral margins are rounded, and come into relation with the raised edges of the centrum next below.
The pedicles are directed obliquely outwards, and the intervertebral notch is narrower above than below. The laminae are long and narrow. The spinous process is short, and bifid at the extremity.
The superior articular processes look backwards and upwards; the inferior are directed forwards and downwards.
The process behind the foramen has a shallow groove for the corresponding spinal nerve. The extremity of the transverse process is bifid; each arm is terminated by a tubercle referred to as anterior and posterior. The costo-transverse foramen is very characteristic of a cervical vertebra. It is bounded posteriorly by the pedicle, externally by the rudimentary transverse process, and anteriorly by a narrow bar of bone springing from the centrum posterior to the neuro-central suture. This thin bar is a vestigial rib, and will be referred to as the costal process.
The spinal foramen of all the cervical vertebrae is large, and somewhat triangular in form.
Peculiar cervical vertebrae
The various cervical vertebrae possess distinguishing features. The first, second, and seventh have characters so different from their fellows as to render them peculiar.
This vertebra has neither body nor spinous process; it is an irregular ring of bone with two thicker portions, the lateral masses, united anteriorly by a bridge, the anterior arch, which constitutes one-fifth of the entire circumference. This arch presents a tubercle on its anterior face for the anterior vertebral ligament and the longus colli muscle; its posterior surface has a circular facet for the odontoid process of the axis. The upper and lower borders are for ligaments.
The first cervical vertebrae or atlas.
The lateral masses are united posteriorly by a larger arch of bone, forming two-fifths of the circumference. Posteriorly this arch has a tubercle, representing a rudimentary spinous process. The upper and under surfaces of the arch afford attachment to ligaments. At the junction of the arch with the lateral masses there is, on the upper surface, a deep groove which lodges the vertebral artery and the suboccipital (first spinal) nerve. A. bridge of bone sometimes converts this into a foramen. A similar, but much shallower, notch is present on the under surface; this, with the axis, forms an intervertebral foramen for the second nerve. The atlas and axis are peculiar in that the first and second spinal nerves issue behind the articular processes, whereas the remaining spinal nerves emerge in front of the articular facets of the vertebrae. Each lateral mass has, on its upper surface, an elongated, deeply concave articular fossa or cup. These articular cups converge anteriorly. Occasionally each presents two oval facets united by an isthmus. These cups receive the occipital condyles and permit nodding movements of the head. The inferior articular processes are circular and almost flat; they are directed downwards, with an inclination inwards, and rest upon the axis, and permit rotatory movements of the head. Between the upper and lower articular surfaces on the inside of the ring two tubercles exist for the transverse ligament. This ligament divides the space within the ring into an anterior smaller segment for the odontoid process of the axis, and a larger portion - the spinal foramen of other vertebra - for the spinal cord and its membranes.
The transverse processes are large, to serve for the attachment of muscles which help to rotate the head. The costo-transverse foramina are large, but the costal processes are slender.
The Axis is easily recognized by the large rounded odontoid process which surmounts its upper surface. The centrum has a more prominent lip than the other cervical vertebrae, and the anterior surface has a median ridge separating two lateral depressions.
The odontoid process is an irregularly rounded peg of bone. The anterior surface has an oval facet for the anterior arch of the atlas. Posteriorly it presents a deeply cut smooth groove for the transverse ligament. To the apex a thin narrow fibrous band (the suspensory ligament) is attached. On each side of the apex there is an oblique facet for the check ligaments which connect it with the occipital bone. The pedicles are stout and broad; they support the oval, upwardly directed, articular surfaces for the atlas. The inferior articular surfaces do not differ from the cervical type. The transverse are smaller than the costal processes.
The spinous process is stout and strong, deeply concave on its under aspect, and affords firm attachment for muscles, especially those which help to rotate the head.
The Seventh Cervical Vertebrae
This vertebra has a longer spinous process than any other cervical vertebra, hence it is sometimes called prominent vertebra. The extremity of this process is not bifid, but has two small lateral tubercles which give attachment to the nuchal ligament. The transverse processes are of large size; the costal processes are very small; and the costo-transverse foramina are the smallest of the series. Very frequently the costal process is segmented off, and constitutes a cervical rib, sometimes of large size.
Occasionally a demi-facet exists on each side of the lower border of the centrum for the head of the first rib. When this demi-facet is present, there is usually a well-developed cervical rib.
The cervical vertebrae also exhibit great variation in regard to the extremities of their spinous processes. As a rule among Europeans, the second, third, fourth, and fifth vertebrae possess bifid spines. The sixth and seventh exhibit a tendency to bifurcate, their tips presenting two small lateral tubercles ; sometimes the sixth has a bifid spine, and more rarely the seventh presents the same condition. Occasionally all the cervical spines, with the exception of the second, are non-bifid, and even in the axis the bifurcation is not extensive. In the lower races of men the cervical spines are relatively shorter and more stunted than in Europeans generally, and, as a rule, are simple. The only cervical vertebra which presents a bifid spine in all races is the axis; even this may be non-bifid in the African, and occasionally in the European. (Owen, Turner, Cunningham.)
The laminae of the lower cervical vertebrae frequently present over the inferior articular processes distinct tubercles from which fasciculi of the multifidus spinae muscle arise. They are usually confined to the sixth and seventh vertebrae, but are fairly frequent on the fifth, and are occasionally seen on the fourth.
Origini of verticla portion of the longus colli muscle.
It is inserted into the second, third and fourth vertebrae.
A large number of muscles are attached to the cervical vertebrae.
To the atlas
Rectus capitis anticus minor, rectus capitis posticus minor, rectus capitis lateralis, superior oblique, inferior oblique, longus colli, splenius colli, intertransversales, levator anguli scapulae.
To the axis
Rectus capitis posticus major, inferior oblique, longus colli, splenius colli, intertransversales, interspinales, levator anguli scapulae, transversalis cervicis, the scalenus medius, semispinalis colli, and multifidus spinae.
To the seventh
Trapezius, complexus, serratus posticus superior, splenius, rhomboideus minor, multifidus spinse, semispinalis, eight intertransversales, inter-spinales, levator costae, scalenus posticus, aceessorius, scalenus medius, trachelo- mastoid, and the longus colli.
The cervical vertebrae (posterior view.)
The thoracic or dorsal vertebrae
The general characters of the thoracic vertebrae have already been considered in the description of the type vertebra. Their most distinguishing features are the facets on the transverse processes and sides of the bodies for the tubercles and heads of ribs.
Peculiar thoracic vertebrae (modified from Gay.)
Peculiar thoracic vertebrae
Several vertebrae in this series differ from the type form. The exceptional are the first, ninth, tenth, eleventh, and twelfth.
The first has a body resembling a cervical vertebra, the upper surface being concave and lipped laterally; it has two entire facets above for the first pair, and two demi-facets below for the second pair of ribs. The spinous process is thick, strong, almost horizontal, and more prominent than the prominent vertebra. Occasionally the transverse process is perforated near its root.
The ninth has demi-facets above, and usually none below; when the inferior demi-facets are present, this vertebra is not exceptional.
The tenth usually has an entire costal facet at its upper border, on each side, but occasionally only demi-facets. It has no lower demi-facets, and the facets on the transverse processes are usually small.
The eleventh has a large body resembling a lumbar vertebra. The rib facets are on the pedicles; they are complete and of large size. The transverse processes are short and have no facets for the tubercles of the eleventh pair of ribs.
In many mammals the spines of the anterior vertebrae are directed backwards, and those of the posterior directed forwards; in the centre of the column there is usually one spine vertical. This is called the anti-clinal vertebra. It is at this point that the thoracic begin to assume the characters of lumbar vertebrae. In man, the eleventh thoracic is the anti-clinal vertebra.
The twelfth resembles in general characters the eleventh, but may be distin- guished from it in having the inferior articular processes convex and turned outwards as in the lumbar vertebrae. It also resembles a lumbar vertebra by possessing well-marked mammillary and accessory tubercles. These tubercles are occasionally present on the tenth and eleventh vertebrae.
A peculiarity, more frequent in the thoracic and lumbar than in the cervical and sacral regions of the column, is the existence of a half- vertebra. Such specimens have a wedge-shaped half-centrum, to which are attached a lamina, a transverse, superior and inferior articular, and half a spinous process. As a rule, a half- vertebra is ankylosed to the vertebrae above and below.
The distinguishing features of lumbar vertebrae are their large size; the margins of the centrum are prominent; the pedicles are stout and strong; the inferior intervertebral notches are deep, and the laminae are thick and strong. The superior articular processes have concave facets directed backwards and inwards, and their posterior borders are surmounted by rounded mammillary processes or tubercles. The inferior articular processes have facets which look forwards and outwards. The transverse processes are long, slender, and each presents near the base, on the posterior aspect, a small accessory tubercle. The spinous processes are thick, broad, and project horizontally backward.
Lumbar vertebrae (side view.)
The transverse processes of the lumbar vertebrae are more complex than they at first appear. Each is compounded of a transverse and a costal process. The accessory process represents the tip of the partially suppressed transverse process, and the part in front is an undifferentiated rib. Between the transverse and costal elements some large vascular foramina are usually present, representing the costo-transverse foramina of other vertebrae. Occasionally the costal element differentiates and becomes a well-developed lumbar rib.
A lumbar vertebra.
A glance at the spine will show that the accessory tubercles are in line with the thoracic transverse processes, and the costal elements are in series with the ribs.
The fifth lumbar vertebra has several distinguishing features.
The centrum is much thicker in front than behind. The inferior articular processes are widely separated to articulate with the first sacral vertebra. The transverse processes are of large size, and the spinous process is small.
Variation in the fifth lumbar vertebra. (After Turner.)
The pedicles of this vertebra are liable to a remarkable deviation from the conditions found in other parts of the spine. The peculiarity consists of a complete solution in the continuity of the arch immediately behind the superior articular processes. In such specimens the anterior part consists of the body carrying the pedicles, transverse and superior articular processes; whilst the posterior segment is composed of the laminae, spine, and inferior articular processes.
A variation in the fifth limbar verterbra. (After Turner.)
The posterior segment of the ring of this vertebra may even consist of two pieces. There is reason to believe that this abnormality of the fifth lumbar vertebra occurs in five per cent, of all subjects examined. Sir William Turner, in his report on the human skeletons in the ' Challenger ' Reports, found seven examples among thirty skeletons examined. The skeletons in which this occurred were: a Malay, an Andamanese, a Chinese, two Bushmen, an Esquimaux, and an African. Turner has also seen it in the skeleton of a Sandwich Islander.
The sacral and coccygeal vertebra
In the adult skeleton, the five vertebrae succeeding the lumbar series are firmly ankylosed to form a single bone, the sacrum: the components of the sacrum are termed sacral vertebrae. Beyond the fifth sacral, four, and occasionally five, other rudimentary vertebrae to which the adjective coccygeal is applied, are ankylosed in adult life, to form a single piece, the coccyx. In advanced life the coccyx unites with the sacrum.
The sacrum is a large triangular- shaped bone, firmly wedged between the innominate bones. It forms the posterior boundary of the true pelvic cavity. The sacrum is curved upon itself with the concavity looking forwards. The upper end of the curve forms, with the body of the fifth lumbar vertebra, an anterior projection known as the promontory. The middle portion of the anterior face of the sacrum exhibits four transverse ridges corresponding to the intervertebral spaces. The intervening portions are the bodies of the vertebra. The upper two sacral vertebrae are almost equal in size to those of the lumbar series, but the three lower rapidly diminish in size from above downwards. The ridges terminate laterally in the anterior sacral foramina, four pairs in all, which are the intervertebral foramina of the sacral vertebras, and transmit the anterior divisions of the first four sacral nerves. The upper two are also traversed by the lateral sacral arteries. The bone immediately outside the foramina corresponds to the costal processes, and the portion formed by the second, third, and fourth sacral vertebrae gives origin to the pyriformis muscle. The lateral part of the fifth sacral vertebra gives partial insertion to the coccyges.
The posterior surface is strongly convex and rough. The middle line is occupied by four tubercles representing the suppressed spinous processes. Of these the first is the largest, the second and third may be confluent, and the fourth is often absent. The bone on each side of the spines is formed by the ankylosed laminae. In the fourth sometimes, but always in the fifth, the lamina fail to meet in the middle line, and this leaves a gap, the hiatus sacralis. The median borders of this hiatus are prolonged downwards as rounded processes, the sacral cornua, to which the posterior sacro-coccygeal ligaments are attached. External to the laminae is a second series of small prominences: these are the articular processes. The first pair are large for the last lumbar vertebra, the second and third are small, and the fourth and fifth are inconspicuous.
The sacrum and coccyx. (anterior view.)
Immediately external to the articular processes are the posterior sacral foramina, four on each side; they are smaller than the anterior, and give exit to the posterior divisions of the first four sacral nerves. External to the foramina there are five eminences on each side, representing the transverse processes. The first pair are large and conspicuous; the second form part of the articular surface for the ilium; the third, fourth and fifth give attachment to ligaments and muscles. The furrow formed by the laminae, and bounded on the median aspect by the spinous, and externally by the articular processes, is known as the sacral groove, and lodges the spinal erector muscle.
The sacrum (posterior view.)
The upper surface, or base, of the sacrum resembles the corresponding aspect of a lumbar vertebra, and its articular processes have well-marked mammillary tubercles. The conjoint transverse and costal processes form on each side a broad surface, the wing or ala. From its margin the iliacus has a small point of origin.
Base of the sacrum.
The apex is directed downwards and forwards, and is formed by the inferior aspect of the body of the fifth sacral vertebra; it articulates by means of an inter-vertebral disc with the coccyx. In advanced life the coccyx and sacrum ankylose at this spot.
The lateral surface presents in the upper two-thirds a broad irregular tract called the auricular process, which is rough and, in the recent state, covered with fibro- cartilage for union with the ilium. The margins are rough for ligaments. Below the auricular surface the lateral borders are sharp and give attachment to the greater and lesser sacro-sciatic ligaments. Near the extremity it presents a notch which is converted into a foramen by articulation with the coccyx. Through the space thus enclosed the anterior branch of the fifth sacral nerve issues. Some- times the foramen is represented by a notch even when the sacrum and coccyx are articulated. The middle of the sacrum is occupied by a continuation of the spinal canal. It is triangular in form at the base, and flattened towards the apex. It lodges the terminal branches of the cauda equina, the filum terminale, and the lower extremity of the dura mater.
The sacrum exhibits sexual differences. In the female it is usually wider, much less curved, and is directed more obliquely backwards, than in the male.
The following muscles are attached to the sacrum : - Pyriformis, coccygeus, iliacus, latissimus dorsi, multifidus spinse, erector spinse, gluteus maximus ; and the occasional muscles, namely, curvator coccygis, extensor coccygis, and the agitator caud?e.
Anterior and posterior common ligaments of the spine; anterior and posterior sacro-coccygeal; greater and lesser sacro-sciatic, anterior and posterior sacro-iliac, two capsular, ligamenta subflava, and the supraspinous.
The coccyx in the adult is made up of four and occasionally five vestigial vertebrae ankylosed to one another. Rarely the number of segments is reduced to three. The first two segments contain, in addition to the body of a vertebra, traces of articular and transverse processes: the rest are mere nodules of bone, representing centra. The anterior surface gives attachment to the anterior sacro-coccygeal ligament; and near its tip to the levator ani; it is in relation with the lower end of the rectum.
The posterior surface is convex, and along its margin affords attachment to the gluteus maximus muscle.
The lateral borders are thin: they receive parts of the greater sacro-sciatic ligaments and of the coccygeal muscle. The base has an oval facet for the fifth sacral vertebra, and presents the two long coccygeal cornua for the posterior sacro-coccygeal ligament. The junction of the coccyx and sacrum completes the foramen of exit for the fifth sacral nerve. In many skeletons the foramen is incomplete externally. The apex is rounded and gives attachment to the sphincter ani, and in front to the levator ani muscles.
The spinal column in general
When the various vertebrae are in their relative positions, the whole is termed the spinal column. It occupies the median line of the posterior aspect of the trunk. Superiorly it supports the head; laterally it gives attachment to ribs; these in their turn receive the weight of the upper limbs. Inferiorly, the sacrum affords attachment to the innominate bones, by which the weight of the trunk is trans- mitted to the lower limbs. The spinal column is the axis of the skeleton.
The spine (lateral view.)
It varies in length in different persons, but on an average it measures, from the atlas to the tip of the coccyx, following the curve, 70 cm. (28’’). Of this the cervical spine measures 12.5 cm. (5''), the thoracic 27.5 cm. (11"), the lumbar 17.5 cm. (7"), and the sacro-coccygeal portion 12.5 cm. (5").
Viewed in profile, the column presents four curves : the first, or cervical, is convex anteriorly ; the thoracic is much larger and longer, with its concavity for- ward ; the lumbar curve has its convexity directed anteriorly, and ends somewhat abruptly at the sacro-vertebral angle ; and to this succeeds the pelvic curve, which corresponds to the hollow of the sacrum. In addition to these, the whole column has a slight lateral curve with the convexity to the right, probably due to muscular action.
Viewed from the front, the superimposed bodies present three pyramids. The first is formed by the cervical vertebrae from the second to the seventh. The bodies of the lumbar and thoracic vertebrae form a much longer pyramid. The third is inverted, and formed by the sacrum and coccyx.
A divided thoracic vertebra. (After Turner.)
Posteriorly, the column presents a median and two lateral rows of processes. The median row is formed by the spinous processes. In the cervical spine, with the exception of the first and the seventh, they are bifid. In the thoracic set they end in rounded tubercles, are long, and for the most part directed obliquely downwards, but in the lower part they become more horizontal until the eleventh is reached. The spine of the eleventh thoracic vertebra is small and almost horizontal; this is the anti-clinal vertebra. In the lumbar region the spinous processes are short, stout plates of bone, with their borders set vertically; in the sacrum they are vestigial, and in the coccyx completely suppressed.
The lateral rows are formed by the transverse processes, which are most marked in the thoracic region, where they are rib-bearers. In the cervical spine they are in the same plane as the ribs. The articular processes in the cervical region are in series with the transverse processes of the thoracic vertebrae.
Between the ridges formed by the spinous and transverse processes we recognize the vertebral grooves in which muscles are lodged. The floor of each groove is formed by the laminae and articular processes, with their mammillary tubercles in the lumbar and lower thoracic regions. Similar tubercles are present on the inferior articular processes of the three lower cervical vertebrae. The intervertebral foramina, oval in shape, are small in the cervical, but gradually increase in size in the thoracic, and are largest in the lumbar region.
The various ossific centers for the vertebrae are deposited in the cartilage which, very early in embryonic life, surrounds the notochord and gradually encloses the spinal cord.
A typical vertebra arises from three primary and numerous secondary centers. The primary centers appear during the sixth week of embryonic life. In the thoracic region the nucleus for the body is first seen, but in the cervical region the lateral centers make their appearance somewhat earlier. The nucleus for the body is deposited around the center, and quickly becomes bi-lobed. This bi-lobed, or dumb-bell, shape is often so pronounced as to give rise to the appearance of two distinct nuclei. Sometimes the nucleus is double, and remains separate throughout life, the vertebra being divided by a vertical fissure. The bifid character of the nucleus of the vertebral body is further emphasized by the occasional occurrence of half-vertebrae. The lateral centers are deposited near the bases of the superior articular processes, and give rise to the pedicles, laminae, articular processes, and a large part of the transverse and spinous processes.
At birth a vertebra consists of three parts - a body and two lateral masses connected by hyaline cartilage. The line of union of the lateral masses with the bodies is known as the neuro-central suture, and this is not obliterated for several years after birth. An examination of a thoracic vertebra at the fifth year will show that a portion of the body of each vertebra is derived from the lateral masses, and that the demi-facets for the rib-heads are situated behind the neuro- central suture, and therefore belong to the pedicles.
During the early years growth progresses rapidly, and at puberty the secondary centers make their appearance in the cartilaginous tips of the transverse and spinous processes. During the seventeenth year a meniscus of bone forms around the margins of the superior and inferior surfaces of the centra. These are the epiphyseal discs; they are thickest at the periphery, and gradually become thin towards the central perforation. By the twenty-fifth year the various secondary nuclei have coalesced with the main bone, and the vertebra is then complete.
Lumbar vertebra at the eighteen year with secondary centers.
In several vertebrae the mode of ossification deviates from the account given above, and requires separate consideration.
This bone has three primary centers - one for each lateral mass (neural arch) appearing in the sixth week of embryonic life. The third appears a few months after birth for the anterior arch. The lateral portions coalesce posteriorly about the third year; the union with the anterior nucleus is delayed until the sixth year. An additional center occasionally appears for the posterior segment.
Immature atlas (third year.)
This is the most exceptional of all the vertebrae. It has the usual three primary nuclei - one for the body, and one on each side for the neural arch. The center for the body appears in the embryo about the fifth month, and a few weeks later two laterally disposed nuclei are seen for the base of the odontoid process ; these fuse together in the middle line, and by the third year ankylose peripherally to the centrum of the axis.
Development of the atlas.
The line of union between the body of the axis and the odontoid process is indicated even in advanced life by a persistent lenticular-shaped cartilage. During the second year a nucleus appears at the tip of the odontoid process. An epiphyseal meniscus for the inferior and superior surfaces of the centrum of the axis appears about the seventeenth year. As a rule, the superior meniscus is represented by a few earthy granules.
The atlas (from an adult) in section.
The sixth and seventh vertebrae
In the cervical vertebrae the pedicles, or anterior extremities of the neural arches, take a much larger share in forming the centrum than is the case with the remaining vertebrae. The sixth, seventh, and possibly other cervical vertebrae present an additional center on each side of the neural arch for the costal process; it appears before birth. The costal processes of the seventh cervical not infrequently fail to ankylose with the vertebra; when this is the case, the processes become cervical ribs. Sometimes these ribs are of large size.
The lumbar vertebrae
In the lumbar vertebrae two additional canters make their appearance, about puberty, namely, for the mammillary tubercles on the posterior aspect of each superior articular process.
An immature cervical vertebra.
The fifth lumbar occasionally differs in the mode of ossification of its arch; in many skeletons this arch is derived from four nuclei. There is a nucleus on each side for the pedicle, the transverse process, and the superior articular process; and one on each side for the lamina, inferior articular process, and the lateral half of the spinous process. The pedicles may fail to join the laminae; more rarely the laminae fail to fuse.
Ossicfication of the fifth lumbar vertebra.
The sacral vertebrae
In addition to the three primary vertebral canters, the three upper sacral vertebrae have each an extra pair corresponding to the costal processes of the seventh cervical vertebra. These processes are very large in the first sacral, smaller in the second, and very small in the third. Although the various primary canters of the sacral vertebrae appear much later than in other regions of the column, yet they are all visible at birth. The centrum of each sacral vertebra develops a superior and inferior epiphyseal meniscus, and eventually the five vertebrae fuse to form a single bone, the sacrum. Even in advanced life the intervertebral discs between the sacral vertebrae persist in the center of the bone. The ear- shaped lateral articular facet on the side of the sacrum arises from two additional canters on each side, about the eighteenth year. The total number of ossific centers for the sacrum is thirty-five.
The coccygeal vertebrae
These are cartilaginous at birth. A few months later the first segment ossifies. The remaining three ossify from above downwards before the fifteenth year. By the twentieth year the first three have usually coalesced. The fourth fuses with them later, and the coccyx ankyloses with the sacrum, as a rule, late in life.
Morphology of the transverse and articular processes.
The Serial Morphology of the Vertebrae
Although at first sight many of the vertebrae exhibit peculiarities, nevertheless a study of the mode by which they develop, and their variations, indicates the serial homology of the constituent parts of the vertebrae in each region of the column.
The centrum, or body of the vertebra, is that part which immediately surrounds the notochord. This part is present in all the vertebrae of man, but the body of the atlas is dissociated from its neural arch, and ankylosed to the body of the axis. The reasons for regarding the odontoid process as the body of the atlas are these: In the embryo the notochord passes through it on its way to the base of the cranium. Between the odontoid process and the body of the axis there is a swelling of the notochord in the early embryo as in other intervertebral regions. This swelling is later indicated by a small inter-vertebral disc hidden in the bone, but persistent even in old age. The odontoid process arises from primary canters, and in chelonians it remains as a separate ossicle throughout life; in Ornithorhynchus it remains distinct for a long time, and it has been found separate even in an adult man. Lastly, in man and manny mammals, an epiphyseal plate develops between it and the true body of the axis.
The anterior segment of the atlas is in no sense homologous with a centrum, and is most probably an enlarged hypapophysis or sub-vertebral wedge-bone, which, in lizards, exists on the ventral aspect of the column between individual centra. Similar ossicles occur in the lumbar region of the mole.
The neural arches and spinous processes are easily recognized throughout the various parts of the column in which complete vertebrae are present.
The articular processes are of no morphological value, and do not require consideration here.
The transverse processes offer more difficulty. They present themselves in the simplest form in the thoracic series. Here they articulate with the tubercles of the ribs. The transverse process and the neck of the rib enclose an arterial foramen, the costo-transverse. In the cervical region this rib, or costal element, and the transverse process are fused together, but the conjoint process thus formed is pierced by the costo-transverse foramen. The compound nature of the process is indicated by the fact that the anterior or costal processes in the lower cervical vertebrae arise from additional centers and occasionally retain their independence as cervical ribs. These processes in Sauropsida (birds and reptiles) are represented by free ribs. In the lumbar region the compound nature of the transverse process is further marked. The true transverse process is greatly suppressed, and its extremity is indicated by the accessory tubercle. Anterior to this in the adult vertebra a group of holes represent the costo-transverse foramen, and the portion in front of this is the costal element. Occasionally it will persist as an independent ossicle, the lumbar rib.
In the sacral series the costal elements are peculiarly modified in the first three vertebrae to form piers of bone for articulation with the ilium. The costo-transverse foramina are completely obscured. In rare instances the first sacral vertebra will articulate with the ilium on one side, but remain free on the other. Under such conditions the free process exactly resembles the elongated transverse process of a lumbar vertebra. The first three sacral vertebrae which develop a costal process (rib) for articulation with the ilium are true sacral vertebrae. Those ankylosed below these are pseudo-sacral. A glance at the spine will show the homology of the various parts of a vertebra from the cervical, thoracic, lumbar and sacral regions.
The mammillary processes are vestiges of the greatly elongated articular processes of such mammals as the dog, armadillo, &c.