THE SPINE
The Spine is a flexuous column, formed of a series of bones called Vertebra.
The Vertebrae are divided into true and false.
The true vertebrae are twenty-four in number, and have received the names cervical, dorsal, and lumbar, according to the position which they occupy; seven being found in the cervical region, twelve in the dorsal, and five in the lumbar.
The false vertebrae, nine in number, are firmly united, so as to form two bones —five entering into the formation of the upper bone or sacrum, and four into the terminal bone of the spine or coccyx.
GENERAL CHARACTERS OF THE VERTEBRAE
Each vertebra consists of two parts, an anterior solid segment or body, forming the chief pillar of support; a posterior segment, the arch, forming part of a hollow cylinder for protection. The arch is formed of two pedicles and two laminae, supporting seven processes; viz. four articular, two transverse, and one spinous process.The Body is the largest and most solid part of a vertebra, serving to support the weight of the cranium and trunk. Above and below it is slightly concave, presenting a rim around its circumference; and its surfaces are rough, for the attachment of the inter vertebral fibro-cartilages. In front it is convex from side to side, concave from above downwards. Behind, flat from above downwards, and slightly concave from side to side. Its anterior surface is perforated by a few small apertures, for the passage of nutrient Vessels; whilst on the posterior surface is a single irregular-shaped, or occasionally several large apertures, for the exit of veins from the body of the vertebra, the vena basis vertebrae.
The Pedicles project backwards, one on each side, from the upper part of the body of the vertebras, at the line of junction of its posterior and lateral surfaces; they form the lateral parts of the arch, which is completed posteriorly by the two laminae. The concavities above and below the pedicles are the intervertebral notches; they are four in number, two on each side, the inferior ones being always the deeper.
The Laminae consist of two broad plates of bone, which complete the vertebral arch behind, enclosing a foramen which serves for the protection of the spinal cord; they are connected to the body through the intervention of the pedicles. Their upper and lower borders are rough, for the attachment of the ligamenta subflava.
The Spinous Process projects backwards from the junction of the two laminae, and serves for the attachment of muscles.
The Transverse Processes, two in number, project one at each side from the point where the articular processes join the pedicle. They also serve for the attachment of muscles.
The Articular Processes are four in number; two superior, the smooth surfaces of which are directed more or less backwards; and two inferior, the articular surfaces of which look more or less forwards. characters of the cervical vertebrae.
CHARACTERS OF THE CERVICAL VERTEBRAE
The Body is smaller than in any other region of the spine, thicker before than behind, and broader from side to side than from before backwards. Its upper surface is concave transversely, and presents a projecting lip on each side; its lower surface being convex from side to side, concave from before backwards, and presenting laterally a shallow concavity, which receives the corresponding projecting lip of the adjacent vertebra. The pedicles are directed obliquely outwards, and the superior intervertebral notches are slightly deeper, but narrower, than the inferior. The lamina are narrow, long, thinner above than below, and imbricated, i. e. overlapping each other; enclosing the foramen, which is very large, and of a triangular form. The spinous processes are short, bifid at the extremity, the two divisions being often of unequal size. They increase in length from the fourth to the seventh. The transverse processes are short, directed downwards, outwards, and forwards, bifid at their extremity, and marked by a groove along their upper surface, which runs downwards and outwards from the superior intervertebral notch, and serves for the transmission of one of the cervical nerves. The transverse processes are pierced at their base by a foramen, for the transmission of the vertebral artery, vein, and plexus of nerves. Each of these processes is formed by two roots: the anterior or smaller, which is attached to the side of the body, corresponds to the ribs in the dorsal region; the posterior is larger, springs from the pedicle, and corresponds to the true transverse processes. It is by the junction of these two processes, that the vertebral foramen is formed. The extremities of each of these roots form the anterior and posterior tubercles of the transverse processes. The articular processes are oblique: the superior are of an oval form, flattened, and directed upwards and backwards; the inferior downwards and forwards.
Characters of the Cervical Vertebrae
The peculiar vertebrae in the cervical region are the first or Atlas; the second or Axis; and the seventh or Vertebra prominent.
The chief peculiarities of this bone are, that it has neither body, gpinous process, nor pedicles. It consists of an anterior arch, a posterior arch, and two lateral masses. The anterior or lesser arch, which forms about one-fifth of the bone, represents the front part of the body of a vertebra; its anterior surface is convex, and presents about its centre a tubercle for the attachment of the Longus colli muscle; posteriorly it is concave, and marked by a smooth oval surface, for articulation with the odontoid process of the axis. The posterior or greater arch, which forms about two-fifths of the circumference of the bone, terminates behind in a tubercle, which is the rudiment of a spinous process, and gives origin to the Rectus capitis posticus minor. The posterior part of the arch presents above a rounded edge; whilst in front, immediately behind the superior articular processes, are two grooves, sometimes converted into foramina by delicate bony spiculae. These grooves represent the superior intervertebral notches, and are peculiar from being situated behind the articular processes, instead of before them, as in the other vertebrae. They serve for the transmission of the vertebral artery, which, ascending through the foramen in the transverse process, winds around the lateral mass in a direction backwards and inwards. They also transmit the sub-occipital nerves. On the under surface of the posterior arch, in the same situation, are two other grooves, placed behind the lateral masses, and representing the inferior intervertebral notches of other vertebrae. They are much less marked than the superior. The lateral masses, which are the most bulky and solid parts of the Atlas, present two articulating processes above, and two below. The two superior are of large size, oval, concave, and approach towards one another in front, but diverge behind; they are directed upwards, inwards, and a little backwards, forming a kind of cup for the condyles of the occipital bone, and are admirably adapted to the nodding movements of the head; whilst the inferior, which are circular in form, and flattened, are directed downwards, inwards, and a little backwards, articulating with the axis, and permitting the rotatory movements. Just below the inner margin of each superior articular surface, is a small tubercle, for the attachment of a ligament which, stretching across the ring of the Atlas, divides it into two unequal parts; the anterior or smaller segment receiving the odontoid process of the Axis, the posterior allowing the transmission of the spinal cord and its membranes. This ligament and the odontoid process are marked in the figure in dotted outline. The transverse processes are of large size, long, not bifid, perforated at their base by a canal for the vertebral artery, which is directed from below, upwards and backwards.
1st Cervical Vertebra, or Atlas
The Axis (so named from forming the pivot upon which the head rotates). The most distinctive character of this bone is the existence of a strong prominent process, tooth-like in form (hence the name odontoid), which arises perpendicularly from the upper part of the body. The body is of a triangular form; its anterior surface deeper than the posterior, presents a median longitudinal ridge, , separating two lateral depressed surfaces for the attachment of the Longi colli muscles. The odontoid process presents two oval articulating surfaces: one in front, for articulation with the Atlas; another behind, for the transverse ligament; the apex is pointed, and on either side of it is seen a rough impression for the attachment of the odontoid or cheek ligaments; whilst the base, where attached to the body, is constricted, so as to prevent displacement from the transverse ligament, which binds it in this situation to the anterior arch of the Atlas. On each side of this process are seen the superior articular surfaces; they are round, convex, directed upwards and outwards, and are peculiar in being supported on the body, pedicles, and transverse processes. The inferior articular surfaces, which are posterior and external to these, have the same direction as those of the other cervical vertebrae. The superior intervertebral notches are very shallow, and lie behind the articular processes; the inferior in front of them, as in the other cervical vertebrae. The transverse processes are very small, not bifid, and perforated by the vertebral foramen, which is directed obliquely upwards, and outwards. The laminae are thick and strong, and the spinous process is of large size, very strong, deeply channelled on its under surface, and presenting a bifid tubercular extremity for the attachment of muscles.
The Axis
Seventh Cervical. The most distinctive character of this vertebra is the existence of a very large, long, and prominent spinous process; hence the name ' Vertebra pro-minens.' This process is thick, nearly horizontal in direction, not bifurcated, and has attached to it the ligamentum nuchae. The foramina in the transverse processes are small, often wanting, and when present do not give passage to the vertebral artery; their upper surface presents only a slight groove, and generally only a trace of bifurcation at their extremity.
CHARACTERS OF THE DORSAL VERTEBRAE
The Dorsal Vertebrae
The Dorsal Vertebrae are intermediate in size between the cervical and lumbar. The body is somewhat triangular in form, broader in the antero-posterior than in the lateral direction, more particularly in the middle of the dorsal region, thicker behind than in front, flat above and below, deeply concave behind, and marked on each lateral surface, near the root of the pedicle, by two demi-facets, one above, the other below. These are covered with cartilage in the recent state; and, when articulated with the adjoining vertebrae, form oval surfaces for the reception of the heads of the corresponding ribs. The pedicles are strong, and the inferior inter vertebral notches of large size. The laminae are broad and thick, and the spinal foramen small, and of a round or slightly oval form. The articular surfaces are flat, the superior being directed backwards and a little outwards and upwards, the inferior forwards and a little inwards and downwards. The transverse processes are thick, strong, and of great length, directed obliquely backwards and outwards, presenting a clubbed extremity, lipped on its anterior part by a small concave surface, for articulation with the tubercle of a rib. The spinous processes are long, directed obliquely downwards, and terminated by a tubercle.
The peculiar dorsal vertebrae are the first, ninth, tenth, eleventh, and twelfth (fig-5)
The Peculiar Dorsal Vertebrae
The First Dorsal Vertebra may be distinguished by the existence on each side of the body, of a single entire articular facet for the head of the first rib, and a half facet for the upper half of the second. The upper surface of the body is like that of a cervical vertebra, being broad transversely, concave, and lipped on each side. The superior articular surfaces are oblique, and the spinous process thick, long, and almost horizontal.
The Ninth Dorsal has no demi-facet below.
The Tenth Dorsal has an entire articular facet at each side above; no demi-facet below.
In the Eleventh Dorsal, the body approaches in its form to the lumbar; and has a single entire articular surface on each side. The transverse processes are very short, and have no articular surfaces at their extremities.
The Twelfth Dorsal has the same characters as the eleventh; but may be distinguished from it by the transverse processes being quite rudimentary, and the inferior articular surfaces being convex and turned outwards, like those of the lumbar vertebrae.
The smallest dorsal vertebra is the fourth. The vertebras increase in size from that point downwards to the twelfth, and upwards to the first. The spinous processes also, from the eighth downwards, become shorter, and are directed more horizontally.
CHARACTERS OF THE LUMBAR VERTEBRA
The Lumbar Vertebrae
The Lumbar Vertebrae are the largest segments of the vertebral column. The Body is large, broad from side to side, flat above, and below, and thicker before-than behind. The pedicles are very strong, directed backwards; and the inferior intervertebral notches are of large size. The laminae are short, but broad and strong; and the foramen triangular, larger than in the dorsal, smaller than in the cervical region. The superior articular processes are concave, and directed backwards and inwards; the inferior, convex, and directed forwards and outwards. Projecting backwards from each of the superior articular processes is a tubercle, the representative of the transverse processes in the dorsal and cervical regions. The transverse processes are long, slender, directed a little backwards, and present, at the posterior part of their base, a small tubercle, which is directed downwards. The spinous processes are thick and broad, somewhat quadrilateral, horizontal in direction, and thicker below than above.
The Fifth Lumbar vertebra is peculiar from having the body much thicker in front than behind, which accounts for the prominence of the sacro-vertebral articulation.
Structure of the Vertebra. The structure of a vertebra differs in different parts. The Body is composed almost entirely of light spongy cancellous tissue, having a thin coating of compact tissue on its external surface, permeated throughout its interior with large canals for the reception of veins, which converge towards a single large irregular or several small apertures at the posterior part of the body of each bone. The arch and processes projecting from it have, on the contrary, an exceedingly thick covering of compact tissue.
Development. Each vertebra is formed of three primary centres of ossification, one for each lamella and its processes, and one for the body. Those for the lamellae appear about the sixth week of foetal life, in the situation where the transverse processes afterwards project, the ossific granules shooting backwards to the spine, forwards to the body, and outwards into the transverse and articular processes. That for the body makes its appearance in the middle of the cartilage about the eighth week. At birth, these three pieces are perfectly separate. During the first year, the lateral portions become partly united behind, in the situation of the spinous process, and thus the arch is formed. About the third year, the body is joined to the arch on each side, in such a manner, that the body is formed from the three original centres of ossification. Before puberty, no other changes occur, ex-cepting a gradual increase of growth of these primary centres, the upper and under surface of the bodies, and the ends of the transverse and spinous processes, being tipped with cartilage, in which ossific granules are not as yet deposited. At sixteen years, four secondary centres appear, one for the tip of each transverse process, and two (sometimes united into one) for the spinous process. At twenty-one years, two thin circular plates of bone are formed, one for the upper, and one for the under surface of the body. All these become joined, and the bone is' completely formed about the thirtieth year of life.
Exceptions to this mode of development occur in the first, second,and seventh cervical, and in those of the lumbar region.
The Aflas
The Atlas is developed by three centres. One (sometimes two) for the anterior arch, and one for each lateral mass. The ossific centres for each lateral mass commence before birth. At birth, the anterior arch is altogether cartilaginous, and the two lateral pieces are separated from one another behind. The nucleus for the anterior arch appears in the first year, between the second and third years the two lateral pieces unite, and join the anterior part at the age of five or six years. There is frequently a separate epiphysis for the rudimentary spine.
The Axis is developed by f ive centres; three for its anterior part, and two for the posterior. The three anterior centres are, one for the lower part of the body, and two for the odontoid process and upper part of the body; the two posterior ones are, one for each lamella. At about the sixth month of foetal life, those for the body and odontoid process make their appearance, the two for the odontoid process joining before birth. At birth the bone consists of four pieces, two anterior and two lateral. At the fourth year the body and odontoid process are completely joined.
The Seventh Cervical. The anterior or costal part of the transverse process of the seventh cervical, is developed from a separate osseous centre at about the sixth month of foetal life, and joins the body and posterior division of the transverse process between the fifth and sixth years. Sometimes this process continues as a separate piece, and becoming lengthened outwards, constitutes what is known as a cervical rib.
The Lumbar Vertebrce have two additional centres (besides those peculiar to the vertebras generally), for the tubercles, which project from the back part of the superior articular processes. The transverse process of the first lumbar is sometimes developed as a separate piece, which may remain permanently unconnected with the remaining portion of the bone; thus forming a lumbar rib, a peculiarity which is sometimes, though rarely, met with.
Ossification of the laminae of the vertebras commences at the upper part of the spine, and proceeds gradually downwards; hence the frequent occurrence of spina bifida in the lower part of the spinal column. Ossification of the bodies, on the other hand, commences a little below the centre of the spinal column, and extends both upwards and downwards. Although, however, the ossific nuclei make their first appearance in the lower dorsal vertebras (about the ninth), the lumbar and first sacral are those in which these nuclei are largest at birth. NEXT PAGE
