THE SPINE
THE FALSE VERTEBRAE
The False Vertebras consist of nine pieces, which are united so as to form two bones, five entering into the formation of the sacrum, four the coccyx.
The Sacrum is a large triangular bone, situated at the lower part of the vertebral column, and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two ossa innominata; its upper part, or base, articulating with the last lumbar vertebra, its apex with the coccyx. The sacrum is curved upon itself, and placed very obliquely, its upper extremity projecting forwards, forming, with the last lumbar vertebra, a very prominent angle, called the promontory or sacro-vertebral angle, whilst its central part is directed backwards, so as to give increased capacity to the pelvic cavity. It presents for examination an anterior and posterior surface, two lateral surfaces, a base, an apex, and a central canal.
The Sacrum, Anterior Surface
The Anterior Surface is deeply concave from above downwards, and slightly so from side to side. In the middle are seen four transverse lines, indicating the original division of the bone into five separate pieces. The portions of bone intervening between the lines correspond to the bodies of the vertebras; they are slightly concave longitudinally, and diminish in size from above downwards. At the extremities of each of these lines, are seen the anterior sacral foramina, analogous to the intervertebral foramina, four in number on each side, somewhat rounded in form, diminishing in size from above downwards, and directed outwards and forwards; they transmit the anterior branches of the sacral nerves. External to these foramina, is the lateral mass, formed by the coalesced transverse processes of the sacral vertebrae, traversed by four broad shallow grooves, which lodge the anterior sacral nerves as they pass outwards, the grooves being separated by prominent ridges of bone, which give attachment to the slips of the Pyriformis muscle.
The Posterior Surface is convex, and much narrower than the anterior. In the middle line, are three or four tubercles, sometimes connected together, which represent the rudimentary spinous processes. Of these tubercles, the first is usually very prominent, and perfectly separate from the rest; the second, third, and fourth, existing either separate, or united into a ridge, which diminishes in size as it descends; the fifth, and sometimes the fourth, remaining undeveloped, and exposing below, the lower end of the sacral canal. External to the spinous processes on each side, are the lamina, broad and well marked in the three first pieces; the lower part of the fourth, and the whole of the fifth, being, undeveloped: in this situation the sacral canal is exposed. External to the laminae are a linear series of indistinct tubercles representing the articular processes; the upper pair are well developed; the second and third are small; the fourth and fifth (usually blended together) are situated on each side of the sacral canal: they are called the sacral cornua, and articulate with the cornua of the coccyx. External to the articular processes are the four posterior sacral foramina; they are smaller in size, and less regular in form than the anterior, and transmit the posterior branches of the sacral nerves. On the outer side of the posterior sacral foramina are a series of tubercles, representing the rudimentary transverse processes. The first pair of transverse tubercles are very distinct, and correspond with each superior angle of the bone; the second, small in size, enter into the formation of the sacro-iliac articulation; the third give attachment to the oblique sacro-iliac ligaments;
The Sacrum, Posterior Surface
and the fourth and fifth to the great sacro-ischiatic ligaments. The interspace between the spinous and transverse processes of the sacrum, presents a wide shallow concavity, called the sacral groove; it is continuous above with the vertebral groove, and lodges the origin of the Erector Spinae.
The Lateral Surface, broad above, becomes narrowed into a thin edge below. Its upper half presents in front a broad ear-shaped surface for articulation with the ilium. This is called the auricular or ear-shaped surface, and in the fresh state is coated with cartilage. It is bounded posteriorly by deep and rough impressions, for the attachment of the sacro-iliac ligaments. The lower half is thin and sharp, and gives attachment to the greater and lesser sacro-ischiatic ligaments; below, it presents a deep notch, which is converted into a foramen by articulation with the transverse process of the upper piece of the coccyx, and transmits the anterior branch of the fifth sacral nerve.
The Base of the sacrum, which is broad and expanded, is directed upwards and forwards. In the middle is seen an oval articular surface, which corresponds with the under-surface of the body of the last lumbar vertebra, bounded behind by the large triangular orifice of the sacral canal. This orifice is formed behind by the spinous process and lamina? of the first sacral vertebra, whilst projecting from it on each side are the superior articular processes; they are oval, concave, directed backwards and inwards, like the superior articular processes of a lumbar vertebra, in front of each articular process in an intervertebral notch, which forms the lower half of the last intervertebral foramen. Lastly, on each side of the articular surface is a brond and flat triangular surface of bone, called the aim of the sacrum; they extend outwards, and are continuous on each side with the iliac fossas.
The Apex, directed downwards and forwards, presents a small oval concave surface for articulation with the coccyx.
The Sacral Canal runs throughout the greater part of the bone; it is large and triangular in form above, small and flattened from before backwards below. In this situation, its posterior wall is incomplete, from the non-development of the laminae and spiuous processes. It lodges the sacral nerves, and is perforated by the anterior and posterior sacral foramina, through which these pass out.
Structure. It consists of much loose spongy tissue within, invested externally by a thin layer of compact tissue.
Differences in the sacrum of the male and female
The sacrum in the female is usually wider than in the male, and it is much less curved, the upper half of the bone being nearly straight, the lower half presenting the greatest amount of curvature. The bone is also directed more obliquely backwards; which increases the size of the pelvic cavity, and forms a more prominent sacro-vertebral angle. In the male the curvature is more evenly distributed over the whole length of the bone, and is altogether greater than in the female.
Peculiarities of the sacrum
This bone, in some cases, consists of six instead of five pieces; occasionally the number is reduced to four. Sometimes the bodies of the first and second vertebrae are not joined, or the laminae and spinous processes have not coalesced with the rest of the bone. Occasionally the superior transverse tubercles are not joined to the rest of the bone on one or both sides; and, lastly, the sacral canal may be open for nearly the lower half of the bone, in consequence of the imperfect development of the laminae and spinous processes. The sacrum also varies considerably with respect to its degree of curvature. From the examination of a large number of skeletons, it would appear, that, in one set of cases, the anterior surface of this bone was nearly straight, the curvature, which was very slight, affecting only its lower end. In another set of cases, the bone was curved throughout its whole length, but especially towards its middle. In a third set, the degree of curvature was less marked, and affected especially the lower third of the bone.
Development of Sacrum . The sacrum, formed by the union of five vertebrae, has thirty-five centres of ossification. Each of the three first pieces is developed by seven centres; viz., three for the body, one for its central part, one for each epiphysal lamella on its upper and under surface, and one for each of the laminae: so far the first three sacral vertebras, as well as the two last, are developed like the other pieces of the vertebral column. One of the characteristic points in the deve-lopment of this bone, consists in the existence of two additional centres for each of the first three pieces, which appear one on each side, close to the anterior sacral foramina, and correspond to the transverse processes of the lumbar vertebrae.
Development of Sacrum
Each of the two last pieces is developed by five centres: three for the body; viz., one for its central part, and one for each of the epiphysal lamellae; and one for each of the laminae.
A second characteristic point in the development of this bone consists in each lateral surface of the sacrum being developed by two epiphysal points, one for the auricular surface, and one for the thin lateral border of the bone.
Period of Development. At about the eighth or ninth week of foetal life, ossification of the central part of the bodies of the three first vertebrae commences, and, at a somewhat later period, that of the two last. Between the sixth and eighth months, ossification of the lamellae takes place; and at about the same period the characteristic osseous tubercles for the three first sacral vertebrae make their appearance. The lateral pieces join to form the arch, and are united to the bodies, first, in the lowest vertebrae. This occurs about the second year, the uppermost segment appearing as a single piece about the fifth or sixth year. About the sixteenth year the epiphysal lamellae for the bodies are formed; and between the eighteenth and twentieth years those for each lateral surface of the sacrum make their appearance. At about this period, the two last segments are joined to one another; and this process gradually extending upwards, all the pieces become united, and the bone completely formed from the twenty-fifth to the thirtieth year of life.
Articulations. With four bones; the last lumbar vertebra, coccyx, and the two ossa innominata.
Attachment of Muscles. The Pyrifonnis and Coccygeus on either side, behind the Gluteus maximus and Erector Spins.
THE COCCYX
The Coccyx ( cuckoo), so called from resembling a cuckoo's beak, is formed of four small segments of bone, the most rudimentary parts of the vertebral column. Of these, the first is the largest, and often exists as a
separate piece, the three last diminishing in size from above downwards, are blended together so as to form a single bone. The gradual diminution in the size of the pieces gives this bone a triangular form, articulating by its base with the apex of the sacrum. It ,. presents for examination an anterior and posterior surface, two borders, a base, and an apex. The anterior surface is slightly concave, and marked with three transverse grooves, indicating the points of junction of the different pieces. It has attached to it the anterior sacro-coccygeal ligament and levator ani muscle, and supports the lower end of the rectum. The posterior surface is convex, marked by grooves similar to those on the anterior surface, and presents on each side a linear row of tubercles, which represent the articular processes of the coccygeal vertebras. Of these, the superior pair are very large; they are called the cornua of the coccyx, and projecting upwards, articulate with the cornua of the sacrum, the junction between these two bones completing the fifth sacral foramen for the transmission of the posterior branch of the fifth sacral nerve. The lateral borders are thin, and present a series of small eminences, which represent the transverse processes of the coccygeal vertebrae. Of these, the first on each side is of large size, flattened from before backwards, and often ascends upwards to join the lower part of the thin lateral edge of the sacrum, thus completing the fifth sacral foramen: the others diminish in size from above downwards, and are often wanting. The borders of the coccyx are narrow, and give attachment on each side to the sacro-sciatic ligaments and Coccygeus muscle. The base presents an oval surface for articulation with the sacrum. The apex is rounded, and has attached to it the tendon of the external Sphincter ani muscle. It is occasionally bifid, and sometimes deflected to one or other side.
The Coccyx
Development. The coccyx is developed by four centres, one for each piece. Occasionally one of the three first pieces of this bone is developed by two centres, placed side by side. The periods when the ossific nuclei make their appearance is the following: in the first segment, at birth; in the second piece, at from five to ten years; in the third, from ten to fifteen years; in the fourth, from fifteen to twenty years. As age advances, these various segments become united in the following order: the two first pieces join, then the third and fourth; and, lastly the bone is completed by the union of the second and third. At a late period of life, especially in females, the coccyx becomes joined to the end of the sacrum.
Articulation. With the sacrum.
Attachment of Muscles. On either side, the Coccygeus; behind, the Glutens maximus; at its apex, the Sphincter ani; and in front, the Levator ani.
OF THE SPINE IN GENERAL
The spinal column, formed by the junction of the vertebrae, is situated in the median line, at the posterior part of the trunk: its average length is about two feet two or three inches; the lumbar region contributing seven parts, the dorsal eleven, and the cervical five.
Viewed in front, it presents two pyramids joined together at their bases, the upper one being formed by all the true vertebra from the second cervical to the last lumbar; the lower one by the false vertebrae, the sacrum, and coccyx. Viewed somewhat more closely, the uppermost pyramid is seen to be formed of three smaller pyramids. Of these the most superior one consists of the six lower cervical vertebras, its apex being formed by the axis or second cervical, its base by the first dorsal. The second pyramid, which is inverted, is formed by the four upper dorsal vertebrae, the base being at the first dorsal, the smaller end at the fourth. The third pyramid commences at the fourth dorsal, and gradually increases in size to the fifth lumbar.
Viewed laterally, the spinal column presents several curves. In the dorsal region, the seat of the principal curvature, the spine
The Spine Column
is concave anteriorly; whilst in the cervical and lumbar regions it is convex anteriorly, especially in the latter. The spine has also a slight lateral curvature, the convexity of which is directed towards the right side. This is most probably produced, as Bichat first explained, from the effect of muscular action; most persons using the right arm in preference to the left, especially in making long-continued efforts, when the body is curved to the right side. In support of this explanation, it has been found by Beclard, that in one or two individuals who were left-handed, the lateral curvature was directed to the left side.
The spinal column presents for examination an anterior, a posterior, and two lateral surfaces, a base, summit, and vertebral canal.
The anterior surface presents the bodies of the vertebrae separated in the fresh state by the intervertebral discs. The bodies are broad in the cervical region, narrow in the upper part of the dorsal, and broadest hi the lumbar region. The whole of this surface is convex transversely, concave from above downwards in the dorsal region, and convex in the same direction in the cervical and lumbar regions.
The posterior surface presents in the median line the spinous processes. These are short, horizontal, with bifid extremities in the cervical region. In the dorsal region, they are directed obliquely above, assume almost a vertical direction in the middle, and are horizontal, like the spines of the lumbar vertebras, below. They are separated by considerable intervals in the loins, by narrower intervals in the neck, and are closely approximated in the middle of the dorsal region. On either side of the spinous processes; extending the whole length of the column, is the vertebral groove, formed by the laminae in the cervical and lumbar regions, where it is shallow, and by the laminae and transverse processes in the dorsal region, where it is deep and broad. In the recent state, these grooves lodge the deep muscles of the back. External to the vertebral grooves are the articular processes, and still more externally the transverse processes. In the dorsal region, these latter processes stand backwards, on a place considerably posterior to the same processes in the cervical and lumbar regions. The transverse processes in certain regions of the spine are formed of two different parts, or segments. In the cervical region, these two segments are distinct; the one arising from the side of the body, the other from the pedicle of the vertebra; and these uniting, enclose the vertebral foramen. In the dorsal region, the anterior segment is wanting; the posterior segment retaining the name of the transverse process. In the lumbar region, the anterior segments (which are largely developed) are called the transverse processes; but, in reality, they are lumbar ribs, the posterior segments or true transverse processes existing in a rudimentary state, and being developed from the superior articular processes, as in the cervical region. In the cervical region, the transverse processes are placed in front of the articular processes, and between the intervertebral foramina. In the lumbar, they are placed also in front of the articular processes, but behind the intervertebral foramina. In the dorsal region, they are posterior both to the articular processes and foramina.
The lateral surfaces are separated from the posterior by the articular processes in the cervical and lumbar regions, and by the transverse processes in the dorsal. These surfaces present in front the sides of the bodies of the Vertebrae, marked in the dorsal region by the facets for articulation with the heads of the ribs. More posteriorly are the intervertebral foramina, formed by the juxtaposition of the inter-vertebral notches, oval in shape, smallest in the cervical and upper part of the dorsal regions, and gradually increasing in size to the last lumbar. They are situated between the transverse processes in the neck, and in front of them in the back and loins, and transmit the spinal nerves. The base of the vertebral column is formed by the under surface of the body of the fifth lumbar vertebra, and the summit by the upper surface of the atlas. The vertebral canal follows the different curves of the spine; it is largest in those regions in which the spine enjoys the greatest freedom of movement, as in the neck and loins, where it is wide and triangular; and narrow and rounded in the back, where motion is more limited. PREVIOUS PAGE