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level of the third sacral segment is reached, at which point the transverse diameter is somewhat abruptly diminished, a reduction in width which is further suddenly accentuated opposite the fifth sacral segment.
As viewed from the side, the bodies display a gradual increase in their antero-posterior extent until the second lumbar vertebra is reached, below which, that diameter is slightly reduced. In the sacral region the reduction in the antero-posterior diameter is great in the first and second sacral segments, more gradual and less marked in the last three segments. The facets for the heads of the ribs in the upper thoracic region lie on the sides of the bodies; those for the tenth, eleventh, and twelfth are placed farther back on the roots of the vertebral arches.
The intervertebral foramina increase in size from above downwards in the movable part of the column, being largest in the lumbar region. In the sacral region they decrease in size from above downwards. In the cervical region the highest two cervical nerves pass out behind the articular processes of the atlas and epistropheus, and lie, therefore, behind the corresponding transverse processes of those vertebræ. The succeeding cervical nerves pass out through the intervertebral foramina, which are placed between the transverse processes and anterior to the articular processes. In the thoracic and lumbar vertebræ the intervertebral foramina lie anterior to both the articular and transverse processes. The arrangement of the intervertebral foramina in the sacrum has been already sufficiently explained.
The vertebral canal for the lodgment of the spinal medulla and its meninges is largest in the cervical and lumbar regions, in both of which it assumes a triangular form; whilst it is narrow and circular in the thoracic region. These facts are correlated with the movements of the column which are most free in those regions where the canal is largest, i.e. the neck and loins.
The average length of the vertebral column is from 70 to 73 centimetres, or from 27 to 283 inches. Of this the cervical part measures from 13 to 14 cm. ; the thoracic, 27 to 29 cm.; lumbar, 17 to 18 cm. ; and the sacro-coccygeal, 12 to 15 cm. The individual differences in the length of the column are less than one might expect, the variation in height of different individuals being often largely dependent on the length of the lower limbs. In the female the average length of the column is about 60 centimetres, or 233 inches, and the curve in the lumbar region is usually more pronounced.
DEVELOPMENT OF THE VERTEBRAL
The Cartilaginous Column.
As has been already stated (p. 37), the neural tube and the notochord are enveloped by a continuous sheath of mesodermal tissue which forms the membranous vertebral column. It is by the chondrification of this that the cartilaginous column is developed. This process commences about the end of the first or the beginning of the second month of foetal life. In correspondence with each vertebral segment, two symmetrical nodules of cartilage appear on either side of the notochord; these rapidly surround and constrict it. By their fusion they constitute the body of a
cartilaginous vertebra, and are so disposed that they alternate in position with the muscle plates which are lying on either side. In this way a vertebral body corresponds in position to the caudal half of the anterior myotome, and the cephalic half of the posterior myotome, the intermyotomic intervals, which contain the connective tissue plates separating the muscle segments, lie in line laterally with the mid-points of the sides of the cartilaginous vertebræ. It is by chondrification of these intersegmental layers that in certain regions the ribs are ultimately developed. Meanwhile, the scleratogenous tissue between the chondrifying vertebral bodies undergoes little change and persists as the intervertebral fibro-cartilage. Here the embedded notochord undergoes but slight compression and enlarges, so that if a length of the column be examined in longitudinal section the notochord displays a moniliform appearance, the constricted parts corresponding to the bodies, the enlarged portions to the fibro-cartilages. The former disappear at a later stage when ossification begins, but the latter persist in the adult as the pulpy core in the centre of the intervertebral fibro-cartilage.
The portions of the scleratogenous tissue which lie lateral to the notochord have next to be considered; these extend dorsalwards around the vertebral canal, and ventralwards beneath the notochord. The former is sometimes called the vertebral bow, the latter the hypochordal bow. The vertebral bow begins to chondrify on each side, and forms the lateral portions of the cartilaginous vertebral
FIG. 117. THE DEVELOPMENT OF THE MEMBRANOUS BASIS, OF A VERTEBRA (after Keith).
arch, the extremities of which usually unite dorsally about the fourth month of foetal life; if from defective development this union should fail to occur deformity known as spina bifida is the result.
From the cartilaginous vertebral arch, so formed, arise the chondrified rudiments of the spinous, transverse, and articular processes.
The chondrification of the vertebral arch is variously described as being independent of the body or an extension from it; in any case, union between it and the body is rapidly effected.
The scleratogenous tissue between the cartilaginous vertebral arches which does not undergo chondrification persists as the ligaments uniting the vertebral lamina. As regards the so-called hypochordal bow, for the most part it disappears. By some it is regarded as being represented by a fibrous strand in the intervertebral fibro-cartilage on the cephalic side of the vertebra to which it belongs. It is, however, noteworthy that in the case of the atlas vertebra there is an exception to this arrangement; for here the hypochordal bow chondrifies and subsequently by ossification forms the anterior arch of that bone-an arch which lies ventral to, and embraces the dens of the epistropheus (q.v. p. 91).
It is only in the thoracic region that the ribs, developed as stated above by the chondrification of the intersegmental septa, attain their full dimensions. In the cervical, lumbar, and sacral regions they exist only in a rudimentary or modified form, as has been described elsewhere. In the construction of the chest wall the ribs are supported ventrally by the sternum, as to the development of which there is some difference of opinion. Ruge has described this bone as formed by the fusion of two cartilaginous bands produced by the coalescence of the expanded ends of the first five
or seven cartilaginous ribs. Paterson, on the other hand, regards the sternum as arising independently of the ribs by the union of a right and left sternal bar in the median ventral line. There are also reasons for supposing that the presternum is intimately associated with the development of the ventral part of the shoulder girdle.
Ossification of the Vertebræ.-The vertebræ are developed by ossification of the cartilage which surrounds the notochord and which passes dorsally over the sides of the vertebral canal. The centres for the bodies first appear in the lower thoracic vertebræ about the tenth week. An oval nucleus develops in each body. At first it is placed dorsal to the notochord, but subsequently surrounds and causes the disappearance of that structure. Occasionally, however, the primitive centre appears to be formed by the coalescence of two primary nuclei. Support is given to this view by the occasional occurrence of vertebræ in which the body is developed in two collateral halves, or in cases where only one-half of the body persists (Turner); normally, however, it is impossible to make out this division. From these single nuclei the bodies are developed, the process extending up and down the column until, by the fifth month, all the bodies possess ossific nodules, except the coccygeal segments. About the seventh week a single centre appears in the vertebral arch on either side. These commence first to ossify in the upper cervical region and extend rapidly downwards throughout the column. They first appear near the bases of the superior articular processes, and extend backwards into the laminæ, laterally into the transverse processes, and forwards into the roots of the vertebral arches. These latter project anteriorly and form a considerable portion of the postero-lateral aspects of the body, from which, however, they are separated by a cartilaginous strip-the neuro-central synchondrosis --which does not entirely disappear until about the fifth or sixth year. It is important to note that in the thoracic region the costal facets lie Centre for behind the neuro-central synchondrosis, and are therefore borne on the lateral aspects of the roots of the vertebral arches. Fusion of the laminæ in the median plane posteriorly begins, after birth, in the lumbar region and extends upwards, so that by the fifteenth month or thereabouts the arches in the cervical region are completed posteriorly. In the sacral region ossification is slower, the vertebral canal not being enclosed till the seventh to the tenth year. The spinous processes are cartilaginous at birth, but they become ossified by the extension into them of the bony laminæ.
At puberty certain secondary or epiphyseal centres make their appearance; these are five in number. One caps the summit of the spinous process, except in the cervical region. A single centre on each side appears at the extremity of the transverse process, and in the thoracic region assists in forming the articular surface for the tubercle of the rib. Two epiphysial plates are formed-one for the superior, and the second for the inferior surface of the body, including also that part which lies posterior to the neuro-central synchondrosis and is formed by the root of the vertebral arch; from these the thickened circumference of both upper and lower aspects of the body are derived. Fusion of these centres with the rest of the bone is not complete till the twenty-fifth year.
In the cervical region independent centres are described as occurring in the anterior roots of the transverse processes of the sixth and seventh vertebræ. These correspond to the costal element, and may occasionally persist in the form of cervical ribs. Elsewhere they are formed by lateral extensions from the root of the vertebral arch.
In the lumbar region the transverse process of the first lumbar vertebra is occasionally associated with an independent costal centre, which may blend with it, or persist as a lumbar rib. The mamillary processes are derived from separate centres. The vertebral arch of the fifth lumbar vertebra is occasionally developed from two centres on each side, as is demonstrated by the fact that the arch is sometimes divided by a synchondrodial joint running obliquely across between the superior and inferior articular processes. (See ante, p. 91; also Fortschritte auf dem Gebiete der Röntgenstrahlen. Ergänzungsheft i.; "die Entwickelung des menschlichen Knochengerüstes während des fötalen Lebens," von Lambertz.) At the eighteenth year there are two epiphyses at the end of the costotransverse process of the fifth lumbar vertebra; one caps the transverse element, the other caps the costal element (Fawcett).
Atlas.-The lateral masses, transverse processes, and posterior arch are developed from two centres-one on each side-which correspond with the centres from which the vertebral arches of the other members of the series are developed. These make their appearance about the seventh week, and do not unite posteriorly till after the third Their point of union is sometimes preceded by the formation of a distinct spinal
nucleus (Quain). The transverse processes are completed by epiphyses about the eighteenth year (Fawcett). The anterior arch is developed from centres variously described as single or double, which appear in the hypochordal arch of cartilage described by Froriep (Arch. f. Anat. u. Physiol., Anat. Abth. 1886) which here persists. In this cartilage ossification commences during the first year of life. Union with the lateral masses is delayed till six or eight years after birth. The lateral extremities of the anterior arch assist in forming the anterior part of the superior articular processes.
Epistropheus. The epistropheus ossifies from five primitive centres. Of these, two --one on each side-appear about the seventh week, and form the articular and transverse processes, together with the lamina and spinous process. One, or it may be two, nuclei appear in the inferior part of the body about the fifth month. The superior part of the body, including a small part of the superior articular process, and the base of the dens,
16. Appears at puberty; unites at 25th year.
17. Appears at puberty; unites at 25th year. 18. Appears about 6th week.
19. Centre for transverse process and vertebral arch;
appears about 8th week.
20. Synchondroses close about 3rd year.
21. Centre for summit of dens; appears 3rd to 5th
22. Appears about 5th or 6th month; unites with
24. Inferior epiphysial plate; appears about puberty,
unites about 25th year.
25. Single or double centre for body; appears about
26. Posterior arch and lateral masses developed from a single centre on either side, which appears about 7th week. In this figure the posterior arch is represented complete by the union posteriorly of its posterior elements.
27. Anterior arch and portion of superior articular surface developed from single or double centre, appearing during 1st year.
28. Epiphysis for transverse process; appears about
Centre for vertebral arch on either side; appears about 6th or 7th week, the laminæ unite from birth to 15th month. The arch is here shown complete posteriorly.
31. Centre for body; appears about 6th week, unites with vertebral arch from 5th to 6th year.
are developed from two laterally-placed nuclei which appear shortly after, and fuse together at the seventh or eighth month, so that at birth the bone consists of four pieces. Fusion between these parts takes' place in the following order :-The dens unites with the body and lateral parts about the third or fourth year; union between the two lateral portions posteriorly and the body and lateral parts anteriorly, is complete at from four to six years. The summit of the dens is developed from a separate centre, occasionally double, which appears from the third to the fifth year, and fuses with the rest of the bone from the eighth to the twelfth year. About puberty an annular epiphysis is developed on the inferior surface of the body, with which it is completely united during the twentieth to the twenty-fifth year. Some authorities state that a few granules between the base of the dens and the superior surface of the body represent the superior epiphysial plate; but
as fusion between the dens and the body occurs before the time for the appearance of these secondary epiphysial plates, this can hardly be regarded as correct. The line of fusion of the dens with the body is defined by a small disc of cartilage which persists within the substance of the bone till an advanced period of life.
A pair of epiphyses placed over the tubercles of the spinous process, if not always present, are at least frequent.
Sacrum. Each of the sacral segments is ossified from three centres: one for the body, and two for the vertebral arch-that for the body, which makes its appearance in the first three sacral vertebræ about the end of the third month, about the fifth to the eighth month for the last two segments. From the two centres for the vertebral arches, which make their appearance about the fifth or sixth month in the higher segments, the laminæ, articular processes, and the posterior half of the alæ on either side are developed. The sacral canal is not enclosed till the seventh to the tenth year, the laminæ usually failing to meet in the lowest segment, and occasionally, to a greater or less extent, in some of the higher segments. The anterior portion of the lateral parts is developed from separate centres which represent the costal elements (Gegenbauer). These appear about the sixth to the eighth month, and may develop in relation to the upper four sacral segments; more usually they are met with in connexion with the first three, and exceptionally they may be found only in the upper two. It is by fusion of these with the posterior arches that the lateral parts, which support the hip bones, are formed. The costal elements fuse about the second to the fifth year with the vertebral arches, prior to their union with the bodies; and the segments of the lateral parts unite with each other sooner than the union of the bodies is effected. The latter only takes place after puberty by the fusion of the epiphysial plates, a pair of which make their appearance between the bodies of each segment. The lower segments begin to unite together about FIG. 120.—DIAGRAM (after Fawcett) OF THE and second sacral vertebræ is not completed till the eighteenth year, but fusion between the first
ARRANGEMENT OF THE COSTAL AND TRANS-
T., Epiphysis of transverse process.
C. V., Ventral epiphysis of costal process.
the twenty-fifth year or after. In addition to the foregoing there are costal and transverse epiphyses. According to Fawcett they are arranged as follows. Costal epiphyses: The costal processes of the I. and II. sacral segments bear at their The numbers indicate the segments to which lateral ends inferiorly two such epiphyses, one the epiphyses belong. dorsal and one ventral; these, by their fusion and expansion mainly in an upward direction, form a plate-the auricular facet. The III. and IV. costal processes have only one epiphysis each, viz., the ventral. All these appear about the eighteenth year. Transverse epiphyses: Epiphyses are developed on all the transverse processes of the sacral vertebræ except the II. Those of the IV. and V. play an important part in the moulding of the lower lateral region of the sacrum. Thus, the transverse epiphysis of the IV. segment becomes comma-shaped by downward and lateral growth, the head of the comma fuses with the costal epiphysis of the III. sacral segment, which in turn unites with the epiphysis of the transverse process of the V. segment, the ultimate result being a Z-like arrangement on the posterior and inferior aspect of the sacrum. The extremities of the superior spinous processes are occasionally developed from independent epiphyses. On making a median section of an adult bone the persistence of the intervertebral fibro-cartilages between the bodies is indicated by a series of oval cavities.
Coccygeal Vertebræ. These are cartilaginous at birth. Each has a separate centre; the first appears from the first to the fourth year, the second from the sixth to the tenth year, the third and fourth segments at or about puberty. Secondary centres, for the coccygeal cornua and epiphysial plates for the bodies are also described. Fusion of the various segments begins below and proceeds upwards, but is liable to great individual variation. In advanced life the coccyx is often ossified to the sacrum.
The sternum occupies the middle of the upper part of the thoracic wall anteriorly. It is connected on each side with the cartilages of the first seven ribs, and supports, superiorly, the clavicles. It consists of three parts, named respectively