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iliac joint; this feature is common in the Simiidæ and some of the lower races of mankind (Paterson). The surface of bone between and lateral to the first, second, third, and fourth foramina affords attachment to the fibres of origin of the piriformis, which may in some instances extend on to the bodies of the second and third segments (Adolphi), whilst on the edge lateral to and below the fourth foramen the coccygeus is inserted.
The posterior surface is rough and irregular. Convex from above downwards, it displays in the median plane the crista sacralis media, a crest whereon are seen four elongated tubercles-the spines of the upper four sacral vertebræ. Lateral to these the bone forms a groove-the sacral groove-the floor of which is made up of the confluent laminae of the corresponding vertebræ. In line with the intervals between the spines, and wider apart above than below, another series of tubercles is to be Superior articular processes Transverse process of first sacral vertebra
seen. These are due to the fusion of the articular processes of the sacral vertebræ, which thus form faint interrupted ridges on each side of the bone (crista sacrales articulares). Normally, the spine of the lowest sacral segment is absent, and the laminæ do not coalesce medially, thus leaving a gap in which the sacral canal is exposed (hiatus sacralis); whilst inferiorly the tubercles corresponding to the inferior articular processes of the last sacral vertebra form little down-projecting processes the sacral cornua-by means of which the sacrum is in part united the coccyx. Just wide of the articular tubercles are the posterior sacral foramina, for the transmission of the posterior rami of the sacral nerves. These are in correspondence with the anterior foramina, so that a probe can be passed directly through both openings; but be it noted that the posterior are much smaller, and their margins much sharper, than is the case with the anterior. The surface of the pars lateralis (lateral mass) lateral to the posterior sacral foramina is rough and irregular, owing to the presence of four more or less elevated tubercles, which constitute the lateral ridges on either side of the bone (crista sacrales
laterales), and which are serially homologous with the true transverse processes of the lumbar vertebræ. The posterior surface of the bone furnishes an extensive surface for the origin of the sacro-spinalis, whilst the edge of the bone lateral to the third and fourth foramen gives attachment to the gluteus maximus.
The base of the bone displays features more in accordance with a typical vertebra. Centrally, and in front, is placed the body, the superior surface of which articulates with the last lumbar vertebra through the medium of an intervertebral fibro-cartilage. The anterior margin is thin and projecting, overhanging the general concavity of the pelvic surface of the bone, and forming what is called the promontory. Posterior to the body, the sacral canal, of triangular form but slightly compressed dorso-ventrally, is seen, whilst still more posteriorly is the short spinous
process, forming the highest tubercle of the median crest. Spreading out from the sides, and partly from the back of the body on each side, is a fan-shaped mass of bone, the upper surface of which is slightly concave from side to side, and convex from above and behind downwards and forwards. This, the ala sacralis, corresponds to the thick upper border of the lateral part, and is formed, as will be explained hereafter, by elements which correspond to the roots of the vertebral arches (O.T. pedicles) and the transverse processes of the sacral vertebræ, together with superadded structures the sacral ribs. The lateral margin of the lateral part, as seen from above, is sharp and laterally convex, terminating posteriorly in a prominent tuberclethe highest of the series of elevations seen on the posterior surface of the bone, which have been already described as serially homologous with the true transverse processes of the lumbar vertebra. Fused with the dorsal surface of each lateral part, and separated from it laterally by a narrow but deep notch, is the superior articular process. This supports a vertical articular surface, which is of circular or oval form, and concave from side to side, having a general direction backwards and a little medially. The borders of the sacrum are thick above, where they articulate with the ilia,
thin and tapering below, where they furnish attachments for the powerful sacrotuberous ligaments (O.T. great sacro-sciatic). The iliac articular surfaces are described as auricular in shape (facies auricularis), and overlie the lateral parts formed by the first three sacral vertebræ, though this arrangement is liable to considerable variation. Posterior to the auricular surface the bone is rough and pitted by three distinct depressions for the attachment of the strong sacro-iliac ligaments. Inferiorly, the edge formed by the lateral parts of the fourth and fifth sacral vertebræ becomes gradually thinner, and at the inferior lateral angle changes its direction and sweeps medially towards the body of the fifth sacral segment.
The apex, or lower end of the sacrum, is formed by the small oval body of the fifth sacral vertebra, which articulates with the coccyx.
The sacral canal follows the curve of the bone; more or less triangular in shape above, it becomes compressed and flattened dorso-ventrally below. Inferiorly, its posterior wall is deficient owing to the imperfect ossification of the lamina of the fifth, and, it may be, of the fourth sacral segments. Passing obliquely downwards and laterally from this canal into the lateral parts on either side are the four pairs of intervertebral foramina, each of which is connected laterally with a V-shaped canal which terminates in front and behind in the anterior and posterior sacral foramina. The posterior limb of the V is shorter and narrower than the anterior. The female sacrum is proportionately broader than the male, its curves are liable to great individual variation; usually it is flattened above, and somewhat abruptly curved below, as contrasted with the male sacrum, in which the curve is more uniformly distributed throughout the bone. In the female the absolute depth of the curve is less than in the male. The iliac articular surface of the female sacrum is smaller than, and of a different shape from, that of the male; in the majority of cases it only extends over two sacral segments, whereas in the male it invariably includes a part, and at times the whole of the third segment (Derry). The variation in the proportions of the breadth to the length of the sacrum is
expressed by the formula
breadth x 100
Sacral Index. Sacra with an index above
100 are platyhieric and are generally characteristic of the higher races, those with an index below 100 are dolichohieric and are more commonly met with in the lower races of men. The average European index is 112-4 for males and 116-8 for females.
The coccyx consists of four-sometimes five, less frequently three-rudimentary vertebræ, which tend to become fused. The first piece is larger than the others; it has an oval hollow
facet on its superior sur
face, which articulates
with the body of the last
processes of the sacrum, extend upwards and unite with the sacral cornua, thus bridging Over the notch for the exit of the fifth sacral
nerve, and converting it into a foramen, the
A. Posterior Surface. B. Anterior Surface.
3. For Sacrum. 4. Cornu.
last of the intervertebral 1. Transverse process.
slight traces of a transverse process and the rudiments of roots of the vertebral arch. The succeeding segments are mere rounded or oval-shaped nodules of bone.
Fusion between the lower elements occurs normally in middle life, whilst union between the first and second segments occurs somewhat later. It is not unusual, however, to find that the first coccygeal vertebra remains separate from the others. Though very variable, as a rule, fusion occurs more commonly in the male, and at an earlier age, than in the female. Szawlowski has recorded a case in which a curved process arose from the ventral surface of the first coccygeal segment. He regards this as possibly the homologue of a ventral arch (Anat. Anz. Jena, vol. xx. p. 320).
From the posterior surface of the coccyx the gluteus maximus arises, whilst to it is attached the filum terminale of the spinal medulla. To its borders are attached the coccygei and levatores ani muscles; and from its tip spring the fibres of the sphincter ani externus.
THE VERTEBRAL COLUMN AS A WHOLE.
When all the vertebræ are articulated together, the resulting column displays certain characteristic features. The division of the column into a true or movable part, comprising the members of the cervical, thoracic, and lumbar series, and a false or fixed portion, including the sacrum and coccyx, can be readily recognised. The, vertebræ are so disposed that the bodies form an interrupted column of solid parts anteriorly, which constitutes the axis of support for the head and trunk; whilst the vertebral arches posteriorly provide a canal for the lodgment and protection of the spinal medulla and its membranes. In the movable part of the column both the anterior supporting axis and the vertebral canal are liable to changes in their disposition, owing to the movements of the head and trunk. Like the bodies and vertebral arches, the spinous and transverse processes are also superposed, and fall in line, forming three series of interrupted ridgesone (the spinous) placed centrally and behind, the others (the transverse) placed laterally. In this way two vertebral grooves are formed which lie between the central and lateral ridges. The floor of each groove is formed by the lamina and articular processes, and in these grooves are lodged many of the muscles which serve to support and control the movements of the column.
Further, the column so constituted is seen to display certain curves in an antero-posterior direction. These curves are, of course, subject to very great variation according to the position of the trunk and head, and can only be satisfactorily studied in a fresh specimen; but if care is exercised in the articulation of the vertebræ, the following characteristic features may be observed, assuming, of course, that the column is erect and the head so placed that the axis of vision is directed towards the horizon. There is a forward curve in the cervical region, which gradually merges with the backward thoracic curve; this becomes continuous below with an anterior convexity in the lumbar region, which ends more or less abruptly at the union of the fifth lumbar with the first sacral vertebra, where the sacrum slopes suddenly backwards, causing the column to form a marked projection-the sacro-vertebral angle. Below this, the anterior concavity of the front of the sacrum is directed downwards as well as forwards. Of these four curves, two-the thoracic and sacral-are primary, they alone exist during fœtal life; whilst the cervical and lumbar forward curves only make their appearance after birth-the former being associated with the extension and elevation of the head, whilst the latter is developed in connexion with the use of the hind limb in the hyper-extended position, which in man is correlated with the assumption of the erect posture; this curve, therefore, only appears after the child has begun to walk. For these reasons the cervical and lumbar curves are described as secondary and compensatory.
Not infrequently there is a slight lateral curvature in the thoracic region, the convexity of the curve being usually directed towards the right side. This may be associated with a greater use of the muscles of the right upper limb, or may depend on the pressure exercised by the upper part of the thoracic aorta on the
vertebræ of the thoracic region, thus causing a slight lateral displacement, together with a flattening of the side of the fifth thoracic vertebra (impressio aortica) as was first pointed out by Wood (Journ. Anat. and Physiol.
vol. iii.). Above and below this curve there are slight compensatory curves in the opposite direction.
The line which unites the tips of the spinous processes is not a repetition of the curves formed by the bodies. This is due to the fact that the length and direction of the spinous processes vary much in different regions; thus, in the neck, with the exception of the second, sixth, and seventh, they are all short (absent in the case of the atlas). In the thoracic region the spinous processes, though long, are obliquely placed-a circumstance which much reduces their prominence; that of the seventh thoracic vertebra is usually the longest and most slanting. Below that point their length gradually decreases, and their position more nearly approaches the horizontal. In the loins the spinous processes have all a slight downward direction.
The spinous processes of the upper three or four sacral vertebræ form an osseous ridge with interrupted tubercles. The ridge formed by the vertebral spines is an important determinant of the surface form, as it corresponds to the median furrow of the back, and there the individual spines may be felt and counted from the seventh cervical down to the sacral region. That is best, done when the back is well bent forwards.
Taken as a whole, the spinous processes of the movable vertebræ in man have a downward inclination-a character which he shares with the anthropoid apes and a few other animals. This character serves to distinguish his column from those of lower mammals in which the spines of the lumbar vertebræ are directed headwards towards the "centre of motion," which is usually situated near the caudal extremity of the thorax, where a vertebra is placed the direction of whose spine is vertical to the horizontally disposed column; this. vertebra is often referred to as the anticlinal vertebra.
As viewed from the front, the vertebral bodies increase in width from the second cervical to the first thoracic; thence a reduction in breadth takes place to the level of the fourth thoracic, below which there is a gradual increase in their transverse diameters until the sacrum is reached. There a rapid reduction in width takes place, terminating inferiorly in the nodules of the coccyx.
The transverse processes of the atlas are wide and outstanding. The succeeding four cervical vertebræ have transverse processes of nearly equal width; the seventh, however, displays a marked increase in its transverse diameter, and is about equal in width to the first thoracic vertebra. Below this a gradual and regular diminution in width characterises the transverse processes of the thoracic vertebræ, until in the case of the eleventh and twelfth they are merely represented by the small lateral tubercles. In the lumbar region the transverse processes again appear outstanding, and of nearly equal length.
FIG. 115.-VERTEBRAL COLUMN
The transverse diameter of the lateral parts of the first sacral vertebra forms the widest part of the column. Below that, a decrease in width occurs until the