General features,–The sacrum (figs. 279-283) is a large bone, triangular in shape, formed by the fusion of the five sacral vertebra. It is situated at the upper and posterior part of the pelvic cavity, where it is inserted like a wedge between the two hip-bones.
Its narrow, blunted apex is situated at the inferior end of the bone and articulates with the coccyx. At the opposite end the wide base projects upwards and forwards to articulate with the fifth lumbar vertebra, with which it forms the sacrovertebral angle. The bone is placed very obliquely and is curved longitudinally so that its dorsal aspect is convex and its ventral aspectis concave (fig. 281). This ventral concavity serves to increase the capacity of the true pelvis. In addition to a base and an apex the sacrum possesses dorsal, pelvic, and lateral surfaces and encloses a bony canal.
In the child the individual sacral vertebra are connected by cartilage and can be separated by maceration. The adult bone shows many signs of its vertebral constitution, especially on its basal aspect.
The base (fig. 282) is formed by the upper surface of the first sacral vertebra and presents all the features of a typical vertebra in a slightly modified form. The body is large and much wider from side to side than from before backwards. Its anterior projecting edge is named the sacral promontory. The vertebral foramen is triangular, and its shape is explained by the fact that the pedicles are short, widely separated, and directed backwards and laterally. The lamina are very oblique and incline downwards, medially and backwards. Where they meet, the spine is represented by a spinous tubercle. The superior articular processes project upwards and bear concave articular facets, which are directed medially and backwards to articulate with the inferior articular processes of the fifth lumbar vertebra. The posterior part of the process projects backwards, and its lateral aspect bears a roughened area which corresponds to the mamillary process of a lumbar vertebra. The region of the transverse process shows important modifications. A broad, sloping mass of bone projects from the lateral side of the body, pedicle and superior articular process.
The mamillary and accessory processes are merely muscular processes which. represented and conjoined in the thoracic region, become separated in the lumbar region by the passage of the internal (medial) branch of the posterior division (posterior primary ramus) of the lower thoracic and lumbar nerves between them, a feature which is not found in any of the other vertebrae, although it is foreshadowed in the fifth lumbar vertebra. It is formed by the transverse process and the costal element fused to each other and to the rest of the vertebra, and forms the upper surface of the lateral mass of the sacrum.
The pelvic surface, of the sacrum is concave and is directed downwards and forwards. It is marked by four pairs of anterior sacral foramina, which communicate through the intervertebral foramina with the sacral canal. They transmit the anterior primary rami of the sacral nerves. The large area which lies between the foramina of the right and left sides is formed by the flattened anterior surfaces of the bodies of the sacral vertebrae, and the lines of fusion of contiguous vertebra are clearly visible as four raised transverse ridges. The bars of bone which separate the foramina from one another on each side represent the costal elements, which are fused to the vertebrae. Lateral to the foramina the costal elements of each side unite with one another to form the lateral mass of the sacrum.
Lateral to this area the dorsal surface of the sacrum presents on each side four posterior sacral foramina. Like the anterior foramina they communicate with the sacral canal through the intervertebral foramina, and each transmits the posterior primary ramus of a sacral nerve. Medial to the foramina, and in line with the superior articular processes of the first sacral vertebra, the bone is marked by a row of four small tubercles, which represent contiguous articular processes fused together. The inferior articular processes of the fifth sacral vertebra are free and project downwards at the sides of the sacral hiatus. They are termed the sacral cornea and are connected to the cornea of the coccyx. The roughened area to the lateral side of the posterior sacral foramina is formed by the fused transverse processes and presents a row of transverse tubercles.
The lateral surface (fig. 281) of the sacrum represents the fused transverse processes and costal elements. It is wide above but rapidly diminishes in breadth in its lower part. The broad upper part bears an ear-shaped surface, termed the auricular surface, for articulation with the ilium, and the area behind it is rough and deeply pitted for the attachment of ligaments. In order to ensure the stability of the body in the erect posture, the sacro-iliac joint, through which one half of the weight of the trunk is transmitted to the lower limb, must provide a good bearing surface. This is obtained by the fusion of the sacral vertebrae and by the persistence of substantial portions of the costal elements. The auricular surface is borne by the costal elements and is shaped like the inverted letter L. The horizontal limb is the shorter and is restricted to the first sacral vertebra : the vertical limb extends downwards to the lower limit of the second or to the middle of the third sacral vertebra. The lower part of the lateral surface takes no part in the transmission of the weight of the body and is consequently reduced in breadth. At its lower end it bends, or curves, medially to reach the side of the body of the fifth sacral vertebra. The point at which the change of direction occurs is termed the inferior lateral angle. Below the angle the lateral surface forms a thin border.
The sacral canal (fig. 283) is formed by the vertebral foramina of the sacral vertebrae and is triangular on transverse section. Its upper opening, seen on the basal surface, appears to be set obliquely but, owing to the inclination of the sacrum, it is directed upwards in the living subject. The lateral wall of the canal presents four intervertebral foramina, through which the canal is connected with both the anterior and the posterior sacral foramina. The lower opening is formed by the sacral hiatus.
Particular features.—The anterior and posterior parts of the body of the first sacral vertebra. give attachment to the lowest fibers of the anterior and posterior longitudinal ligaments respectively. The upper borders of the laminae of the first sacral vertebra give attachment to the lowest ligaments flava. The upper surface of the lateral mass is smooth and slightly concave in its medial part but is irregularly roughened in its lateral part. It is covered almost entirely by the psoas major muscle. The smooth area is in contact: with the anterior primary ramus of the fifth lumbar nerve, as it runs downwards and forwards from its intervertebral foramen to join a branch of the fourth lumbar nerve to form the lumbosacral trunk. The rough area gives attachment to the lumbosacral ligament which lies on the; lateral side o� the fifth lumbar nerve, and to the anterior ligament o� the sacroiliac joint. The anterolateral part of the area gives origin to a portion of the iliacus muscle.
The dorsal surface of the sacrum is rough and irregular. The sacrospinalis (erector spinae) arises by an elongated U-shaped origin from the spinous and transverse tubercles, and covers the multifidus which arises from the intervening area. The posterior primary rami of the upper three sacral nerves pierce these muscles after they emerge from the posterior sacral foramina. It not infrequently happens that the laminae of the fourth sacral vertebra fail to meet in the median plane behind. The sacral hiatus is then elongated considerably.
The sacral canal contains the cauda equina, including the filum terminale and the spinal meninges. Opposite the middle of the sacrum the subarachnoid and subdural spaces become closed, and the lower sacral nerve roots and the filum terminals pierce the arachnoid and dura mater at that level. The filum terminals emerges below at the sacral hiatus and passes downwards across the posterior surface of the fifth sacral vertebra and the sacro coccygeal joint to reach the coccyx. The fifth sacral nerve also emerges through the sacral hiatus close to the medial side of the sacral cornu and grooves the lateral part of the body of the fifth sacral vertebra.
Differences in the sacrum of the male and female.-In the female the sacrum is shorter and wider than in the male, reflecting the necessity for a wider and a shallower pelvic cavity. The upper part of the bone is flattened, and the lower part curved abruptly forwards, whereas in the male curvature is more evenly distributed over the whole length of the bone. It should be remembered, however, that the curvature of the bone may vary considerably in different specimens of the same sex. In the female the bone is directed more obliquely backwards than in the male; this increases the size of the pelvic cavity and renders the sacrovertebral angle more prominent. In the female the auricular surface for articulation with the ilium is shorter than that in the male, extending along the sides of the first and second sacral vertebra only : in the male it is continued down to the middle or lower limit of the third vertebra. No difficulty will be experienced in distinguishing a typical male or a typical female sacrum, but, as the sexual characters are not always pronounced, there are many cases in which the sexing is by no means easy. When there is any difficulty in determining the sex of a sacrum, greatest stress should always be laid on the relationship between the length and the breadth of the bone.
Structure.–The sacrum consists of spongy substance enveloped by a thin layer of compact bone.
Variations.–Either the fifth lumbar or, more commonly, the first coccygeal vertebra may become incorporated in the sacrum, which then consists of six vertebrae. The inclusion of the fifth lumbar vertebra is usually incomplete and it may be limited to one or other side. In the most minor degree of the abnormality the transverse process of the fifth lumbar vertebra, is unusually large, and articulates with the sacrum at the posterolateral part of the upper surface of its lateral mass. Reduction of the number of the constituents of the sacrumis less common. The transverse process of the first sacral segment may not be joined to the rest of the lateral mass on one or both sides, and a considerable part of the posterior wall of the sacral canal may be wanting, in consequence of the imperfect development of the laminae and spines.
Coccyx
General features. The coccyx (figs. 284, 285) is a small bone, triangular in shape, which consists usually of four rudimentary vertebra; fused together, but the number may be increased to five or reduced to three. Not infrequently, the first coccygeal vertebra exists as a separate piece. The bone is directed downwards and forwards from the apex of the sacrum, so that its pelvic surface is directed upwards and forwards and its dorsal surface downwards and backwards.
The base of the coccyx, formed by the upper surface of the body of the first coccygeal vertebra, presents an oval, articular facet for articulation with the apex of the sacrum. Posterolateral to the facet, two processes, named the coccygeal cornua, project upwards to articulate with the sacral cornua ; they are the homologues of the pedicles and superior articular processes of the movable vertebrae. A rudimentary transverse process projects laterally and slightly upwards from each side of the body of the first coccygeal vertebra and may ascend to articulate or fuse with the inferior lateral angle of the sacrum.In that event five pairs of foramina are found in the sacrum.
The second, third and fourth coccygeal vertebrae diminish successively in size and are usually fused with one another. They are mere nodules of bone, which represent the rudimentary bodies of the vertebrae, although the second may show traces of transverse processes and pedicles.
Particular features.-The lateral parts o� the anterior surface, including the rudimentary transverse process, give insertion to the, levatores ani and the coccygei. The anterior sacrocoecygeal ligament is attached to the front of the body of the first and may extend downwards to reach the second. coccygeal vertebra (fig. 545). The cornua give attachment to the intercornual ligaments. The interval between the body of the fifth sacral vertebra and the articulating sacral and coccygeal cornua on each side represents the intervertebral foramen between the fifth sacral and the first coccygeal vertebra, and transmits the fifth sacral nerve. The posterior primary ramus of that nerve descends behind the rudimentary transverse process, but its anterior primary ramus passes forwards through a foramen, placed between the transverse process and the sacrum and bounded laterally by the lateral sacrocoecygeal ligament, which connects the process to the inferior lateral angle of the sacrum. The posterior aspect of the coccyx gives origin, on each side, to the gluteus maximus muscle and its tip, to the sphincter ani externus. The median area gives attachment to the deep and superficial posterior sacrocoecygeal ligaments. The latter extends downwards from the dorsal margins of the sacral hiatus and may close the lower end of the sacral canal. The filum terminale, which is situated between the two ligaments, blends with them on the posterior surface of the first coccygeal vertebra.
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