This articulation is a cartilaginous joint, between the apex of the sacrum and the base of the coccyx, the bones being united by anterior, posterior and lateral sacrocoecygeal ligaments, and by a disc of fibrocartilage.
The anterior sacrocoecygeal ligament (fig. 545) consists of a few irregular fibers which descend from the anterior surface of the sacrum to the front of the coccyx.
The superficial posterior sacrocoecygeal ligament is a flat band which arises from the margin of the sacral hiatus, and descends to be inserted into the posterior surface of the coccyx. This ligament completes the lower part of the sacral canal.
The deep posterior sacrocoecygeal ligament extends from the back of the fifth sacral vertebra to the back of the coccyx.
The intercornwal ligaments connect the cornua of the sacrum and coccyx on each side.
A lateral sacrococcygeal ligament exists on each side and connects the transverse process of the coccyx to the inferior lateral angle of the sacrum; it completes the foramen for the fifth sacral nerve.
A thin disc of fibrocartilage is interposed between the contiguous surfaces of the sacrum and coccyx; it is somewhat thicker in front and behind than at the sides. Occasionally the coccyx is freely movable on the sacrum; in such cases an articular capsule lined with synovial membrane is present.
In the young subject the different segments of the coccyx are connected together by the extension downwards of the anterior and posterior sacrococcygeal ligaments, thin annular discs of fibrocartilage being interposed between the segments. In the adult male, all the pieces become ossified together at a comparatively early period; but in the female, this does not commonly. occur until a later period of life. At a more advanced age the joint between the sacrum and coccyx is obliterated.
Backward and forward movements take place between the sacrum and coccyx; their extent increases during pregnancy.
Movements of the vertebral column.—The movements permitted in the vertebral column are : flexion, extension, lateral movement, circumduction, and rotation.
In flexion, or movement forwards, the anterior longitudinal ligament is relaxed and the anterior parts of the intervertebral discs are compressed while the posterior longitudinal ligament, the ligaments flava, and the interspinous and supraspinous ligaments are stretched, as well as the posterior fibers of the intervertebral discs. The interspaces between the laminae are widened, and the inferior articular processes glide upwards upon the superior articular processes of the subjacent vertebrae. Flexion is most extensive in the lumbar region.
In extension, or movement backwards, an exactly opposite disposition of the parts takes place. This movement is limited by the anterior longitudinal ligament, and by the approximation of the spines, it is freest in the cervical region.
In lateral movements, the sides of the intervertebral discs are compressed, the extent of motion being limited by the resistance offered by the surrounding ligaments. Lateral movements may take place in any part of the column, but are freest in the cervical and lumbar regions.
Circumduction is very limited, and is merely a succession of the preceding movements.
Rotation is produced by the twisting of the vertebra on the intervertebral discs; this, although only slight between any two vertebral, allows of a considerable extent of movement when it takes place in the whole length of the column, the front of the upper part of the column being turned to one or other side. This movement occurs to a slight extent in the cervical region, is freer in the upper part of the thoracic region, and absent in the lumbar region.
The extent and variety of the movements are influenced by the shape and direction of the articular facets. In the cervical region the upward inclination of the superior articular facets allows of free flexion and extension. Extension can be carried farther than flexion; at the upper end. of the region it is checked by the locking of the posterior edges of the superior atlantal facets in the condylar fossae of the occipital bone; at the lower end it is limited by a mechanism whereby the inferior articular processes of the seventh cervical vertebra slip into grooves behind and below the superior articular processes of the first thoracic. Flexion is arrested just beyond the point where the cervical convexity is straightened; the movement is checked by the apposition of the projecting lower lips of the bodies of the vertebra; with the shelving surfaces on the bodies of the subjacent vertebrae. Lateral flexion and rotation are free in the cervical region, and are always combined; the upward and medial inclinations of the superior articular facets impart a rotatory movement during lateral flexion. In the thoracic region, notably in its upper part, all the movements are limited in order to reduce interference with respiration to a minimum. The almost complete absence of an upward inclination of the superior articular facets prohibits any marked flexion, while extension is checked by the contact of the inferior articular margins with the lamina:, and the contact of the spines with one another. Rotation is free in the thoracic region: the superior articular processes are segments of a cylinder whose axis is in the paid-ventral line of the vertebral bodies. The direction of the articular facets would allow of free lateral flexion, but this movement is considerably limited in the upper part of the region by the resistance of the ribs and sternum. In the lumbar region flexion and extension are free. The inferior articular facets are not in close apposition with the superior facets of the subjacent vertebrae, and on this account a considerable amount of lateral flexion is permitted. For the same reason a slight amount of rotation can be carried out, but this is so soon checked by the interlocking of the articular surfaces that it is negligible.
Muscles producing the movements.—-The vertebral column may be moved either by (a) muscles attached to it and acting directly on it, or by (b) muscles attached to other bones and acting indirectly on the column.
a. Muscles acting directly on the vertebral column.
- Flexion- Longus cervicis (Longus colli), Scaleni’ Quadratus lumborum, Psoas major and Psoas minor.
- Extension- Interspinales, Multifidus, Spinales, Semispinales thoracis et cervicis, Costocervicalis (Iliocostales cervicis), Longissimi thoracis, cervicis and Splenius cervicis.
- Lateral flexion.- Intertransversarii, Multifidus, Costocervicalis (Iliocostalis cervicis), Longissimus cervicis, Splenius cervicis, Levatores costarum, Longus cervicis (Longus colli), Scaleni, Quadratus lumborum and Psoas major.
- Rotation.–Rotatores, Multifidus, Splenius cervicis, Semispinalis thoracis et cervicis, Levatores costarum and Longus cervicis.
b. Muscles acting indirectly on the vertebral column.
- Flexion- Sternomastoid, Longus capitis and the abdominal muscles.
- Extension.-Splenius capitis, Semispinalis capitis, Iliocostales, Costales (haocostales dorsi), and Longissimi thoracis et capitis.
- Lateral flexion and rotation.-Sternomastoid, External oblique, Internal oblique, Iliocostales, Costales (Iliocostales dorsi), and Longissimi thoracis et capitis.
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