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Basic Anatomy 693 Fractures of the Odontoid Process of the Axis Fractures of the odontoid process are relatively common and result from falls or blows on the head (Fig. 12.8). Excessive mobility of the odontoid fragment or rupture of the transverse ligament can result in compression injury to the spinal cord. Fracture of the Pedicles of the Axis (Hangman’s Fracture) Severe extension injury of the neck, such as might occur in an automobile accident or a fall, is the usual cause of hangman’s fracture. Sudden overextension of the neck, as produced by the knot of a hangman’s rope beneath the chin, is the reason for the common name. Because the vertebral canal is enlarged by the forward displacement of the vertebral body of the axis, the spinal cord is rarely compressed (Fig. 12.8). Congenital Spondylolisthesis In congenital spondylolisthesis, the body of a lower lumbar vertebra, usually the fifth, moves forward on the body of the vertebra below and carries with it the whole of the upper Muscles of the Back ■■ Degenerative Spondylolithesis This condition is common in the elderly and involves degeneration of the intervertebral discs in the lumbar region and osteoarthritis of the intervertebral joints. Anterior slippage of the fifth lumbar vertebra often occurs, and the lumbar nerve roots may be pressed upon causing low back pain and pain down the leg in the distribution of the involved nerve. ■■ The muscles of the back may be divided into three groups: ■■ portion of the vertebral column. The essential defect is in the pedicles of the migrating vertebra. It is now generally believed that, in this condition, the pedicles are abnormally formed and accessory centers of ossification are present and fail to unite. The spine, laminae, and inferior articular processes remain in position, whereas the remainder of the vertebra, having lost the restraining influence of the inferior articular processes, slips forward. Because the laminae are left behind, the vertebral canal is not narrowed, but the nerve roots may be pressed on, causing low backache and sciatica. In severe cases, the trunk becomes shortened, and the lower ribs contact the iliac crest. The superficial muscles connected with the shoulder girdle. They are described in Chapter 9. The intermediate muscles involved with movements of the thoracic cage. They are described with the thorax in Chapter 2. The deep muscles or postvertebral muscles belonging to the vertebral column. Deep Muscles of the Back (Postvertebral Muscles) In the standing position, the line of gravity (Fig. 12.9) passes through the odontoid process of the axis, posterior site of nipping of spinal nerve site of destruction of spinal cord B A anterior arch of atlas transverse ligament of atlas C waist fracture of odontoid process odontoid process of atlas base fracture of odontoid process fracture of pedicle D posterior arch of atlas E FIGURE 12.8 Dislocations and fractures of the vertebral column. A. Unilateral dislocation of the fifth or the sixth cervical vertebra. Note the anterior displacement of the inferior articular process over the superior articular process of the vertebra below. B. Bilateral dislocation of the fifth or the sixth cervical vertebra. Note that 50% of the vertebral body width has moved forward on the vertebra below. C. Flexion compression–type fracture of the vertebral body in the lumbar region. D. Jefferson’stype fracture of the atlas. E. Fractures of the odontoid process and the pedicles (hangman’s fracture) of the axis. 694 CHAPTER 12 The Back semispinalis capitis longissimus capitis longissimus cervicis iliocostalis cervicis spinalis thoracis iliocostalis thoracis semispinalis thoracis multifidus longissimus thoracis iliocostalis lumborum B A FIGURE 12.9 A. Arrangement of the deep muscles of the back. B. Lateral view of the skeleton showing the line of gravity. Because the greater part of the body weight lies anterior to the vertebral column, the deep muscles of the back are important in maintaining the normal postural curves of the vertebral column in the standing position. to the centers of the hip joints, and anterior to the knee and ankle joints. It follows that when the body is in this position, the greater part of its weight falls in front of the vertebral column. It is, therefore, not surprising to find that the postvertebral muscles of the back are well developed in humans. The postural tone of these muscles is the major factor responsible for the maintenance of the normal curves of the vertebral column. The deep muscles of the back form a broad, thick column of muscle tissue, which occupies the hollow on each side of the spinous processes of the vertebral column (Fig. 12.9). They extend from the sacrum to the skull. It must be realized that this complicated muscle mass is composed of many separate muscles of varying length. Each individual muscle may be regarded as a string, which, when pulled on, causes one or several vertebrae to be extended or rotated on the vertebra below. Because the origins and insertions of the different groups of muscles overlap, entire regions of the vertebral column can be made to move smoothly. The spines and transverse processes of the vertebrae serve as levers that facilitate the muscle actions. The muscles of longest length lie superficially and run vertically from the sacrum to the rib angles, the transverse processes, and the upper vertebral spines (Fig. 12.9). The muscles of intermediate length run obliquely from the transverse processes to the spines. The shortest and deepest muscle fibers run between the spines and between the transverse processes of adjacent vertebrae. The deep muscles of the back may be classified as follows: Superficial Vertically Running Muscles ■ ■■ Erector spinae éiliocostalis ê ê longissimus êspinalis ë Intermediate Oblique Running Muscles ■ ■■ Transversospinalis Deepest Muscles Interspinales ■■ Intertransversarii ■■ é Semispinalis ê ê multifidus ê Rotatores ë Basic Anatomy 695 Knowledge of the detailed attachments of the various muscles of the back has no practical value to a clinical professional, and the attachments are therefore omitted in this text. ■■ Splenius ■■ The splenius is a detached part of the deep muscles of the back. It consists of two parts. The splenius capitis arises from the lower part of the ligamentum nuchae and the upper four thoracic spines and is inserted into the superior nuchal line of the occipital bone and the mastoid process of the temporal bone. The splenius cervicis has a similar origin but is inserted into the transverse processes of the upper cervical vertebrae. Nerve Supply All the deep muscles of the back are innervated by the posterior rami of the spinal nerves. Muscular Triangles of the Back Auscultatory Triangle The auscultatory triangle is the site on the back where breath sounds may be most easily heard with a stethoscope. The boundaries are the latissimus dorsi, the trapezius, and the medial border of the scapula. Lumbar Triangle The lumbar triangle is the site where pus may emerge from the abdominal wall. The boundaries are the latissimus dorsi, the posterior border of the external oblique muscle of the abdomen, and the iliac crest. Deep Fascia of the Back (Thoracolumbar Fascia) The lumbar part of the deep fascia is situated in the interval between the iliac crest and the 12th rib. It forms a strong aponeurosis and laterally gives origin to the middle fibers of the transversus and the upper fibers of the internal oblique muscles of the abdominal wall (see page 117). Medially, the lumbar part of the deep fascia splits into three lamellae. The posterior lamella covers the deep muscles of the back and is attached to the lumbar spines. The middle lamella passes medially, to be attached to the tips of the transverse processes of the lumbar vertebrae; it lies anterior to the deep muscles of the back and posterior to the quadratus lumborum. The anterior lamella passes medially and is attached to the anterior surface of the transverse processes of the lumbar vertebrae; it lies anterior to the quadratus lumborum muscle. Blood Supply of the Back Arteries ■■ In the cervical region, branches arise from the occipital artery, a branch of the external carotid; from the vertebral artery, a branch of the subclavian; and from the deep cervical artery, a branch of the costocervical trunk. ■■ In the thoracic region, branches arise from the posterior intercostal arteries. In the lumbar region, branches arise from the subcostal and lumbar arteries. In the sacral region, branches arise from the iliolumbar and lateral sacral arteries, branches of the internal iliac artery. Veins The veins draining the structures of the back form plexuses extending along the vertebral column from the skull to the coccyx. ■■ ■■ The external vertebral venous plexus lies external and surrounds the vertebral column. The internal vertebral venous plexus lies within the vertebral canal but outside the dura mater of the spinal cord (Fig. 12.10). The external and internal vertebral plexuses form a capacious venous network whose walls are thin and whose channels have incompetent valves or are valveless. They communicate through the foramen magnum with the venous sinuses within the skull. Free venous blood flow may therefore take place between the skull, the neck, the thorax, the abdomen, the pelvis, and the vertebral plexuses, with the direction of flow depending on the pressure differences that exist at any given time between the regions. This fact is of considerable clinical significance (see carcinoma of the prostate, page 696). The internal vertebral plexus receives tributaries from the vertebrae by way of the basivertebral veins (Fig. 12.10) and from the meninges and spinal cord. The internal plexus is drained by the intervertebral veins, which pass outward with the spinal nerves through the intervertebral foramina. Here, they are joined by tributaries from the external vertebral plexus and in turn drain into the vertebral, intercostal, lumbar, and lateral sacral veins. Lymph Drainage of the Back The deep lymph vessels follow the veins and drain into the deep cervical, posterior mediastinal, lateral aortic, and sacral nodes. The lymph vessels from the skin of the neck drain into the cervical nodes, those from the trunk above the iliac crests drain into the axillary nodes, and those from below the level of the iliac crests drain into the superficial inguinal nodes (see page 127). Nerve Supply of the Back The skin and muscles of the back are supplied in a segmental manner by the posterior rami of the 31 pairs of spinal nerves. The posterior rami of the first, sixth, seventh, and eighth cervical nerves and the fourth and fifth lumbar nerves supply the deep muscles of the back and do not supply the skin. The posterior ramus of the second cervical nerve (the greater occipital nerve) ascends over the back of the head and supplies the skin of the scalp. The posterior rami run downward and laterally and supply a band of skin at a lower level than the intervertebral