The anterior muscles of the spine are less well developed because they are assisted in their primary action of flexion by gravity. In the cervical region, the anterior spinal muscles include the longus colli, longus capitis muscles, and the rectus capitis anterior and lateralis muscles. The lateral attachments of these muscles overlie the transverse processes of cervical vertebrae and must be considered during an anterior approach to the intervertebral discs, the uncovertebral joints, or the vertebral artery in the transverse canal (8,9). As a group, they produce flexion of the head and neck if acting bilaterally, and lateral flexion if acting on one side only. All receive motor innervation from anterior primary rami of cervical spinal nerves.
The longus colli muscles (right and left) each have three parts: a vertical, a superior oblique, and an inferior oblique part. The fibers of the longus colli are arranged symmetrically around the transverse process of C5. The longus capitis muscles (right and left) are slightly anterior and lateral to the superior oblique fibers of the longus colli muscle. The longus capitis muscles arise by thin slips from the transverse processes of C3-C6. The tendons unite and form a distinct band that attaches to the basilar part of the occipital bone, between the anterior edge of the foramen magnum and the pharyngeal tubercle. The rectus capitis anterior and lateralis muscles lie anterior to the anterior atlantooccipital membrane and the atlantooccip-ital joint capsules, and help fill the gap between the atlas and the occipital bone.
The scalene muscles attach directly to the cervical spine, and when acting from their inferior attachment on the ribs can flex the spine. When the spine is fixed, the scalene muscles raise the ribs in inspiration. These muscles are critical landmarks in the neck. The anterior scalene muscle attaches to the anterior tubercles of the transverse processes of C3-C6. They descend to their attachment on the first rib at the scalene tubercle. The
middle scalene muscle is the largest of the three scalene muscles. It attaches to the posterior tubercles of C3-C7 and passes downward to its insertion on the first rib just posterior to the insertion of the anterior scalene muscle. The posterior scalene is the smallest of the three, arises from the posterior tubercles of the transverse processes of C4-C6, and passes inferiorly to the lateral aspect of the second rib. Branches of the anterior primary rami of cervical nerves innervate all the scalene muscles. The phrenic nerve is formed on the surface of the anterior scalene muscle and descends through the superior thoracic aperture on the medial aspect of the anterior scalene. The subclavian vein passes anterior to the anterior scalene muscle, and the subclavian artery and roots of the brachial plexus pass between the anterior and middle scalene muscles. For descriptive purposes, the anterior scalene muscle divides the subclavian artery into three parts, with the vertebral artery, the thyrocervical trunk, and the internal thoracic artery associated with the first part. The costocervical trunk is associated with the second part, and the dorsal scapular artery is associated with the third part.
The anterior flexors of the spine in the lumbar region are the psoas major and minor muscles and the iliacus. These muscles are often described as muscles of the posterior abdominal wall, but they attach directly to the spine and have a direct effect on the position of the spine. The psoas major arises from the sides of the bodies of T12-L4, the intervertebral discs between the bones, and the transverse processes of all lumbar vertebrae. The muscle crosses the pelvic brim under the inguinal ligament and, after passing anterior to the capsule of the hip joint, attaches distally to the lesser trochanter of the femur. The psoas minor arises from the sides of the bodies of T12 and L1 and the intervening disc and attaches distally to the pectin pubis and the iliopubic eminence. The iliacus arises from the inner lip of the iliac crest, the upper two thirds of the iliac fossa, and the superolateral part of the sacrum. Its muscle fibers blend with those of the psoas major to insert on the lesser trochanter. The psoas muscles and the iliacus flex the thigh on the hip, but when the thigh is fixed, flex the trunk on the thigh. These muscles are innervated by the anterior rami of L1-L3.
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