Muscles
The muscles of the erector spinae
group are covered in the lumbar region by the
thoracolumbar fascia, in the lumbar and thoracic regions by the serratus
posterior muscles and in the neck by the splenius muscles.
In the
lumbar region, the thoracolumbar fascia is trilaminar and encloses erector spinae
posteriorly and quadratus lumborum anteriorly (Fig. 8.28). At the lateral
margins of both muscles, the laminae of the fascia fuse and give attachment to
the transversus abdominis (Fig. 8.28).
Serratus posterior superior and
inferior (Figs 8.29 & 8.30) are weak respiratory muscles, which connect
lower cervical and upper thoracic spines with upper ribs, and lower thoracic
and upper lumbar spines with lower ribs, respectively. They are supplied by
branches of anterior rami of thoracic spinal nerves (intercostal nerves).
Splenius muscle
Splenius (Fig. 8.31) attaches medially
to the lower part of the ligamentum nuchae and to the upper thoracic spinous
processes. The superior part, splenius capitis, attaches laterally to the
mastoid process and superior nuchal line. The inferior part, splenius cervicis,
attaches laterally to the transverse processes of the upper cervical vertebrae.
Erector spinae
On each side, erector spinae (Fig.
8.33) occupy the groove between the spinous and transverse processes of
vertebrae. The muscle attaches inferiorly to the posterior surface of the
sacrum and the posterior part of the iliac crest. In the lumbar region it forms
a prominent vertical muscle mass (Fig. 8.37) and attaches to the spinous and
transverse processes of the lumbar vertebrae. In the upper lumbar region the muscle
divides into three columns. The lateral column forms ili- ocostalis, which
attaches to the angles of the ribs and transverse processes of the lower
cervical vertebrae. The intermediate column forms the longissimus, which
attaches to the ribs and the transverse processes of the thoracic and cervical
vertebrae. The medial part of the erector spinae forms the spinalis, which
attaches to the spinous processes of the upper lumbar, thoracic and lower
cervical vertebrae.
The muscles of this group lie
obliquely (Fig. 8.32), covered by erector spinae (Fig. 8.7). They interconnect transverse
processes and spinous processes of vertebrae at higher levels. Semispinalis
forms the longest and most superficial member of this group and interconnects
lower thoracic transverse processes and upper thoracic and cervical spinous
processes. Multifidus lies deep to semispinalis. Its fibres attach the
transverse process of each vertebra to the spinous processes of one to three
vertebrae above. The deepest fibres of transversospinalis form the rotatores,
which connect the lamina of one vertebra to the transverse process of the
vertebra below. The rotatores are best developed in the thoracic region.
The transversospinalis muscles are
supplied by the posterior rami of the spinal nerves.
Actions
Simultaneous contraction of the deep
muscles of both sides extends the vertebral column and regulates or prevents
flexion. Unilateral contraction produces lateral flexion and rotation, assisted
by the abdominal oblique muscles. The deep muscles have a very important role
in achieving and maintaining the fully upright posture.
