MUSCLES


The musculature of the back is arranged in a series of layers, of which only the deeper are true, intrinsic, back muscles. These true back muscles are characterized by their position and by their innervation by branches of the posterior (dorsal) rami of the spinal nerves. Those below the neck lie deep to posterior layer of the thoracolumbar fascia. In the lumbar region, where the layers of the thoracolumbar fascia are well-defined, they occupy the compartment between its posterior and middle layers (p. 734).
Lying superficial to the true, intrinsic muscles are the extrinsic, 'immigrant' muscles. The most superficial of these run between the upper limb and the axial skeleton, and consist of trapezius, latissimus dorsi, levator scapulae and the rhomboid muscles. Beneath this layer lie the serratus posterior group, superior and inferior, which are variably developed but usually thin, muscles, whose function may be respiratory or possibly proprioceptive. All the extrinsic muscles are innervated by anterior (ventral) rami.
Trapezius, latissimus dorsi, levator scapulae, rhomboid major and rhomboideus minor are described on pages 836-838; serratus posterior muscles are described on page 963.
The muscles of the posterior abdominal wall are described on page 1115.
The intrinsic muscles also have superficial and deep layers. The more superficial layers contain the splenius muscles in the neck and upper thorax, and the erector spinae group in the trunk as a whole. The deeper layers include the transversospinal group, which is itself layered into semispinalis, multifidus and the rotatores, and the suboccipital muscles. Deepest of all lie the interspinal and intertransverse muscles. The latter group are not all innervated by dorsal rami: lumbar intertransversarii mediales, thoracic intertransversarii and medial parts of cervical posterior intertransversarii are so innervated, but the others are supplied by ventral rami and are thus not true muscles of the back.


VASCULAR SUPPLY

The deep muscles of the back receive their blood supply from the following arteries: vertebral artery; deep cervical artery; superficial and deep descending branches of the occipital artery; deep branch of the transverse cervical artery, when present; superior intercostal artery via dorsal branches of the upper two posterior intercostal arteries; posterior intercostal arteries of the lower nine spaces via dorsal branches; dorsal branches of the subcostal arteries; dorsal branches of the lumbar arteries; dorsal branch of arteria lumbalis ima; dorsal branches of the lateral sacral arteries.
Superficial (extrinsic) muscles of the back.
    Figure 45.50 Superficial (extrinsic) muscles of the back.
The detailed pattern of the arterial supply of the deep muscles of the back has been described by Michel Salmon (Taylor & Razaboni 1994). These muscles are supplied by dorsal branches of the posterior intercostal and lumbar arteries. In the thoracic and upper lumbar regions, where the components of the erector spinae run in well-defined longitudinal columns, arterial trunks from these branches run in the sulci between the columns and between the erector spinae and multifidus, giving off branches to supply the muscles. In the lumbar region, where the erector spinae is more of a common muscle mass, this vascular pattern is less regular.


SPLENIUS CAPITIS

Attachments
Splenius capitis arises from the dorsal edge of the lower half of the ligamentum nuchae, the spines of the seventh cervical and upper three or four thoracic vertebrae, and their supraspinous ligaments. The muscle passes upwards and laterally to be attached to the mastoid process and the rough surface on the occipital bone just below the lateral third of the superior nuchal line.
Relations
The upper part of splenius capitis lies beneath sternocleidomastoid and the remainder lies deep to serratus posterior superior, the rhomboids and trapezius. Between sternocleidomastoid and trapezius it forms part of the floor of the posterior triangle of the neck, above and behind levator scapulae. Deep to splenius lie the upper parts of the erector spinae complex and the semispinalis cervicis.
Vascular supply
See page 762.
Innervation
Splenius capitis is innervated by medial branches of the dorsal rami of the middle cervical spinal nerves.
Actions
The action of splenius capitis is described under splenius cervicis.
SPLENIUS CERVICIS
Attachments
Splenius cervicis is attached to the spines of the third to the sixth thoracic vertebrae. It ascends to the posterior tubercles of the transverse processes of the upper two or three cervical vertebrae, immediately anterior to the attachment of levator scapulae. The splenii may be absent or vary in their vertebral attachments. Accessory slips also occur.
Relations
Splenius cervicis lies deep to serratus posterior superior, the rhomboids and trapezius. Its deep relations include the upper parts of the erector spinae complex and the lower semispinalis muscles.
Vascular supply
See page 762.

Innervation
Splenius cervicis is innervated by the medial branches of the dorsal rami of the lower cervical and upper thoracic spinal nerves.
Actions
  Rhomboid muscles.
    Figure 45.51 Rhomboid muscles.
  Splenius cervicis and splenius capitis.
    Figure 45.52 Splenius cervicis and splenius capitis.
Acting together, the splenii of the two sides draw the head directly backwards. Acting separately, they draw the head to one side, and rotate it slightly, turning the face to the same side. Each is therefore synergistic with the contralateral sternocleidomastoid.


ERECTOR SPINAE (Figs 45.53, 45.54, 68.1, 68.2, 68.3)

Iliocostalis Longissimus Spinalis
Iliocostalis lumborum Longissimus thoracis Spinalis thoracis
Iliocostalis thoracis Longissimus cervicis Spinalis cervicis
Iliocostalis cervicis Longissimus capitis Spinalis capitis
   
The erector spinae (sacrospinalis) muscle complex lies on either side of the vertebral column. It forms a large musculotendinous mass, which varies in size and composition at different levels. In the sacral and lower lumbar regions it narrows and becomes increasingly strong and tendinous as it approaches its attachments. In the upper lumbar region it expands to form a thick fleshy mass which divides into three columns which are, from lateral to mid-line, iliocostalis, longissimus and spinalis. The columns may be subdivided as follows: The main muscle mass can readily be felt in the lumbar region in the living subject. Its lateral border is flanked by a visible groove (Fig. 44.3), which ascends over the back of the thorax, traversing the ribs at their angles and running first laterally, then vertically, and finally medially until it is obscured by the scapula.
Attachments
Erector spinae arises from the anterior surface of a broad, thick tendon or aponeurosis, which is attached in the midline to the median sacral crest, the spines of the lumbar and the eleventh and twelfth thoracic vertebrae, their supraspinous ligaments, and laterally to the medial aspect of the posterior iliac crest and to the lateral sacral crest, where it blends with the sacrotuberous and dorsal sacroiliac ligaments. Some of its fibres are continuous with gluteus maximus and multifidus.
Iliocostalis
Iliocostalis lumborum is attached, by flattened tendons, to the inferior borders of the angles of the lower six or seven ribs.
 Erector spinae muscle group.
    Figure 45.53 Erector spinae muscle group.
Iliocostalis thoracis attaches below to the upper borders of the angles of the lower six ribs medial to the tendons of insertion of iliocostalis lumborum, and above to the superior borders of the angles of the upper six ribs and the back of the transverse process of the seventh cervical vertebra.
Iliocostalis cervicis attaches below to the angles of the third to the sixth ribs, medial to the tendons of insertion of iliocostalis thoracis, and above to the posterior tubercles of the transverse processes of the fourth, fifth and sixth cervical vertebrae.
Longissimus
Longissimus thoracis is the largest of the continuations of the erector spinae. In the lumbar region, where it blends with iliocostalis lumborum, some of its fibres are attached to the whole length of the posterior surfaces of the transverse processes and the accessory processes of the lumbar vertebrae, and to the middle layer of the thoracolumbar fascia. In the thoracic region it is attached, by rounded tendons, to the tips of the transverse processes of all the thoracic vertebrae, and by fleshy slips to the lower nine or ten ribs between their tubercles and angles.
Longissimus cervicis lies medial to longissimus thoracis. It is attached by long thin tendons to the transverse processes of the upper four or five thoracic vertebrae, and again by tendons to the posterior tubercles of the transverse processes of the second to the sixth cervical vertebrae.
Longissimus capitis lies between longissimus cervicis and semispinalis capitis. It is attached below by tendons to the transverse processes of the upper four or five thoracic vertebrae and the articular processes of the lower three or four cervical vertebrae and above to the posterior margin of the mastoid process, deep to splenius capitis and sternocleidomastoid. It is usually traversed by a tendinous intersection near its upper end.
Spinalis
Spinalis thoracis, the medial continuation of erector spinae, is barely separable as a distinct muscle. It lies medial to longissimus thoracis, and blends intimately with it. It is attached below by three or four tendons to the eleventh and twelfth thoracic and the first and second lumbar vertebral spines: these unite in a small muscle which is attached above by separate tendons to the spines of the upper thoracic vertebrae (the number varies from four to eight). It blends closely with semispinalis thoracis, which lies anterior to it.
Axial MRI of lumbar spine showing erector spinae.
 
    Figure 45.54 Axial MRI of lumbar spine showing erector spinae.
Spinalis cervicis, when it is present, is attached to the lower part of the ligamentum nuchae and the spine of the seventh cervical vertebra (and sometimes to the first and second thoracic vertebrae), and to the spine of the axis. Occasionally it is also attached to the spines of the two vertebrae immediately below it.
Spinalis capitis usually blends to some extent with semispinalis capitis (see below), but can be separate.
Relations
Erector spinae is covered in the lumbar and thoracic regions by the thoracolumbar fascia (p. 734), and by serratus posterior inferior below and the rhomboids and splenii above. In the lumbar region it lies in the compartment between the posterior and middle layers of the thoracolumbar fascia.
Vascular supply
See page 762.
Innervation
Erector spinae is innervated by lateral and intermediate branches of the dorsal rami of the lower cervical, thoracic and lumbar spinal nerves.
Actions
Erector spinae as a group extends and laterally flexes the vertebral column when acting against gravity. It contracts eccentrically to control the movement as the column is flexed forwards or laterally with the aid of gravity.
Contraction of the erectores spinae extends the trunk, a movement controlled largely by opposing activity of the abdominal muscle complex (rectus abdominus and the oblique abdominals). Flexion of the trunk is initiated by flexor muscles such as rectus abdominis: as the centre of gravity moves forward control is transferred to the erectores spinae which then contract eccentrically. When the trunk is fully flexed the erectores spinae are relaxed and electromyographically quiet: in this position, flexion may be limited by passive forces generated by tension in the thoracolumbar fascia and in the spinal ligaments and by resistance to deformation of the intervertebral discs. Electromyographic activity in the erector spinae group is greater when work is carried out on a low surface from a standing position. Lateral flexion is controlled by the contralateral erector spinae, with input from the oblique muscles.
Longissimus capitis extends the head and turns the face to the ipsilateral side.


TRANSVERSOSPINALIS

Semispinalis thoracis Multifidus Rotatores thoracis
Semispinalis cervicis Rotatores cervicis
Semispinalis capitis Rotatores lumborum
   
  Attachments of semispinalis.
    Figure 45.55 Attachments of semispinalis.
The transversospinalis muscular group consists of the following muscles: These muscles run obliquely upwards and medially from transverse processes to adjacent, and sometimes more distant, spinous processes. Bogduk and his co-workers believe that, in the lumbar region at least, multifidus should be considered to run downwards and laterally (Macintosh et al 1986).
Semispinalis (Fig. 45.55)
Semispinalis thoracis consists of thin, fleshy fasciculi interposed between long tendons. It is attached below by a series of tendons to the transverse processes of the sixth to the tenth thoracic vertebrae, and above, again by tendons, to the spines of the upper four thoracic and lower two cervical vertebrae.
Semispinalis cervicis, a thicker muscle, is attached below by a series of tendinous and fleshy fibres to the transverse processes of the upper five or six thoracic vertebrae, and above to the spines of the second to the fifth cervical vertebrae. The fasciculus connected with the axis is the largest, and is composed chiefly of muscle.
Semispinalis capitis is attached by a series of tendons to the tips of the transverse processes of the upper six or seven thoracic and seventh cervical vertebrae, to the articular processes of the fourth, fifth, and sixth cervical vertebrae and, occasionally, to the spine of the seventh cervical or first thoracic vertebra. The tendons come together in a broad muscle which attaches above to the medial part of the area between the superior and inferior nuchal lines of the occipital bone. The medial part of the muscle, which is usually more or less distinct from the rest, is sometimes called biventer cervicis, because it is traversed by an incomplete tendinous intersection.
Multifidus (Fig. 45.56)



 
    Figure 45.56 Multifidus. A, cervicothoracic. B, lumbosacral parts.
 Rotatores (thoracic region)
    Figure 45.57 Rotatores (thoracic region). (By permission from Benninghoff, Anatomie, 15th edition © Urban and Schwarzenberg, 1994.)
Multifidus consists of a number of fleshy and tendinous fasciculi which lie deep to the foregoing muscles and fill the groove at the side of the spines of the vertebrae from the sacrum to the axis. Its fasciculi attach as follows: most caudally, to the back of the sacrum as low as the fourth sacral foramen, to the posterior superior iliac spine and dorsal sacroiliac ligaments; in the lumbar region, to all the mammillary processes; in the thoracic region, to all the transverse processes; in the cervical region, to the articular processes of the lower four vertebrae. In the lumbar region a few fibres may attach to the tendon (aponeurosis) of the erector spinae, and to the capsules of the facet joints. Each fasciculus is attached to the spinous process of one of the vertebrae above. Some fasciculi attach to the base of the spinous process, while others reach its tip. The fasciculi vary in length. Thus the most superficial connect one vertebra to the third or fourth above, those next in depth connect one vertebra to the second or third above, and the deepest connect adjacent vertebrae. For further detail of the structure of this muscle, see Kalimo et al (1989).
Rotatores (Fig. 45.57)

Rotatores thoracis consists of eleven pairs of small roughly quadrilateral muscles. Each connects the upper and posterior part of the transverse process of one vertebra to the lower border and lateral surface of the lamina of the vertebra immediately above. Some fibres may extend to the base of the spinous process of the vertebra above (rotatores longi). The first is found between the first and second thoracic vertebrae, and the last between the eleventh and twelfth thoracic vertebrae. One or more may be absent from the upper or lower ends of the series.
Rotatores cervicis and lumborum are represented only by irregular and variable muscle bundles, whose attachments are similar to those of rotatores thoracis.
Relations
The transversospinalis group lie deep to erector spinae, except in the neck where semispinalis lies mainly deep to splenius and trapezius. A small section of semispinalis capitis may lie even more superficially, forming the uppermost part of the floor of the posterior triangle of the neck. In the lumbosacral region multifidus lies immediately deep to the erector spinae tendon (aponeurosis). The components of transversospinalis themselves lie in three planes, semispinalis is the most superficial and the rotatores are the most deeply placed. Semispinalis is absent in the lumbar and sacral regions, and the rotatores are well represented only in the thoracic region.
Vascular supply
See page 762.
Innervation
All of the transversospinalis group is innervated by dorsal rami of spinal nerves, usually by medial branches.
Actions
Semispinales thoracis and cervicis extend the thoracic and cervical regions of the vertebral column, and rotate them towards the opposite side. Semispinalis capitis extends the head, and turns the face slightly towards the opposite side.


INTERSPINALES

Interspinales are short paired muscular fasciculi attached above and below to the apices of the spines of contiguous vertebrae, one on either side of the interspinous ligament. They are most distinct in the cervical region, where they consist of six pairs, the first between the axis and third vertebra, and the last between the seventh cervical and first thoracic vertebrae. In the thoracic region they occur between the first and second vertebrae (sometimes between the second and third), and the eleventh and twelfth vertebrae. In the lumbar region there are four pairs between the five lumbar vertebrae. A pair is occasionally found between the last thoracic and first lumbar vertebrae, and another between the fifth lumbar vertebra and the sacrum. Sometimes cervical interspinales span more than two vertebrae.
INTERTRANSVERSARII
Intertransversarii are small muscles between the transverse processes of the vertebrae. They are best developed in the cervical region, where they consist of posterior and anterior sets of muscles separated by the ventral rami of spinal nerves. Posterior intertransverse muscles are divisible into medial and lateral slips, which are supplied by the dorsal and ventral rami of the spinal nerves, respectively. Each medial slip, the intertransverse muscle 'proper', is often further subdivided into medial and lateral parts by the passage through it of the dorsal ramus of a spinal nerve. Anterior intertransverse muscles and lateral parts of the posterior muscles connect the costal processes of contiguous vertebrae and medial parts of the posterior muscles connect true transverse processes. There are seven pairs of these muscles, the highest between the atlas and axis, and the lowest between the seventh cervical vertebra and the first thoracic: the anterior muscles between atlas and axis are often absent. In the thoracic region they consist of single muscles, which are present between the transverse processes of only the last three thoracic and first lumbar vertebrae. In the lumbar region they again consist of two sets of muscles. One set, intertransversarii mediales, connects the accessory process of one vertebra with the mammillary process of the next. The other set, intertransversarii laterales, can be divided into ventral and dorsal parts: the ventral parts connect the transverse processes (costal elements) of the lumbar vertebrae, and the dorsal parts connect the accessory processes to the transverse processes of succeeding vertebrae. Both ventral and dorsal lumbar intertransversarii are innervated by ventral primary rami (Bogduk 1997).
Thoracic intertransverse muscles and ligaments are homologous with the medial slips of the 'proper' posterior intertransverse muscles of the cervical region, and levatores costarum (p. 962) are homologous with their lateral slips. The lateral branch of the dorsal ramus of a spinal nerve separates thoracic intertransverse from levator costae. The lumbar levatores costarum are represented by the medial intertransverse muscles; the lateral intertransverse are homologous with the intercostal muscles. For other views on the homologies and classification of transversospinal musculature consult Sato (1973).
Actions of the short muscles of the back
The short muscles of the back probably function, for the most part, as postural muscles. In effect, the vertebral column consists of a series of short, jointed levers. A mechanical arrangement of this type is unstable under compression and will tend to buckle unless movement at the individual joints is controlled. The short muscles may serve to stabilize adjoining vertebrae, controlling their movement during motion of the vertebral column as a whole, and providing for more effective action of the long erector spinae muscles. In theory, the short muscles are capable of producing extension (multifidus, interspinales), lateral flexion (multifidus, intertransversarii) and rotation (multifidus and rotatores), but their detailed patterns of activity remain unknown. The deep muscles of the back as a whole are certainly involved in the control of posture: they contract intermittently during the swaying movements that take place from an upright position.
SUBOCCIPITAL MUSCLES (Fig. 45.58)
The suboccipital muscles are four small muscles which connect the occipital bone, atlas and axis posteriorly. They lie inferior to the anterior part of the occipital bone, where three of the muscles form the boundaries of the suboccipital triangle. Above and medially lie rectus capitis posterior major; above and laterally, obliquus capitis superior, and below and laterally, obliquus capitis inferior. With the head in the anatomical position the suboccipital triangle lies almost in the horizontal plane.
Rectus capitis posterior major
Rectus capitis posterior major is attached by a pointed tendon to the spine of the axis, becomes broader as it ascends, and is attached to the lateral part of the inferior nuchal line and the occipital bone immediately below it. As the muscles of the two sides pass upwards and laterally, they leave between them a triangular space in which parts of the recti capitis posteriores minores are visible.
Rectus capitis posterior minor
Rectus capitis posterior minor is attached by a narrow pointed tendon to the tubercle on the posterior arch of the atlas. As it ascends it broadens before attaching to the medial part of the inferior nuchal line and to the occipital bone between the inferior nuchal line and the foramen magnum (p. 463). Either muscle may be doubled longitudinally. There may be an attachment to the dura mater.
Obliquus capitis inferior
Obliquus capitis inferior, the larger of the two oblique muscles, passes laterally and slightly upwards from the lateral surface of the spine and the adjacent upper part of the lamina of the axis to the inferoposterior aspect of the transverse process of the atlas.
Obliquus capitis superior
Obliquus capitis superior is attached by tendinous fibres to the upper surface of the transverse process of the atlas. It expands in width as it ascends dorsally, and is attached to the occipital bone between the superior and inferior nuchal lines, lateral to semispinalis capitis and overlapping the insertion of rectus capitis posterior major.
Relations of the suboccipital triangle
Medially the suboccipital triangle is covered by a layer of dense adipose tissue, deep to semispinalis capitis. Laterally it lies under longissimus capitis and sometimes splenius capitis, both of which overlap obliquus capitis superior. The 'floor' of the triangle is formed by the posterior atlanto-occipital membrane and the posterior arch of the atlas. The vertebral artery and the dorsal ramus of the first cervical nerve lie in a groove on the upper surface of the posterior arch of the atlas.
Vascular supply
The suboccipital muscles receive their blood supply from the vertebral artery and deep descending branches of the occipital artery.
Innervation
All the suboccipital muscles are supplied by the dorsal ramus of the first cervical spinal nerve.
 Posterior view of the left suboccipital triangle.
 
    Figure 45.58 Posterior view of the left suboccipital triangle.
Actions of the suboccipital triangle
The suboccipital muscles are involved in extension of the head at the atlanto-occipital joints and rotation of the head and atlas on the axis. Obliquus capitis superior and the two recti are probably more important as postural muscles than as prime movers, but this is difficult to confirm by direct observation. Rectus capitis posterior major extends the head and, acting with obliquus capitis inferior, rotates the face towards the ipsilateral side. Rectus capitis posterior minor extends the head. Obliquus capitis superior extends the head and laterally flexes it to the ipsilateral side.