The muscles of the trunk may be arranged in six groups
| I. Deep muscles of the back | IV. Muscles of the abdomen |
| II. Suboccipital muscles | V. Muscles of the pelvis |
| III. Muscles of the thorax | VI. Muscles of the perineum |
I. THE MUSCLES OF THE BACK (fig. 795)
The deep or intrinsic muscles of the back consist of a complex group of muscles extending from the pelvis to the skull. They are:
| Splenius capitis | Multifidus |
| Splenius cervicis | Rotatores |
| Sacrospinalis | Interspinales |
| Semispinalis | Intertransversarii |
The lumbar fascia (lumbodorsal fascia) covers the deep muscles of the back of the trunk. Above, it passes in front of the Serratus posterior superior and is continuous with the investing fascial layer on the back of the neck.
In the thoracic region the lumbar fascia is a thin fibrous lamina covering the extensor muscles of the vertebral column and separating them from the muscles connecting the vertebral column to the upper extremity. It contains both longitudinal and transverse fibers, and is attached, medially, to the spines of the thoracic vertebrae; laterally, to the angles of the ribs.
In the lumbar region the lumbar fascia is in three layers, anterior, middle and posterior (fig. 594). The posterior layer is attached to the spines of the lumbar and sacral vertebra and to the supraspinous ligament; the middle layer is attached, medially, to the tips of the transverse processes of the lumbar vertebrae, and to the intertransverse ligaments, below, to the iliolumbar ligament, and above, to the lumbocostal ligament. The anterior layer covers the Quadratus lumborum and is attached to the anterior aspects of the transverse processes of the lumbar vertebrae under cover of the lateral part of the Psoas major muscle. The posterior and middle layers unite at the lateral margin of the Sacrospinalis, to form the tendon of origin of the Transversus abdominis.
The Splenius capitis (fig. 618) arises from the lower half of the ligamentum nuchae, from the spine of the seventh cervical vertebra, and from the spines of the upper three or four thoracic vertebrae The fibers of the muscle are directed upwards and laterally and are inserted, under cover of the Sternocleidomastoid muscle, into the mastoid process of the temporal bone, and into the rough surface on the occipital bone just, below the lateral third of the superior nuchal line.
Nerve-supply.-The Splenius capitis is supplied by the lateral branches of the posterior primary rami of the middle cervical nerves.
Actions-The Splenius capitis acts in conjunction with the Splenius cervicis.
The Splenius cervicis (fig. 618) arises from the spines of the third to the sixth thoracic vertebrae; it is inserted into the posterior tubercles of the transverse processes of the upper two or three cervical vertebrae.
Nerve-supply.-The Splenius cervicis is supplied by lateral branches of the posterior primary rami of the lower cervical nerves.
Actions.-The Splenii of the two sides, acting together; draw the head directly backwards; acting separately, they draw the head to one side, and slightly rotate it, turning the face to the same side,
The Sacrospinalis (fig. 595), and its prolongations in the thoracic and cervical regions, lie in the groove on the side of the vertebral column. They are covered in the lumbar and thoracic regions by the lumbar fascia, and in the cervical region by the nuchal fascia. They forms a large muscular and tendinous mass, which varies in size and structure at different parts of the vertebral column. In the sacral region it is narrow and pointed, and at its origin chiefly tendinous in structure. In the lumbar region it forms a thick fleshy mass which, on being followed upwards, divides into three columns; these gradually diminish in size as they ascend to be inserted into the vertebrae and ribs.
Longissimus Spinalis (a) L. thoracis (a) S. thoracis (b) L. cervicis (b) S. cervicis (c) L. capitis (c) S. capitis
| Lateral Column | Intermediate Column | Medial Columnn | ||
| Iliocostocervicalis | ||||
| (a) Iliocostalis | ||||
| (b) Costalis | ||||
| (c) Costocervicalis |
The Iliocostalis (Iliocostalis lumborum) is inserted, by flattened tendons, into the inferior borders of the angles of the lower six or seven ribs.
The Costalis (Iliocostalis dorsi) arises from the upper borders of the angles of the lower six ribs medial to the tendons of insertion of the Iliocostalis; it is inserted into the upper borders of the angles of the upper six ribs and into the back of the transverse process of the seventh cervical vertebra.
The Costocervicalis (Iliocostalis cervicis) arises from the angles of the third, fourth, fifth and sixth ribs medial to the tendons of insertion of the costalis, and is inserted into the posterior tubercles of the transverse processes of the fourth, fifth and sixth cervical vertebrae.
Nerve-supply.—These three muscles are supplied by the posterior primary rami of the lower cervical, the thoracic and the upper lumbar nerves.
Actions.-These muscles are extensors of the vertebral column; they also bend it to one side. The slips attached to the ribs act as depressors of the thorax.
The Longissimus thoracis is the intermediate and largest of the continuations of the Sacrospinalis. In the lumbar region; where it is as yet blended with the Iliocostalis, some of its fibers 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 lumbar fascia. In the thoracic region it is inserted, by rounded tendons, into the tips of the transverse processes of all the thoracic vertebrae, and by fleshy processes into the lower nine or ten ribs between their tubercles and angles.
The Longissimus cervicis, situated medial to the Longissimus thoracis, arises by long thin tendons from the summits of the transverse processes of the upper four or five thoracic vertebrae, and is inserted by similar tendons into the posterior tubercles of the transverse processes of the cervical vertebrae from the second to the sixth inclusive.
The Longissimus capitis lies between the Longissimus cervicis and the Semispinalis capitis. It arises by tendons from the transverse processes of the upper four or five thoracic vertebrae, and the articular processes of the lower three or four cervical vertebrae, and is inserted into the posterior margin of the mastoid process, deep to the Splenius capitis and Sternocleidomastoid. It is usually crossed by a tendinous intersection near its insertion.
Nerve-supply.-The Longissimi are supplied by the posterior primary rami of the lower cervical, the thoracic and the lumbar nerves.
Actions.-The Longissimus thoracis et cervicis bend the vertebral column backwards and laterally; the Longissimus capitis extends the head, and turns the face towards the same side.
The Spinalis thoracis, the medial continuation of the Sacrospinalis, is scarcely separable as a distinct muscle. It is situated at the medial side of the Longissimus thoracis, and is intimately blended with it; it arises by three or four tendons from the spines of the eleventh and twelfth thoracic, and first and second lumbar vertebrae; these, uniting, form a small muscle which is inserted by separate tendons into the spines of the upper thoracic vertebrae, the number varying from four to eight. It is intimately united with the Semispinalis thoracis, which lies deep to it.
The Spinalis cervicis is an inconstant muscle, which arises from the lower part of the ligamentum nuchm, the spine of the seventh cervical, and sometimes from the spines of the first and second thoracic vertebrae, and is inserted into the spine of the axis, and occasionally into the spines of the two vertebrae below it. The Spinalis capitis is usually more or less blended with the Semispinalis capitis.
Nerve-supply.-The Spinales are supplied by the posterior primary rami of the lower cervical and thoracic nerves.
Actions.—The Spinales extend the vertebral column.
The Semispinalis thoracis consists of thin, fleshy fasciculi, interposed between tendons of considerable length. It arises by a series of tendons from the transverse processes of the thoracic vertebrae from the sixth to the tenth inclusive, and is inserted, by tendons, into the spines of the upper four thoracic and lower two cervical vertebrae.
The Semispinalis cervicis, thicker than the preceding, arises by a series of tendinous and fleshy fibers from the transverse processes of the upper five or six thoracic vertebrae, and is inserted into the cervical spines, from the axis to the fifth inclusive. The fasciculus connected with the axis is the largest, and is chiefly muscular in structure.
Nerve -supply.-The Semispinales are supplied by the posterior primary rami of the cervical and thoracic nerves.
Actions.—The Semispinales thoracis et cervicis extend the thoracic and cervical portions of the vertebral column, and rotate them towards the opposite side; the Semispinalis capitis extends the head, and turns the face slightly towards the opposite side.
The Multifidus consists of a number of fleshy and tendinous fasciculi, which fill the groove at the side of the spines of the vertebrae, from the sacrum to the axis. In the sacral region, the fasciculi arise from the back of the sacrum as low as the fourth sacral foramen; from the aponeurosis of origin of the Sacrospinalis, from the medial surface of the posterior superior iliac spine and from the posterior sacro-iliac ligaments; in the lumbar region, from all the mamillary processes; in the thoracic region, from all the transverse processes; and in the cervical region, from the articular processes of the lower four vertebrae, Each fasciculus passes obliquely upwards and medially, and is inserted into the whole length of the spine of one of the vertebrae above. The fasciculi vary in length; the most superficial pass from one vertebra to the third or fourth above; those next in depth run from one vertebra to the second or third above; while the deepest connect contiguous vertebrae.
Nerve-supply.-The Multifidus is supplied by the posterior primary rami of the spinal nerves.
Actions.-The fasciculi of the Multifidus bend the segments of the vertebral column backwards and laterally, and rotate them towards the opposite side.
The Rotatores lie deep to the Multifidus and are found only in the thoracic region; they are eleven in number on each side, and are small and somewhat quadrilateral in form. Each arises from. the upper and posterior part of the transverse process of one vertebra, and is inserted into the lower border and lateral surface of the lamina of the vertebra next above, the fibers extending as far as the root of the spine. The first is found between the first and second thoracic vertebrae; the last, between the eleventh and twelfth. Sometimes the number of these muscles is diminished by the absence of one or more from the upper or lower end of the series.
Nerve-supply.-The Rotatores are supplied by the posterior primary rami of the spinal nerves.
Actions.-The Rotatores mainly rotate the individual vertebrae towards the opposite side.
The Interspinales are short muscular fasciculi, placed in pairs between the spines of contiguous vertebrae, one on each side of the interspinous ligament. In the cervical region they are most distinct, and consist of six pairs; the first is situated between the axis and third vertebra, and the last between the seventh cervical and the first thoracic vertebrae. They are small narrow bundles, attached, above and below, to the apices of the spines. In the thoracic region they are found between the first and second vertebra;, and sometimes between the second and third, and the eleventh and twelfth vertebrae. In the lumbar region there are four pairs in the intervals 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.
Nerve-supply.—–The Interspinales are supplied by the posterior primary rank of the spinal nerves.
Actions.-The Interspinales extend those segments of the vertebral column to which they are attached.
The Extensor coccygis is a slender muscular fasciculus, which is not always present; it extends over the lower part of the posterior surface of the sacrum and coccyx. It arises by tendinous fibers from the last segment of the sacrum, or first piece of the coccyx, and passes downwards to be inserted into the lower part of the coccyx. It is a rudiment of the Levator caudae muscle of the lower animals.
The Intertransversarii are small muscles placed between the transverse processes of the vertebrae. In the cervical region they are best developed, and consist of anterior and posterior fasciculi, which are separated by the anterior primary rami of the spinal nerves. The anterior intertransverse muscles connect the costal processes, and the posterior intertransverse muscles the transverse processes of contiguous vertebrae. There are seven pairs of these muscles, the highest between the atlas and axis, the lowest between the seventh cervical and first thoracic vertebrae, but the anterior intertransverse muscle between the atlas and the axis is often absent. Tn the thoracic region they consist of single muscles, which are present between the transverse processes of the lower three thoracic vertebrae, and between the transverse processes of the last thoracic and first lumbar vertebrae. In the lumbar region they again consist of two sets of muscles, one, named Intertransversarii lateralles, between the transverse processes of the lumbar vertebrae, the other, named Intertransversarii mediales, connecting the accessory process of one vertebra with the mamillary process of the next. The posterior intertransverse muscles of the cervical region are homologous with the intertransverse muscles of the thoracic region and with the lateral intertransverse muscles of the lumbar region, but the anterior intertransverse muscles of the cervical regions are not represented elsewhere.
Nerve-supply. -The medial intertransverse muscles are supplied by posterior primary rami of the spinal nerves; all the others are supplied by anterior primary .rami.
Actions.—The intertransverse muscles act as lateral flexors of those segments of the vertebral column to which they are attached.
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