The deep or intrinsic muscles of the back consist of a complex group of muscles extending from the pelvis to the skull. They include:
The lumbodorsal fascia is a deep investing membrane, which covers the deep muscles of the back of the trunk. Above, it passes in front of the Serratus posterior superior and is continuous with a similar investing layer on the back of the neck – the nuchal fascia.
In the thoracic region the lumbodorsal fascia is a thin fibrous lamina, which serves to bind down the Extensor muscles of the vertebral column and to separate 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 spinous processes of the thoracic vertebrae; laterally to the angles of the ribs.
In the lumbar region the fascia (lumbar aponeurosis) is in two layers, anterior and posterior. The posterior layer is attached to the spinous processes of the lumbar and sacral vertebrae and to the supraspinal ligament; the anterior 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 two layers unite at the lateral margin of the Sacrospinalis, to form the tendon of origin of the Transversus abdominis. The aponeurosis of origin of the Serratus posterior inferior and the Latissimus dorsi are intimately blended with the lumbodorsal fascia.
The Splenius capitis arises from the lower half of the ligamentum nuchae, from the spinous process of the seventh cervical vertebra, and from the spinous processes of the upper three or four thoracic vertebrae. The fibers of the muscle are directed upward and lateralward and are inserted, under cover of the Sternocleidomastoideus, 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.
The Splenius cervicis arises by a narrow tendinous band from the spinous processes of the third to the sixth thoracic vertebræ; it is inserted, by tendinous fasciculi, into the posterior tubercles of the transverse processes of the upper two or three cervical vertebrae.
The Splenii are supplied by the lateral branches of the posterior divisions of the middle and lower cervical nerves.
The Splenii of the two sides, acting together, draw the head directly backward, assisting the Trapezius and Semispinalis capitis; acting separately, they draw the head to one side, and slightly rotate it, turning the face to the same side. They also assist in supporting the head in the erect position.
The Sacrospinalis (Erector spinæ) 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 lumbodorsal fascia, and in the cervical region by the nuchal fascia. This large muscular and tendinous mass 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 is larger, and forms a thick fleshy mass, which, on being followed upward, is subdivided into three columns; these gradually diminish in size as they ascend to be inserted into the vertebrae and ribs.
The Sacrospinalis arises from the anterior surface of a broad and thick tendon, which is attached to the medial crest of the sacrum, to the spinous processes of the lumbar and the eleventh and twelfth thoracic vertebrae, and the supraspinal ligament, to the back part of the inner lip of the iliac crests and to the lateral crests of the sacrum, where it blends with the sacrotuberous and posterior sacroiliac ligaments. Some of its fibers are continuous with the fibers of origin of the Glutaeus maximus. The muscular fibers form a large fleshy mass, which splits, in the upper lumbar region into three columns, viz., a lateral, the Iliocostalis, an intermediate, the Longissimus, and a medial, the Spinalis.
The Iliocostalis lumborum (Iliocostalis muscle; Sacrolumbalis muscle) is inserted, by six or seven flattened tendons, into the inferior borders of the angles of the lower six or seven ribs.
The Iliocostalis dorsi (Musculus accessorius) arises by flattened tendons from the upper borders of the angles of the lower six ribs medial to the tendons of insertion of the Iliocostalis lumborum; these become muscular, and are 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 Iliocostalis cervicis (Cervicalis ascendens) arises from the angles of the third, fourth, fifth, and sixth ribs, and is inserted into the posterior tubercles of the transverse processes of the fourth, fifth, and sixth cervical vertebrae.
The Longissimus dorsi is the intermediate and largest of the continuations of the Sacrospinalis. In the lumbar region, where it is as yet blended with the Iliocostalis lumborum, 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 anterior layer of the lumbodorsal 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 (Transversalis cervicis), situated medial to the Longissimus dorsi, 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 (Trachelomastoid muscle) lies medial to the Longissimus cervicis, between it 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, beneath the Splenius capitis and Sternocleidomastoideus. It is almost always crossed by a tendinous intersection near its insertion.
The Spinalis dorsi, the medial continuation of the Sacrospinalis, is scarcely separable as a distinct muscle. It is situated at the medial side of the Longissimus dorsi, and is intimately blended with it; it arises by three or four tendons from the spinous processes of the first two lumbar and the last two thoracic vertebrae: these, uniting, form a small muscle which is inserted by separate tendons into the spinous processes of the upper thoracic vertebrae, the number varying from four to eight. It is intimately united with the Semispinalis dorsi, situated beneath it.
The Spinalis cervicis (Spinalis colli) is an inconstant muscle, which arises from the lower part of the ligamentum nuchæ, the spinous process of the seventh cervical, and sometimes from the spinous processes of the first and second thoracic vertebrae, and is inserted into the spinous process of the axis, and occasionally into the spinous processes of the two vertebrae below it.
The Spinalis capitis (Biventer cervicis) is usually inseparably connected with the Semispinalis capitis.
The Semispinalis dorsi consists of thin, narrow, fleshy fasciculi, interposed between tendons of considerable length. It arises by a series of small tendons from the transverse processes of the sixth to the tenth thoracic vertebrae, and is inserted, by tendons, into the spinous processes of the upper four thoracic and lower two cervical vertebrae.
The Semispinalis cervicis (Semispinalis colli), 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 spinous processes, from the axis to the fifth inclusive. The fasciculus connected with the axis is the largest, and is chiefly muscular in structure.
The Semispinalis capitis (Complexus) is situated at the upper and back part of the neck, beneath the Splenius, and medial to the Longissimus cervicis and capitis. It arises by a series of tendons from the tips of the transverse processes of the upper six or seven thoracic and the seventh cervical vertebrae, and from the articular processes of the three cervical above this. The tendons, uniting, form a broad muscle, which passes upward, and is inserted between the superior and inferior nuchal lines of the occipital bone. The medial part, usually more or less distinct from the remainder of the muscle, is frequently termed the Spinalis capitis; it is also named the Biventer cervicis since it is traversed by an imperfect tendinous inscription.
The Multifidus (Multifidus spinæ) consists of a number of fleshy and tendinous fasciculi, which fill up the groove on either side of the spinous processes of the vertebræ, from the sacrum to the axis. In the sacral region, these 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 sacroiliac 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, passing obliquely upward and medialward, is inserted into the whole length of the spinous process of one of the vertebrae above. These fasciculi vary in length: the most superficial, the longest, pass from one vertebra to the third or fourth above; those next in order run from one vertebra to the second or third above; while the deepest connect two contiguous vertebrae.
The Rotatores (Rotatores spinæ) lie beneath the Multifidus and are found only in the thoracic region; they are eleven in number on either side. Each muscle is small and somewhat quadrilateral in form; it arises from the upper and back part of the transverse process, and is inserted into the lower border and lateral surface of the lamina of the vertebra above, the fibers extending as far as the root of the spinous process. 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.
The Interspinales are short muscular fasciculi, placed in pairs between the spinous processes of the contiguous vertebrae, one on either side of the interspinal ligament. In the cervical region they are most distinct, and consist of six pairs, the first being situated between the axis and third vertebra, and the last between the seventh cervical and the first thoracic. They are small narrow bundles, attached, above and below, to the apices of the spinous processes. In the thoracic region, they are found between the first and second vertebrae, and sometimes between the second and third, and between the eleventh and twelfth. In the lumbar region there are four pairs in the intervals between the five lumbar vertebrae. There is also occasionally one between the last thoracic and first lumbar, and one between the fifth lumbar and the sacrum.
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 downward to be inserted into the lower part of the coccyx. It is a rudiment of the Extensor muscle of the caudal vertebrae of the lower animals.
The Intertransversarii (Intertransversales) are small muscles placed between the transverse processes of the vertebrae. In the cervical region they are best developed, consisting of rounded muscular and tendinous fasciculi, and are placed in pairs, passing between the anterior and the posterior tubercles respectively of the transverse processes of two contiguous vertebrae, and separated from one another by an anterior primary division of the cervical nerve, which lies in the groove between them. The muscles connecting the anterior tubercles are termed the Intertransversarii anteriores; those between the posterior tubercles, the Intertransversarii posteriores; the anterior divisions of the spinal nerves supply both sets. There are seven pairs of these muscles, the first pair being between the atlas and axis, and the last pair between the seventh cervical and first thoracic vertebrae. In the thoracic region they are present between the transverse processes of the lower three thoracic vertebrae, and between the transverse processes of the last thoracic and the first lumbar. In the lumbar region they are arranged in pairs, on either side of the vertebral column, one set occupying the entire interspaced between the transverse processes of the lumbar vertebrae, the Intertransversarii laterales; the other set, Intertransversarii mediales, passing from the accessory process of one vertebra to the mammillary of the vertebra below. The anterior divisions, and the Intertransversarii mediales supply the Intertransversarii laterales by the posterior divisions of the spinal nerves.