deep to the splenius capitis muscle. In the neck the muscle is placed between the splenius capitis and semispinalis capitis.

M. Spinalis Dorsi.-The spinalis dorsi forms the medial column of the sacrospinalis. It lies in the thoracic region, and arises by tendinous fibres from the lower two thoracic and upper two lumbar spinous processes, and also directly from the tendon of the longissimus dorsi.

It is a narrow muscle which, lying close to the thoracic spinous processes medial to the longissimus dorsi, and it is inserted into the upper (four to eight) thoracic spines. It is not prolonged into the neck.

The semispinalis capitis (O.T. complexus) closely resembles in position and attachments the longissimus capitis.

It takes origin from the transverse processes of the upper six thoracic and the articular processes of the lower four cervical vertebræ, medial to the longissimus cervicis and longissimus capitis. It has an additional origin also from the spinous process of the last cervical vertebra.

It forms a broad muscular sheet which extends upwards in the neck, to be inserted into the medial impression between the superior and inferior nuchal lines of the occipital bone (Fig. 396, p. 444). The medial portion of the muscle is separate, and forms the biventer cervicis, consisting of two fleshy bellies with an intervening tendon, placed vertically in contact with the ligamentum nucha. The muscle is covered mainly by the splenius and longissimus capitis muscles. It conceals the semispinalis cervicis and the muscles of the suboccipital triangle.

Nerve-Supply.-Posterior rami of spinal nerves.

Actions. The several parts of the sacrospinalis muscle have a complex action, on the vertebral column, head, ribs, and pelvis. The muscle serves as an extensor of the vertebral column, and assists in lateral movement and rotation. The longissimus capitis and semispinalis capitis assist in extension, lateral movement and rotation of the head. The iliocostales and longissimus are accessory muscles of inspiration. The whole muscle helps in extension and lateral movement of the pelvis in the act of walking.

Third Group.

This group comprises the semispinales (dorsi and cervicis) and multifidus. They occupy the vertebral furrow, under cover of the sacrospinalis and semispinalis capitis muscles. They are only incompletely separate from one another. The semispinales, dorsi and cervicis, form a superficial stratum, the multifidus being more deeply placed. The more superficial muscles have the longer fibres; the fibres of the multifidus pass over fewer vertebræ. Both muscles extend obliquely upwards from transverse to spinous processes.

M. Semispinalis. The semispinalis muscle extends from the loin to the second cervical vertebra. Its fibres are artificially separated into an inferior part, the semispinalis dorsi, and a superior part, the semispinalis cervicis.

The semispinalis dorsi arises from the transverse processes of the lower six thoracic vertebræ.

It is inserted into the spinous processes of the last two cervical and first four thoracic vertebræ.

The semispinalis cervicis arises from the transverse processes of the upper six thoracic, and the articular processes of the lower four cervical vertebræ.

It is inserted into the spines of the cervical vertebræ from the second to the fifth.

M. Multifidus. The multifidus (O.T. multifidus spinæ) differs from the previous muscle in extending from the sacrum to the second cervical vertebra, and in the shortness of its fasciculi, which pass over fewer vertebræ to reach their insertion.

It arises from the sacrum, from the posterior sacro-iliac ligament (Fig. 395, p. 443), from the mamillary processes of the lumbar vertebræ, from the transverse processes of the thoracic vertebræ, and from the articular processes of the lower four cervical vertebræ.

It is inserted into the spines of the vertebræ up to and including the second cervical.

FIG. 394.-THE SUBOCCIPITAL TRIANGLE OF THE LEFT SIDE.

Lying in contact with the vertebral laminæ, the muscle is covered in the neck and back by the semispinalis, and in the loin by the sacrospinalis muscle.

Nerve-Supply.-Posterior rami of the spinal nerves.

Actions. These muscles are concerned in extension, lateral movement and rotation of the spine.

Fourth Group.

This group includes several sets of small muscles, which are vertebro-cranial or intervertebral in their attachments.

The muscles bounding the suboccipital triangle are four in number-obliqui capitis, inferior and superior, and recti capitis posteriores, major and minor.

These muscles are concealed by the semispinalis capitis and splenius capitis; they enclose a triangular space (the suboccipital triangle) in which the vertebral artery, the posterior ramus of the suboccipital nerve, and the posterior arch of the atlas are contained.

The obliquus capitis inferior arises from the spine of the epistropheus, and is inserted into the transverse process of the atlas.

Nerve Supply.-Posterior ramus of the first cervical (suboccipital) nerve.
Actions. Extension, lateral flexion and rotation of the atlas in the axis.

M. Obliquus Capitis Superior.-The obliquus capitis superior arises from the transverse process of the atlas, and is inserted into the occipital bone deep and lateral to the semispinalis capitis and above the inferior nuchal line (Fig. 396). Nerve-Supply.-Posterior ramus of the first cervical (suboccipital) nerve. Actions. Elevation, lateral movement and rotation of the head on the atlas.

M. Rectus Capitis Posterior Major.-The rectus capitis posterior major arises from the spine of the second cervical vertebra, and is inserted into the occipital bone deep to the obliquus capitis superior and semispinalis capitis and below the inferior nuchal line (Fig. 396).

Nerve-Supply.-Posterior ramus of the first cervical (suboccipital) nerve.
Actions. Elevation, lateral movement and rotation of the head.

M. Rectus Capitis Posterior Minor. The rectus capitis posterior minor arises deep to the preceding muscle from the posterior tubercle of the atlas, and is

inserted into the occipital bone below the inferior nuchal line medial to and beneath the rectus capitis posterior major (Fig. 396, p. 444).

Nerve-Supply. Posterior ramus of the first cervical (suboccipital) nerve.
Actions. Elevation, lateral movement and rotation of the head.

Mm. Rotatores.-The rotatores are eleven pairs of small muscles occupying the vertebral groove in the thoracic region, deep to the semispinalis dorsi, of which they form the deepest fibres. Each consists of a small slip arising from the · transverse process and inserted into the lamina of the vertebra directly above. Nerve-Supply.-Posterior rami of the thoracic nerves.

Actions. Extension and rotation of the vertebral column.

Mm. Interspinales.-The interspinales are bands of muscular fibres connecting together the spinous processes of the vertebræ.

Nerve-Supply.-Posterior rami of the spinal nerves.
Action.-Extension of the vertebral column.

Mm. Intertransversarii.—The intertransversarii are slender slips extending between the transverse processes. They are double in the neck, the anterior branches of the spinal nerves passing between them. In the loin the inter-transverse muscles are usually double, but they are often absent, or are replaced by membrane. Nerve-Supply.-Anterior rami of the spinal nerves.

Actions. Lateral movement and rotation of the vertebral column.

Mm. Rectus Capitis Lateralis.-The rectus capitis lateralis, extending from the transverse process of the atlas to the jugular process of the occipital bone (Fig. 396, p. 444), is homologous with the posterior of the two inter-transverse muscles. Nerve-Supply.-Anterior ramus of the first cervical (suboccipital) nerve.

Action.-Lateral movement and rotation of the head. The action of these muscles is extremely complex. Not only do they act on the vertebral column, ribs, head, and pelvis, in conjunction with other muscles, but some of them act also in relation to the movements of the limbs as well. In this section will be given an analysis of their movements in relation to the vertebral column, head, and pelvis. The movements of the limbs and of the ribs (respiration) are dealt with in other sections. The chief muscles are engaged in preserving the erect position, and in the movements of the trunk they are assisted in large measure by muscles whose chief actions are referred to elsewhere.

1. Movements of the Vertebral Column.-The movements of the vertebral column are flexion, extension, and lateral movement or rotation. These movements occur in all regionsneck, thorax, and loin; flexion and extension and lateral movement are most limited in the region of the thorax; while rotation is most limited in the region of the loin.