II. THE SUBOCCIPITAL MUSCLES (fig. 596)
| Rectus capitis posterior major /td> | Obliquus capitis inferior |
| Rectus capitis posterior minor | Obliquus capitis superior |
The Rectus capitis posterior major arises by a pointed tendon from the spine of the axis, and, becoming broader as it ascends, is inserted into the lateral part of the inferior nuchal line of the occipital bone, and into the bone immediately below the line. As the muscles of the two sides pass upwards and laterally, they leave between there a triangular space, in which parts of the Recti capitis posteriores minores are seen.
Nerve-supply.—-The Rectus capitis posterior major is supplied by the posterior primary ramus of the suboccipital nerve.
Actions.-The Rectus capitis posterior major extends the head, and turns the face towards the same side.
The Rectus capitis posterior minor arises by a narrow pointed tendon from the tubercle on the posterior arch of the atlas, and, widening as it ascends, is inserted into the medial part of the inferior nuchal line of the occipital bone and also into the bone between that line and the foramen magnum.
Nerve-supply.—The Rectus capitis posterior minor is supplied by the posterior primary ramus of the suboccipital nerve.
Action.-The Rectus capitis posterior minor extends the head.
The Obliquus capitis inferior, the larger of the two Oblique muscles, arises from the lateral aspect of the spine of the axis, and passes laterally and slightly upwards, to be inserted into the lower and back part of the transverse process of the atlas.
Nerve-supply.-The Obliquus capitis inferior is supplied by the posterior primary ramus of the suboccipital nerve.
Action.-The Obliquus capitis inferior turns the face towards the same side. Owing to the length of the transverse process of the atlas the muscle is enabled to act to good mechanical, advantage.
The Obliquus capitis superior, narrow below, wide and expanded above, arises by tendinous fibers from the upper surface of the transverse process of the atlas. It passes upwards and backwards, and is inserted into the occipital bone, between the superior and inferior nuchal lines, lateral to the Semispinalis capitis and overlapping the insertion of the Rectus capitis posterior major.
Nerve-supply.-The Obliquus capitis superior is supplied by the posterior primary ramus of the suboccipital nerve.
The suboccipital triangle.-This triangle is bounded, above and medially, by the Rectus capitis posterior major; above and laterally, by the Obliquus capitis superior; below and laterally, by the Obliquus capitis inferior. Medially, the roof is formed by a layer of dense fibrofatty tissue, situated deep to the Semispinalis capitis, and, laterally, by the Longissimus capitis. 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 posterior primary ramus of the first cervical nerve (fig. 596) lie in the groove on the upper surface of the posterior arch of the atlas.
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