9. =Obliquus capitis posterior= (s. caudalis) (great oblique muscle of
the head).—This is a strong, quadrilateral muscle, which covers the
dorso-lateral aspect of the atlas and axis.
_Origin._—The side of the spine and the posterior articular process of
the axis.
_Insertion._—The dorsal surface of the wing of the atlas.
_Action._—Chiefly to rotate the atlas, and with it the head, to the same
side; also to assist in extending and fixing the atlanto-axial joint.
_Structure._—The muscle is composed almost entirely of parallel fleshy
fibers directed obliquely forward and outward.
_Relations._—Superficially, the skin, the splenius, complexus,
trachelo-mastoideus, and mastoido-humeralis muscles; deeply, the arch
and spine of the axis, the wing of the atlas, the atlanto-axial joint,
the multifidus cervicis, the posterior straight muscles, the occipital
and vertebral vessels, and the first and second cervical nerves. The
terminal part of the vertebral artery joins the posterior branch of the
occipital artery under cover of the muscle.
_Blood-supply._—Occipital and vertebral arteries.
_Nerve-supply._—Dorsal branch of the second cervical nerve.
10. =Obliquus capitis anterior= (s. cranialis) (small oblique muscle of
the head).—A short, thick, quadrilateral muscle which lies on the side
of the occipito-atlantal articulation.
_Origin._—The anterior edge of the wing of the atlas and the fossa
atlantis.
_Insertion._—The styloid process and crest of the occipital bone, and
the mastoid process.
_Action._—Acting together, to extend the head on the atlas; acting
singly, to flex the head laterally.
_Structure._—The muscle contains a good deal of tendinous tissue. The
direction of its fibers is forward, upward, and inward.
_Relations._—Superficially, the posterior auricular muscles, artery, and
nerve, the aponeurosis of the splenius, trachelo-mastoideus and
mastoido-humeralis, and the parotid gland; deeply, the straight muscles,
the complexus, the occipito-hyoideus, the occipito-atlantal
articulation, branches of the occipital artery, and a branch of the
occipital nerve.
_Blood-supply._—Occipital artery.
_Nerve-supply._—Dorsal branch of the first cervical nerve.
11. =Rectus capitis posterior major= (s. dorsalis major) (great
posterior straight muscle of the head).—This muscle extends from the
axis to the occipital bone, in contact with the ligamentum nuchæ.
_Origin._—The edge of the spinous process of the axis.
_Insertion._—The occipital bone, below and external to the complexus and
ligamentum nuchæ.
_Action._—To extend the head.
[Illustration: