4. =Splenius.=—This is an extensive, flat, triangular muscle, partly
covered by the preceding three muscles.
_Origin._—The second, third, and fourth thoracic spines by means of the
dorso-scapular ligament, and the funicular portion of the ligamentum
nuchæ.
_Insertion._—The occipital crest, the mastoid process, the wing of the
atlas, and the transverse processes of the third, fourth, and fifth
cervical vertebræ.
_Action._—Acting together, to elevate the head and neck; acting singly,
to incline the head and neck to the side of the muscle acting.
_Structure._—The muscle arises in the withers from the anterior part of
the dorso-scapular ligament, which also affords attachment to the
rhomboideus, serratus anticus, and complexus muscles. The fibers pass
upward and forward toward the head and the first cervical vertebra. The
insertion on the occipital bone and the mastoid process is by means of a
thin aponeurosis common to the splenius, trachelo-mastoideus, and
mastoido-humeralis. The atlantal insertion is by a strong, flat tendon,
also in common with these muscles. The remaining insertions are fleshy
digitations.
_Relations._—Superficially, the skin and fascia, the trapezius
rhomboideus, cervicalis, serratus magnus, and posterior auricular
muscles; deeply, the complexus, trachelo-mastoideus, and longissimus
muscles.
_Blood-supply._—Deep cervical and dorsal arteries.
_Nerve-supply._—Dorsal branches of the last six cervical nerves.