_Action._--Lateral flexor of the head. The two together elevate the head.
=M. longissimus capitis= (=trachelomastoideus=) (Fig. 73, _g_; Fig. 69, _e_).--This is a slender muscle lying close against the lateral border of the splenius (Fig. 73, _b_), and sometimes fused with the splenius at its craniad end. It is a cranial continuation of the longissimus dorsi (Fig. 69, _f″_).
_Origin_ by five slips which are attached by strong tendons to the anterior articular processes of the last four cervical vertebræ. The tendons are common to this muscle and the complexus. The five slips unite to form a flat belly which has its
_Insertion_ by a strong rounded tendon into the mastoid process of the temporal bone. The tendon may be closely united to that of the splenius.
_Relations._--Lateral surface with the sternomastoid (Fig. 68, _c_), the levator scapulæ (Fig. 73, _h_), and longissimus dorsi (Fig. 69, _f″_). Medial surface with the splenius (Fig. 73, _b_) (to which it is partly united), the complexus (Fig. 69, _b_), and the longus atlantis (Fig. 69, _d_).
=M. biventer cervicis= (Fig. 69, _a_) (=medial portion of M. semispinalis cervicis et capitis=).--The biventer cervicis is a flat muscle which lies beneath the splenius (Fig. 73, _b_), next the median line of the neck, with its fibres longitudinal.
_Origin_ in three or four slips from the surface of the three or four aponeurotic arches which take origin from the tips of the spinous processes of the last cervical and the first three thoracic vertebræ and pass laterad and caudad to the anterior processes of the second, third, fourth, and fifth thoracic vertebræ. These arches are deviated from their straight course by four aponeurotic bands which pass from them obliquely caudad and dorsad to the sides of the spinous processes of the vertebræ into which the arches are inserted. The arches form the beginning of the sheet of fascia which passes outside of the longitudinal supraspinous muscles, and the anchoring bands form the beginning of a similar sheet which passes within the longitudinal supraspinous muscles. An additional slip may take origin from the cervical ligament between the last cervical and the first thoracic vertebræ. The fibres form a flat band divided by two or three oblique tendinous intersections. It adheres closely to the cervical supraspinous ligament.