_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_). _Action._--Lateral flexor of the head. =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. _Insertion_ by a strong tendon into the medial part of the lambdoidal crest. _Relations._--Outer surface with the splenius (Fig. 73, _b_) and at the caudal end with the longissimus dorsi (Fig. 69, _f″_). Inner surface with the spinalis dorsi (Fig. 69, _g_), the semispinalis cervicis (Fig. 71, _c_), and the rectus capitis posterior major (Fig. 71, _a_). Medial edge with the muscle of the opposite side; lateral edge with the complexus (Fig. 69, _b_). _Action._--Raises the snout. =M. complexus= (Fig. 69, _b_) (=lateral portion of M. semispinalis cervicis et capitis=).--The complexus is closely associated with the biventer cervicis (_a_) and lies on its lateral side beneath the splenius (Fig. 73, _b_). _Origin_ by tendinous bands from the anterior articular processes of the last five or six cervical and the first one, two, or three thoracic vertebræ. The same bands give origin externally to the fibres of the longissimus capitis (Fig. 69, _e_), and internally to those of deeper muscles of the neck. The six or more muscular slips continued from these tendons unite to form a flat band, the