The panniculus, except the cervical part, has been removed. _a_, Trapezius cervicalis; _a′_, trapezius thoracalis; _c_, _c′_, mastoido-humeralis; _d_, sterno-cephalicus; _e_, deltoid; _f_, long head of triceps; _f′_, external head of triceps; _g_, anterior superficial pectoral; _h_, posterior deep pectoral; _h′_, anterior deep pectoral; _i_, serratus thoracis; _i′_, serratus cervicis; _k_, latissimus dorsi; _l_, obliquus abdominis externus; _l′_, aponeurosis of _l_; _m_, serratus posticus; _m′_, lumbo-dorsal fascia; _o_, tensor fasciæ latæ; _o′_, fascia lata; _o″_, gluteus superficialis; _q_, _q′_, _q″_, biceps femoris; _r_, semitendinosus; _s_, sacro-coccygeus superior; _t_, sacro-coccygeus lateralis; _u_, coccygeus; _v_, cervical panniculus; _w_, splenius; _x_, rhomboideus; _y_, tendon of trachelo-mastoideus and mastoido-humeralis; _z_, suprasinatus; _X_, wing of atlas; _2_, spine of scapula; _4′_, external epicondyle of humerus; _6_, deltoid tuberosity; _8_, olecranon; _16_, external angle of ilium; _20_, patella; _21′_, external condyle of tibia. (After Ellenberger-Baum, Anat. für Künstler.) ] _Action._—Acting together, to extend the head and neck; acting singly, to flex the head and neck laterally or to rotate the atlas. _Structure._—The origin from the thoracic vertebræ is by aponeurotic slips which blend with the complexus. The succeeding fleshy portion, in passing along the neck, receives fasciculi from each of the cervical vertebræ except the first two. The dorsal division of the muscle is inserted into the mastoid process by a flat tendon which fuses with that of the splenius; the ventral division is inserted into the wing of the atlas by a ribbon-like tendon in common with the splenius and mastoido-humeralis. _Relations._—Superficially, the splenius muscle and dorsal branches of the cervical nerves; deeply, the complexus, the spinalis colli, and the oblique muscles of the head. The superior cervical vessels cross the deep face of the muscle obliquely at the level of the sixth and seventh cervical vertebræ. _Blood-supply._—Vertebral and superior cervical arteries. _Nerve-supply._—Dorsal branches of the last six cervical nerves. 6. =Complexus= (M. semispinalis capitis).—This is a large triangular muscle which lies chiefly on the ligamentum nuchæ, under cover of the splenius and trachelo-mastoideus muscles. _Origin._—(1) The second, third, and fourth thoracic spines, in common with the splenius and serratus anticus; (2) the transverse processes of the first six or seven thoracic vertebræ; (3) the articular processes of the cervical vertebræ. _Insertion._—The posterior surface of the occipital bone, external to the funicular portion of the ligamentum nuchæ. _Action._—It is the chief extensor of the head and neck. Acting singly, the muscle inclines the head to the same side.