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. FOURTH LAYER 5. =Trachelo-mastoideus= (M. longissimus capitis et atlantis).—This muscle consists of two parallel, fusiform portions. It lies between the deep face of the splenius and the ventral part of the complexus. _Origin._—(1) The transverse processes of the first two thoracic vertebræ; (2) the articular processes of the cervical vertebræ. _Insertion._—(1) The mastoid process; (2) the wing of the atlas. [Illustration: FIG. 178.—SUPERFICIAL MUSCLES OF HORSE.