_Structure._—The muscle arises by a short, thin aponeurosis, from which the fibers of the flat fleshy portion converge to the spine of the scapula and the aponeurosis which separates the two portions. The cervical fascia joins the ventral edge of the cervical portion to the mastoido-humeralis, or the two muscles may unite here. _Relations._—Superficially, the skin and fascia; deeply, the rhomboideus, latissimus dorsi, supraspinatus, infraspinatus, deltoid, splenius, serratus magnus, and anterior deep pectoral muscles, and the cartilage of the scapula. _Blood-supply._—Deep cervical and intercostal arteries. _Nerve-supply._—Spinal accessory nerve. SECOND LAYER This consists of two muscles—the rhomboideus and the latissimus dorsi. 2. =Rhomboideus.=—This consists of two portions: (=a=) =Rhomboideus cervicalis s. cervicis.=—_Origin._—The funicular portion of the ligamentum nuchæ, from the second cervical to the second thoracic vertebra. _Insertion._—The internal surface of the cartilage of the scapula. (=b=) =Rhomboideus thoracalis s. dorsalis.=—_Origin._—The spinous processes of the second to the seventh thoracic vertebra by means of the dorso-scapular ligament. _Insertion._—The inner surface of the cartilage of the scapula. _Action._—To draw the scapula upward and forward. When the limb is fixed the cervical portion will elevate the neck. _Structure._—The cervical portion is narrow, pointed at its anterior extremity, and lies along the funicular part of the ligamentum nuchæ, to which it is attached by short tendon bundles. The fibers are directed for the most part longitudinally. The thoracic portion is quadrilateral in shape, and its fibers are nearly vertical. Its deep face is intimately attached to the dorso-scapular ligament. _Relations._—Superficially, the skin and fascia (over a small area), the trapezius, and the cartilage of the scapula; deeply, the dorso-scapular ligament, the splenius, complexus, longissimus dorsi, and serratus anticus. _Blood-supply._—Dorsal and superior cervical arteries. _Nerve-supply._—Sixth cervical nerve. 3. =Latissimus Dorsi.=—This is a wide muscle which has the form of a right-angled triangle. It lies for the most part under the skin and panniculus, on the lateral wall of the thorax, from the spine to the arm. _Origin._—The lumbo-dorsal fascia—and by this means from the lumbar and thoracic spines as far forward as the highest point of the withers. _Insertion._—The internal tubercle of the humerus, in common with the teres major. _Action._—To draw the humerus upward and backward and flex the shoulder joint. If the limb is advanced and fixed, it draws the trunk forward.