_Structure._—The muscle is composed of a succession of bundles. The largest of these constitute the thoracic part of the muscle, which has a strong tendon inserted into the last two cervical vertebræ. A bursa is interposed between the tendon and the spine at the first costo-vertebral articulation. The cervical portion consists of a number of smaller bundles, each of which passes from its origin on the transverse process of one vertebra forward and inward to its insertion into a vertebra further forward. The most anterior bundle is inserted by a strong tendon into the ventral tubercle of the atlas. _Relations._—The principal relations of the two muscles in the thorax are: ventrally, the pleura, and, further forward, the trachea and œsophagus; dorsally, the vertebræ and the costo-vertebral joints; laterally, the dorsal, superior cervical, and vertebral vessels, the sympathetic nerve, and the thoracic roots of the brachial plexus. In the neck, important relations are: ventrally, the trachea and œsophagus, the carotid artery, the vagus, sympathetic and recurrent nerves; dorsally, the vertebræ and, in the middle third of the neck, the intertransversales muscles; laterally, the scalenus, the rectus capitis anticus major, and the intertransversales (in the anterior third). The terminal part of the muscle is separated from the trachea by the œsophagus, which is here median in position. _Blood-supply._—Subcostal and vertebral arteries. _Nerve-supply._—Ventral branches of the spinal nerves. 11. =Intertransversales colli= (Mm. intertransversarii cervicis).—These are six fasciculi which occupy the spaces between the lateral aspects of the vertebræ and the transverse and articular processes. There is thus a bundle for each intervertebral articulation except the first. Each bundle consists of a dorsal and ventral portion. _Attachments._—The dorsal bundles pass from transverse process to articular process; the ventral bundles extend between adjacent transverse processes. _Action._—To bend the neck laterally. _Structure._—They contain strong tendinous intersections. _Relations._—Superficially, the mastoido-humeralis, rectus capitis anterior major, complexus, trachelo-mastoideus, splenius, scalenus, and longissimus muscles; deeply, the vertebræ, the longus colli muscle, and the vertebral vessels. The muscles are perforated by branches of these vessels and by the primary branches of the cervical nerves. _Blood-supply._—Vertebral artery. _Nerve-supply._—The cervical nerves. LATERAL MUSCLES This group consists of twelve pairs of muscles arranged in layers. FIRST LAYER 1. =Trapezius cervicalis.=—Described on p. 250. SECOND LAYER 2. =Rhomboideus cervicalis.=—Described on p. 251. 3. =Serratus magnus= (M. serratus cervicis).—Described with the thoracic part on p. 254. THIRD LAYER