SPINAL). The posterior triangle is formed by the sterno-mastoid in front, the trapezius behind, and the clavicle below; in its floor from above downward part of the following muscles are seen : complexus, splenius, levator anguli scapulae, scalenus medius and scalenus anticus. Sometimes a small piece of the scalenus posticus is caught sight of behind the scalenus medius. The splenius rotates the head to its own side, the levator anguli scapulae raises the up per angle of the scapula, while the three scalenes run from the transverse processes of the cervical vertebrae and fix or raise the upper ribs. The trapezius (Pl. I.-5) arises from the spines of the thoracic vertebrae and the ligamentum nuchae, and is inserted into the outer third if the clavicle and the spine of the scapula ; it is used in shrugging the shoulders and in drawing the upper part of the scapula toward the mid-dorsal line. Its nerve supply is the spinal accessory and third and fourth cervical nerves. When the superficial muscles and complexus are removed from the back of the neck, the sub-occipital triangle is seen beneath the occipital bone. Externally it is bounded by the superior oblique, running from the transverse process of the atlas to the lateral part of the occipital bone, internally by the rectus capitis posticus major, passing from the spine of the axis to the lateral part of the occipi tal bone, and inferiorly by the inferior oblique joining the spine of the axis to the transverse process of the atlas. These muscles move the head on the atlas and the atlas on the axis. They are supplied by the posterior branch of the first cervical nerve. Muscles of the Trunk.—The trapezius has already been de scribed as a superficial muscle of the upper part of the back; in the loin region the latissimus dorsi (P1. 1.-5) is the superficial muscle, its origin being from the lower thoracic spines, lower ribs and lum bar fascia, and it is inserted into the upper part of the arm bone. When the trapezius is cut, the rhomboid muscles (major and minor) passing from the upper thoracic spines to the vertebral border of the scapula are seen, and deep to these is the serratus posticus superior passing from nearly the same spines to the upper ribs. On reflecting the latissimus dorsi the serratus posticus in ferior is seen running from the lower thoracic spines to the lower ribs. When these muscles are removed the great mass of the erector spinae is exposed, familiar to every one as the upper cut of the sirloin or ribs of beef ; it runs all the way up the dorsal side of the vertebral column from the pelvis to the occiput, the com plexus already mentioned being its extension to the head. It is longitudinally segmented into many different bundles to which special names are given, and it is attached to the various verte brae and ribs as it goes up, thus straightening the spinal column. Deep to the erector spinae are found shorter bundles passing from one vertebra to another and forming the semispinalis and multi fidus spinae muscles. The latissimus dorsi and rhomboids are sup
plied by branches of the brachial plexus of nerves, while the deeper muscles get their nerves from the posterior primary divisions of the spinal nerves (see NERVE, SPINAL). On the anterior part of the thoracic region the pectoralis major runs from the clavicle, sternum and ribs, to the humerus (Pl. 1.-4) ; deep to this is the pectoralis minor, passing from the upper ribs to the coracoid process. The serratus magnus is a large muscle rising by serrations from the upper eight ribs, and running back to the vertebral bor der of the scapula, which it draws forward as in the fencer's lunge. Between the ribs are the external and internal intercostal muscles; the former beginning at the tubercle and ending at the junctions of the ribs with their cartilages, while the latter only begin at the angle of the ribs but are prolonged on to the sternum, so that an interchondral as well as an intercostal part of each muscle is rec ognized. The fibres of the external intercostals run downward and forward, those of the internal downward and backward (see RES PIRATION). The abdominal walls are formed of three sheets of muscle, of which the most superficial or external oblique (P1. is attached to the outer surfaces of the lower ribs ; its fibres run downward and forward to the pelvis and mid-line of the abdomen, the middle one or internal oblique is on the same plane as the ribs, and its fibres run downward and backward, while the transversalis is attached to the deep surfaces of the ribs, and its fibres run hori zontally forward. Below, all these muscles are attached to the crest of the ilium and to Poupart's ligament, which is really the lower free edge of the external oblique, while, behind, the two deeper ones, at all events, blend with the fascia lumborum. As they approach the mid-ventral line they become aponeurotic and form the sheath of the rectus. The rectus abdominis (P1. 1.-4) is a flat muscular band which runs up on each side of the linea alba or mid-ventral line of the abdomen from the pubis to the ribs and sternum. This muscle has certain tendinous intersections or lineae transversae, the positions of which are noticed in the article ANATOMY (Superficial and Artistic), and the morphology of which is referred to later. In front of the lowest part of the rectus is sometimes a small triangular muscle called the pyramidalis. The quadratus lumborum is a muscle at the back of the abdominal wall which runs between the last rib and the crest of the ilium. In front of the bodies of the vertebrae is a prevertebral or hypaxial musculature, of which the rectus capitis anticus major and minor muscles and longus colli in the neck and the psoas in the loins form the chief parts, the latter being familiar as the undercut of the sir loin of beef, while the pelvis is closed below by a muscular floor formed by the levator ani and coccygeus muscles. The diaphragm g explained in a separate article.