Page:EB1911 - Volume 19.djvu/69

 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. .—The trapezius has already been described as a superficial muscle of the upper part of the back; in the loin region the latissimus dorsi (fig. 5) is the superficial muscle, its origin being from the lower thoracic spines, lower ribs and lumbar fascia, and it is inserted into the upper part of the arm bone or humerus. 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 inferior 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 complexus 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 vertebrae 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 multifidus spinae muscles. The latissimus dorsi and rhomboids are supplied 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 ). On the anterior part of the thoracic region the pectoralis major runs from the clavicle, sternum and ribs, to the humerus (fig. 6); 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 border 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 recognized. The fibres of the external intercostals run downward and forward, those of the internal downward and backward (see ). The abdominal walls are formed of three sheets of muscle, of which the most superficial or external oblique (fig. 6) 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