Muscles of thoracic cage
Note the multiple attachments of large sheets of muscle. Lateral to the sternum, pectoralis major has slips of origin from the upper six costal cartilages. Serratus anterior is attached to the upper eight ribs; the first rib has only a thin external border available, so the first digitation spans the intercostal space along a fibrous band and is attached, with the second digitation, to a rough plateau which characterizes the second rib. The next digitations arise behind pectoralis minor, and the last four interdigitate with external oblique. So the eight digitations at the border of serratus anterior form a bold forward convexity. Pectoralis minor is attached to the third to fifth ribs (often also to the second) just in front of serratus anterior. External oblique arises from the anterior angles of the lower eight ribs; below serratus anterior it interdigitates with latissimus dorsi on the last four ribs. Rectus abdominis is attached to the anterior surfaces of the fifth to seventh costal cartilages and the lower border of the seventh. Lateral to this the lower borders of the last six costal cartilages receive the internal oblique. Transversus abdominis and the diaphragm interdigitate on the internal surfaces of the lower six costal cartilages. The serratus posterior muscles, lying on the thoracolumbar fascia, are inserted just lateral to the ridges at the posterior angles of their respective ribs, the second to fifth for the superior and the last four ribs for the inferior muscle. The scalene muscles and quadratus lumborum are attached to the thoracic cage, above and below; they belong with the intercostal muscles to the various layers of the body wall. Their attachments will be noted in the study of the individual ribs. On the internal surfaces of the ribs the subcostal muscles lay in the paravertebral gutter and the innermost intercostals he in the lateral curve of the thoracic wall; each sheet widens from above down, but neither leaves any mark on the ribs. Between these sheets of muscle parietal pleura clothes the ribs and costal cartilages, except at the cardiac notch alongside the sternum behind the fifth and sixth left costal cartilages.