This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
Covering the chest anteriorly are the pectoralis major and pectoralis minor muscles. The serratus anterior (magnus) winds around its side and posteriorly, above is the trapezius and below the latissimus dorsi. Beneath them are the erector spinae (sacrospina/is) muscles on each side of the spinous processes.
The pectoralis major muscle arises from the sternal half of the clavicle, from the sternum and costal cartilages as low as the sixth or seventh rib. and from the 12 aponeurosis of the external oblique muscle and sheath of the rectus muscle. It inserts into the outer lip of the bicipital groove. It is to be noted in regard to this muscle that it is attached only to the inner half of the clavicle and that the clavicular and sternal parts are separated by a cleft. When removing it in excision of the breast for carcinoma one separates the muscle by passing through this cleft and detaching the part below. It forms the anterior fold of the axilla and by following this fold to the chest-wall it leads to the fifth rib, as it is at that rib that the muscle leaves the chest-wall.
Fig. 201. - The clavipectoral fascia.
The pectoralis major is covered by the pectoral fascia. When in removal of the female breast for nonma-lignant growths the breast is raised, the muscle beneath is seen to be covered with a thin fascia continuous with the fascia of the axilla. Beneath the pectoralis major is the clavipectoral fascia continuous with the costocoracoid membrane above and the axillary fascia at the sides. The pectoralis minor passes from the third, fourth, and fifth ribs to the coracoid process. Its origin is well forward toward the anterior extremities of the ribs and, as it is not attached so low on the chest as is the pectoralis major, it is hidden by the latter and does not aid in forming the anterior axillary fold. This muscle is frequently removed in operations for carcinoma of the mammary gland.
Fig. 202. - The serratus anterior muscle arising by ten digitations from the nine upper ribs.
The serratus anterior (magnus) muscle (Fig. 202) passes from the side of the chest to the vertebral or posterior border of the scapula, arising by nine or ten digitations from the eight or nine upper ribs, the second having two. The slip arising from the sixth rib is the one most prominently seen on raising the arm away from the side, it passes the farthest forward. The slips into the fifth, seventh, and eighth ribs may also be seen. This muscle passes across the axilla from in front backward, lying on the chest-wall. It is supplied by the posterior thoracic nerve from the fifth, sixth, and seventh cervicals. This nerve is also called the long external respiratory nerve of Bell. The internal respiratory nerve is the phrenic, which comes from the third, fourth, and fifth cervical nerves. (One of the main functions of this muscle is to keep the scapula applied to the chest and to aid in rotating it in elevation of the arm. When it is paralyzed the arm cannot be raised beyond a right angle and the scapula projects, particularly at its lower angle and posterior edge. This condition is called " winged scapula".
Fig. 203. - Muscles of the back.
The trapezius muscle (Fig.203) has the shape of a triangle, its apex being out on the acromion process and its base in the median line. It arises posteriorly from the inner third of the superior curved line of the occiput, the occipital protuberance, ligamentum nuchae, and the spines of the seventh cervical and all the thoracic vertebras.
It inserts into the outer third of the clavicle and the acromion and spinous processes of the scapula. It aids in rotating the scapula and elevating the shoulder; its paralysis is followed by marked dropping of the shoulder. It is supplied by the spinal accessory nerve, which is sometimes injured in operations for tumors involving the posterior cervical triangle.
The latissimus dorsi muscle arises from the spinous processes of the lower six thoracic vertebrae, from the posterior layer of the lumbar fascia, the outer lip of the posterior third of the iliac crest and by digitations from the lower three or four ribs. Sometimes it is attached to the angle of the scapula. It unites with the tendon of the teres major muscle to be inserted into the bottom of the bicipital groove and extends somewhat higher than the tendon of the pectoralis major. A bursa, which may become inflamed, sometimes lies between the muscle and the inferior angle of the scapula. The latissimus dorsi and teres major muscles form the posterior axillary fold.
The erector spinae (sacrospinalis) muscle fills up the hollows on each side of the spinous processes. As the various muscular bundles are inserted into the vertebrae by innumerable small tendinous slips, in exposing the vertebrae in performing laminectomy it is necessary to cut them with a knife or scissors. One should not attempt to separate them by blunt dissection. These muscles become atrophied in cases in which the spine becomes distorted.