ANTERIOR THORACIC WALL
The anterior thoracic wall is covered by skin and the superﬁcial fascia, which contains the mammary glands. Its framework is formed by the anterior part of the bony thorax, described and illustrated in Plate 1-2.
The muscles here belong to three groups: muscles of the upper extremity, muscles of the anterolateral abdominal wall, and intrinsic muscles of the thorax (see Plates 1-4, 1-5, and 1-6).
MUSCLES OF THE UPPER EXTREMITY
These muscles include the pectoralis major, pectoralis minor, serratus anterior, and subclavius.
The pectoralis major is a thick, fan-shaped muscle that has three areas of origin: clavicular, sternocostal, and abdominal. The clavicular origin is the anterior surface of roughly the medial half of the clavicle. The sternocostal origin is the anterior surface of the manubrium and body of the sternum and the costal cartilages of the ﬁrst six ribs. The small and variable abdominal origin is the aponeurosis of the external abdominal oblique muscle. The pectoralis major inserts onto the crest of the greater tubercle of the humerus.
The pectoralis minor is a thin triangular muscle that lies deep to the pectoralis major. It arises from the superior margins and external surfaces of the third, fourth, and ﬁfth ribs close to their costal cartilages and from the fascia covering the intervening intercostal muscles. The pectoralis minor inserts onto the coracoid process of the scapula. The pectoralis major and minor muscles are supplied by the medial and lateral anterior thoracic (pectoral) nerves, which are branches of the medial and lateral cords of the brachial plexus.
The serratus anterior is a large muscular sheet that curves around the thorax. It arises by muscular digitations from the external surfaces and superior borders of the ﬁrst eight or nine ribs and from the fascia covering the intervening intercostal muscles. It inserts onto the ventral surface of the vertebral border of the scapula. Its nerve supply is the long thoracic nerve, a branch of the brachial plexus (ﬁfth, sixth, and seventh cervical nerves), which courses inferiorly on the external surface of the muscle.
The subclavius is a small triangular muscle tucked between the clavicle and the ﬁrst rib. It has a tendinous origin from the junction of the ﬁrst rib and its costal cartilage, and it inserts into a groove toward the lateral end of the lower surface of the clavicle. It receives its nerve supply from the subclavian branch of the brachial plexus.
MUSCLES OF THE ANTEROLATERAL ABDOMINAL WALL
These muscles, which are partially on the anterior thoracic wall, are the external abdominal oblique and the rectus abdominis.
The external abdominal oblique muscle originates by ﬂeshy digitations from the external surfaces and inferior borders of the ﬁfth to twelfth ribs. The fasciculi from the last two ribs insert into the iliac crest, and the remaining fasciculi end in an aponeurosis that inserts in the linea alba.
The superior end of the rectus abdominis muscle is attached primarily to the external surfaces of the costal cartilages of the ﬁfth, sixth, and seventh ribs. The rectus abdominis muscle is enclosed in a sheath formed by the aponeuroses of the external oblique, the internal oblique, and the transverse abdominis muscles. Its inferior end is attached to the crest of the pubis.
The muscles of the anterolateral abdominal wall are supplied by the thoraco abdominal branches of the lower six thoracic nerves.
INTRINSIC MUSCLES OF THE THORAX
These muscles, which help to form the anterior thoracic wall, are the external and internal intercostal muscles and the transversus thoracis muscle.
The external intercostal muscles each arise from the lower border of the rib above and insert onto the upper border of the rib below. Their ﬁbers are directed down- ward and medially. They extend from the tubercles of the ribs to the beginnings of the costal cartilages, from which they continue medially as the anterior intercostal membranes. The internal intercostal muscles each arise from the inner lip and ﬂoor of the costal groove of the rib above and from the related costal cartilage. They insert onto the upper border of the rib below. These muscles extend from the sternum to the angles of the ribs, from which they continue to the vertebral column as the posterior intercostal membranes. The ﬁbers of the internal intercostal muscles are directed downward and laterally. The innermost intercostal muscles are deep to the internal intercostals, of which they were once regarded a constituent. They attach to the internal aspects of adjoining ribs and their ﬁbers run in the same direction as those of the internal intercostals. The intercostal muscles are supplied by the related intercostal nerves.
A muscle occasionally present, the sternalis, lies on the origin of the pectoralis major muscle parallel to the sternum. Its variable attachments are to the costal cartilages, sternum, rectus sheath, and sternocleidomastoid and pectoralis major muscles.
On the inner surface of the anterior thoracic wall lies a thin sheet of muscular and tendinous ﬁbers called the transversus thoracis muscle. This muscle arises from the posterior surfaces of the xiphoid process, the lower third of the body of the sternum, and the sternal ends of the related costal cartilages. It is inserted by muscular slips onto the inner surfaces of the second or third to the sixth costal cartilages.
NERVES OF THE ANTERIOR THORACIC WALL
The nerve supply of the skin of the anterior thoracic wall has two sources: the anterior and middle supraclavicular nerves (branches of the cervical plexus made up mostly of ﬁbers from the fourth cervical nerve) cross over the clavicle to supply the skin of the infraclavicular area; the anterior and lateral cutaneous branches of the related intercostal nerves pierce the muscles to supply the skin of the remainder of the anterior thoracic wall.
ARTERIES OF THE ANTERIOR THORACIC WALL
Arteries supplying the anterior thoracic wall come from several sources. There is typically an artery in the upper part of the intercostal space and one in the lower part of the space. Posteriorly, nine pairs of intercostal arteries come from the back of the aorta and run forward in the lower nine intercostal spaces. Also posteriorly, the ﬁrst intercostal space receives the highest intercostal branch of the costocervical trunk from the subclavian artery. This same artery anastomoses with the highest aortic intercostal artery, contributing to the supply of the second intercostal space. Near the angle of the rib, each aortic intercostal artery gives off a collateral intercostal branch that descends to run forward along the upper border of the rib below the intercostal space. These arteries anastomose with the intercostal branches of the internal thoracic (internal mammary) artery, of which there are two in each of the upper ﬁve or six spaces.
VEINS OF THE ANTERIOR THORACIC WALL
Similar to venous drainage elsewhere, that of the anterior thoracic wall exhibits considerable variation. The most frequent pattern involves the veins accompanying the internal thoracic (internal mammary) arteries and the azygos, hemiazygos, and accessory hemiazygos veins. The veins accompanying the internal thoracic arteries receive tributaries corresponding to the arterial branches and empty into the brachiocephalic (innominate) veins of the same side. The ﬁrst posterior intercostalvein usually empties into either the brachiocephalic (innominate) or the vertebral vein. The right highest intercostal vein usually drains blood from the second and third intercostal spaces and passes inferiorly to empty into the azygos vein. The left highest intercostal vein also receives the second and third posterior inter- costal veins and empties into the lower border of the left brachiocephalic vein.
The fourth to the eleventh posterior intercostal veins on the right side empty into the azygos vein, which is ordinarily formed by the junction of the right ascending lumbar vein and the right subcostal vein. The latter courses superiorly on the right side of the thoracic vertebrae to the level of the fourth posterior intercostal vein, where it passes in front of the root of the lung to empty into the superior vena cava just before this vessel enters the pericardial sac. On the left side, the ascending lumbar vein and the subcostal vein form the hemiazygos vein, which usually receives the lower four posterior intercostal veins as it runs superiorly to the left of the vertebral column. Here it crosses at about the level of the ninth thoracic vertebra to empty into the azygos vein. The accessory hemiazygos vein receives the fourth to the eighth posterior intercostal veins as it courses inferiorly to the left of the vertebral column before crossing at about the level of the eighth thoracic vertebra, also emptying into the azygos vein.
LYMPHATIC DRAINAGE OF THE ANTERIOR THORACIC WALL
The lymphatic drainage of the anterior thoracic wall involves three general groups of lymph nodes: sternal (internal thoracic), phrenic (diaphragmatic), and intercostal. The sternal nodes lie along the superior parts of the internal thoracic arteries. There are several groups of phrenic nodes on the superior surface of the diaphragm, and there is an intercostal node or two at the vertebral end of each intercostal space. The efferents of the sternal nodes usually empty into the broncho mediastinal trunk. The efferents of the phrenic nodes ordinarily go to the sternal nodes. The upper intercostal nodes send their efferents to the thoracic duct, and the lower ones on each side drain intra vessel that courses inferiorly into the cisterna chyli.