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.
