Thorax
Before describing the anatomy of the
heart, it is helpful to review other anatomic features of the thoracic cavity
and organs.
 |
| LUNGS IN SITU: ANTERIOR VIEW |
The thorax proper constitutes the upper part of the body or trunk, with a shape between a barrel and a truncated cone that is functionally favorable. Although the intrathoracic pressure is often subatmospheric, the chest wall is still able to retain its integrity by means of rather thin, lightweight skeletal elements. The thoracic cavity occupies only the upper part of the thoracic cage. The abdominal (peritoneal) cavity reaches upward as high as the lower tip of the sternum, affording protection to large, easily injured abdominal organs such as the liver, spleen, stomach, and kidneys.
The thoracic and
abdominal cavities are separated by the dome-shaped diaphragm, a sheet
of tissue consisting of a peripheral muscular part and a central tendinous part
that closes the thoracic cavity interiorly. Superiorly, the narrow upper
thoracic aperture bounded by the upper part of the sternum, the short stout
first ribs, and the body of the first thoracic vertebra (T1) gives access to
the root of the neck and is not closed by a specific structure. The thorax is
bounded posteriorly by the bodies of the 12 thoracic vertebrae and the
posterior portions of the ribs, anteriorly by the sternum, costal
cartilages, and anterior portions of the ribs, and laterally by the
remaining parts of the ribs. The spaces between successive ribs are bridged by
the intercostal muscles.
The sternum (breastbone)
lies anterior in the midline and superficially. The clavicles and the
first seven pairs of ribs articulate with it. The sternum consists of three
parts: the bony manubrium and corpus sterni and the small, cartilaginous
xiphoid process. The clavicles articulate with the manubrium on its
upper border, and the notch between these joints is the interclavicular (or
suprasternal) notch. Just below the sternoclavicular joints, the cartilages of
the first ribs are attached to the sternum. No joint spaces are present here.
The manubrium and the body of the sternum are united by fibro cartilage. The
junction between the manubrium and the body of the sternum usually forms a
prominent ridge, accentuated by the two parts of the sternum forming a slight
angle with each other, the sternal angle of Louis. This is an important
landmark because the cartilages of the second ribs articulate with the sternum
at this point. The third, smallest part of the sternum is the xiphoid
cartilage, a thin, spoon-shaped process attached to the lower end of the
sternal body.
Most of the bony thorax
is formed by the ribs, usually 12 on each side of the trunk. The ribs consist of
a series of thin, curved, rather elastic bones that articulate posteriorly with
the thoracic vertebrae and terminate anteriorly in the costal cartilages. The
first seven pairs of ribs attach to the sternum by means of their cartilages, whereas the eighth, ninth, and tenth
pairs articulate with each other and do not reach the sternum. The 11th and
12th pairs are small and poorly developed, ending in free cartilaginous tips.
The ribs are thickest posteriorly; they flatten out and widen as they curve
forward. Along the inferior and inner surface of the posterior part of each
rib, a groove the sulcus costae affords protection to the intercostal vessels
and nerve.
The first two and last
two ribs differ somewhat from the previous description. The first rib (see
Plate 1-2) is very short and relatively heavier than the other ribs. On the superior surface of the first
rib, two grooves are divided by a tubercle the tuberculum scaleni that forms
the point of insertion of the anterior scalene muscle. The groove in front of
the muscle is occupied by the subclavian vein, whereas the subclavian
artery follows the groove behind the tubercle. The second rib is longer
than the first and resembles the other ribs except the small 11th and 12th
ribs.
The spaces between
successive ribs are occupied by intercostal muscles (see Plate 1-1). Each external intercostal muscle
arises from the lower border of the rib above, runs obliquely downward and medially, and
inserts into the upper border of the rib below. Each internal intercostal
muscle arises from the lower border of the rib above and runs downward and
outward to insert on the upper border of the rib below. Between these two
muscle layers lie the intercostal vessels, whereas the intercostal nerves lie
between the internal and the innermost intercostal muscles.
 |
| HEART IN SITU |
Many muscles of the upper
extremities originate from the chest wall, including the pectoralis major (see
Plate 1-1) and pectoralis minor muscles and the serratus anterior muscle, which
originate from the anterior and lateral portions of the chest wall.
Several neck muscles
originate from the upper rim of the thoracic cage. The sternohyoid and sternothyroid
(see Plate 1-1) are thin, straplike muscles that arise from the superior
border and posterior surface of the sternum and insert into the hyoid bone and
the thyroid cartilage, respectively. The sternocleidomastoid muscle (SCM)
arises (see Plate 1-1) as a stout sternal head from the upper border of the
sternum, adjacent to the sternoclavicular joint, and as a second clavicular
head from the medial third of the clavicle. The interval between the two heads
is usually visible as a slight depression, behind which the apex of the lung
rises from the thorax into the root of the neck. Above this interval the two
heads of the SCM unite to form a single muscular belly that passes obliquely
upward, backward, and laterally to insert into the lateral surface of the
mastoid process and occipital bone.
Superficial to the SCM,
the external jugular vein passes perpendicularly downward from its
origin at the lower border of the parotid gland, crosses the SCM, and
penetrates the deep fascia of the neck to empty into the subclavian vein.
Of the deeper neck
muscles, the three scalene muscles originate from the transverse processes of
the cervical vertebrae. The anterior scalene muscle inserts into the
scalene tubercle of the first rib; the medial scalene muscle also attaches to
the upper surface of the first rib, but more posteriorly. The posterior scalene
muscle inserts on the second rib. The components of the cervical nerve plexus
emerge from the groove between the anterior and middle scalene muscles. The
anterior scalene muscle is crossed laterally and anteriorly by the phrenic
nerve, which originates from the cervical plexus and runs downward and
behind the subclavian vein to enter the thoracic cavity. The groove between the
anterior and middle scalene muscles widens inferiorly to form a triangular
opening through which emerge the components of the brachial plexus and the
subclavian artery. After ascending from the thoracic cavity, the
subclavian artery crosses the upper surface of the first rib, lying in the
groove posterior to the scalene muscle, and enters the axilla. The subclavian vein runs
parallel to the subclavian artery but in front of the anterior scalene muscle.
Deep in the lower portion
of the neck under the SCM, a narrow space is bordered anteriorly by the omohyoid
and strap muscles, posteriorly by the anterior scalene muscle and
prevertebral fascia, and medially by the pharynx, esophagus, trachea, and
thyroid gland (see Plate 1-1). In this space the common carotid artery,
internal jugular vein, and vagus nerve are enclosed in a common
connective-tissue sheath; the jugular vein runs most superficially and the vagus nerve lies
beneath, between the common carotid artery and internal jugular veins. On the
left side the thoracic duct (see Plate 1-1) crosses over the subclavian
artery and runs anteriorly to empty into the proximal subclavian vein.
Blood for the chest wall
is supplied by the intercostal arteries and the internal thoracic (internal
mammary) arteries. After originating from the aorta, the
posterior intercostal arteries cross the vertebral bodies and enter their
corresponding intercostal spaces, passing along the inferior border of the ribs
between the internal and external
intercostal muscles. The vessels are well protected posteriorly by the
subcostal groove. The internal thoracic arteries originate from the inferior
surface of the subclavian arteries and run downward, lateral to, and (for a
short distance) with the phrenic nerve, reaching the posterior surface
of the anterior chest wall. The arteries continue their downward course for
approximately 1/4 inch laterally to
the edges of the sternum, dividing just above the diaphragm into their two
terminal branches: the musculophrenic and superior epigastric
arteries. Along their course the internal thoracic arteries give rise to
branches to the thymus, mediastinum, and pericardium posteriorly;
to the perforating branches to the skin and subcutaneous tissues anteriorly;
and finally to the lateral
branches that pass along the rib cartilages and anastomose with the posterior
intercostal arteries. The veins of the thoracic wall correspond in their course
with the arteries. The 10 lower intercostal veins on the right enter the azygos
vein, and the upper two intercostal veins enter either the azygos or the brachiocephalic
(innominate) vein. The lower intercostal veins on the left side enter the hemiazygos
or accessory hemiazygos vein. The three left superior intercostal
veins enter the left brachiocephalic vein by a common stem, the left superior intercostal vein.
The chest wall receives
its nerve supply from the intercostal nerves, which accompany the intercostal
vessels.
Most of the thoracic
cavity is occupied by the two lungs, each of which is enclosed by its pleura.
Each pleura forms a closed sac invaginated by the lung so that part of it
covers (and is adherent to) the inner surface of the chest wall, the diaphragm,
and the mediastinum, known as the costal, the diaphragmatic, and
the mediastinal pleura, respectively, and collectively as the parietal
pleura (see Plate 1-2). That part of the mediastinal pleura that covers the
pericardium is called the pericardial pleura; the remainder (visceral
pleura) covers the lung. The virtual space between the visceral and parietal
pleurae contains a tiny amount of clear fluid. The pleural reflections (see
Plate 1-1), between the costal and diaphragmatic portions of the parietal
pleura, lie lower than the corresponding lower edge of the lung. The resulting
space normally is not completely filled by the lung, even on deep inspiration,
and is called the recessus costodiaphragmaticus.
 |
| MEDIASTINUM: CROSS SECTION |
The right lung consists
of three lobes the superior, middle, and inferior lobes and is somewhat larger
than the left lung, which has two the superior and inferior lobes (see
Plate 1-1). The smaller size of the left lung results from the eccentric
position of the heart, which encroaches on the left pleural cavity. The
two pleural cavities
almost meet behind the upper sternum, but the left costomediastinal
reflection deviates laterally below the fourth rib cartilage, exposing a small
triangular portion of the pericardium that is not covered by pleura. At the
same level, the anteroinferior portion of the left superior lobe recedes even
more, leaving a portion of the pericardial pleura that is not covered by lung tissue.
The central space between
the two pleural cavities is the mediastinum. The mediastinum is divided
arbitrarily into superior, anterior, middle, and posterior mediastina. The
shallow anterior mediastinum contains a portion of the left internal thoracic
vessels and the
vestigial transverse thoracic muscle. The superior mediastinum contains
the thymus gland (see Plate 1-2), which largely disappears by about age 12
years leaving a small pad of fat and areolar tissue, and the brachiocephalic
veins, which join each other on the right to form the superior vena cava (see
Plate 1-5). Posterior to the brachiocephalic veins, the phrenic and vagus
nerves descend from the neck. The phrenic nerves, accompanied by the pericardiacophrenic
vessels, run laterally, anterior to the lung roots and along the
pericardium, until they reach the diaphragm.
The aortic arch ascends
from the heart into the superior mediastinum, almost reaches the upper border
of the manubrium sterni, courses
obliquely backward and to the left over the left main bronchus, and
continues as the descending aorta downward, anteriorly, and slightly to
the left of the vertebral column. Originating from the convexity of the arch,
from the proximal to the distal position, are the brachiocephalic, left common
carotid, and subclavian arteries.
The right vagus nerve (see
Plate 1-5) passes between the subclavian artery and vein and gives off the
right recurrent nerve, which loops around the subclavian artery to ascend along
the trachea. The left vagus nerve runs between the subclavian vein and
the aortic arch, giving rise to the left recurrent nerve (see Plate
1-5), which similarly loops around the arch to ascend along the trachea.
The trachea descends from
the neck behind the aortic arch and bifurcates into right and left main bronchi
at the level of the sternal angle. Behind the trachea runs the normally
collapsed esophagus (see Plate 1-4), joined by the vagus nerves just beyond the
branching off of the recurrent nerves from the vagi. Behind the esophagus,
between the azygos vein and the descending aorta, the thoracic duct (see
Plate 1-2) ascends, coursing behind the aortic arch to enter the neck, where it
empties into the left subclavian vein.
Against the necks of the
ribs, the sympathetic trunks descend from the neck, first giving off the
greater thoracic splanchnic nerve (major splanchnic nerve) (see Plate
1-3) at about the level of the sixth rib and then the minor or lesser
and lowest thoracic splanchnic nerves.
The posterior mediastinum
is a shallow space containing the lower portions of the esophagus, vagus
nerves, descending aorta, azygos and hemiazygos veins, thoracic duct, and
sympathetic nerve chains. The remaining and largest part of the mediastinum,
the middle mediastinum, contains the pericardium, heart, lung roots, and
phrenic nerves.
 |
| PERICARDIAL SAC |
The pericardial cavity
is the third serous cavity contained in the chest, with the two pleural
cavities. The pericardial cavity is conical in shape, with the base of the cone
lying posteriorly to the right and the apex anteriorly to the left. It
completely invests the heart and the proximal portions of the great vessels. As
with the pleura, a visceral portion of the pericardium is distinguished
overlying the heart and proximal great vessels, usually called the epicardium,
as is a parietal portion. The inferior part of the parietal pericardium is
densely adherent to the middle tendinous part of the diaphragm. Most of the
lateral and anterior portions are contiguous but not normally adherent to the
pleura. A small triangular part of the anterior portion of the parietal
pericardium lies directly behind the sternum, separated only by areolar and fatty tissue (endothoracic
fascia) and the transverse thoracic muscle.
The great vessels enter
and leave the pericardial cavity at its base. A curved, transversely running
passageway between the arterial and venous poles of the heart is called the transverse
pericardial sinus. Posteriorly, a blind recess of the pericardial cavity is
bordered by the pericardial reflection between the pulmonary veins and inferior vena cava, called the oblique
pericardial sinus. Small recesses exist between the superior and inferior
pulmonary veins on each side and behind the fold of the left vena cava (ligament
of Marshall), a small crease of pericardium running from the left aspect of the
pulmonary trunk to the left atrium, between the neck of the left auricle and
the left pulmonary veins. The left vena cava fold contains the vestigial
remains of the left common
cardinal vein.
