Thorax

3
Thorax

Regional anatomy

Pectoral region

Breast

Muscles of the pectoral region

Thoracic wall

Superior thoracic aperture

Inferior thoracic aperture

Skeletal framework

Intercostal spaces

Diaphragm

Venous drainage

Innervation

Movements of the thoracic wall and diaphragm during breathing

Pleural cavities

Pleura

Lungs

Mediastinum

Middle mediastinum

Superior mediastinum

Posterior mediastinum

Anterior mediastinum

 

Regional anatomy

The thorax is an irregularly shaped cylinder with a narrow opening (superior thoracic aperture) superiorly and a relatively large opening (inferior thoracic aperture) inferiorly (Fig. 3.1). The thorax consists of:

a wall,

two pleural cavities,

the lungs, and

the mediastinum.

Fig. 3.1 Thoracic wall and cavity.

The thorax:

houses and protects the heart, lungs, and great vessels,

acts as a conduit for structures passing between the neck and the abdomen,

plays a principal role in breathing, and

provides support for the upper limbs.

The thorax also provides support for the upper limb. Muscles anchored to the anterior thoracic wall provide some of this support, and together with their associated connective tissues, nerves, and vessels, and the overlying skin and superficial fascia, define the pectoral region.

PECTORAL REGION

The pectoral region is external to the anterior thoracic wall and anchors the upper limb to the trunk. It consists of:

a superficial compartment containing skin, superficial fascia, and breasts; and

a deep compartment containing muscles and associated structures.

Breast

The breasts consist of mammary glands, associated skin, and connective tissues. The mammary glands are modified sweat glands in the superficial fascia anterior to the pectoral muscles and the anterior thoracic wall (Fig. 3.2).

Fig. 3.2 Breasts.

The mammary glands consist of ducts and associated secretory lobules. These converge, forming 15 to 20 lactiferous ducts, which open independently onto the nipple. The nipple is surrounded by a circular pigmented area of skin, the areola (Fig. 3.2).

A well-developed, connective tissue stroma surrounds the ducts and lobules of the mammary gland. In certain regions, this stroma condenses, forming well-defined ligaments, the suspensory ligaments of breast, which are continuous with the dermis of the skin and support the breast.

In nonlactating women, the predominant component of the breasts is fat, whereas glandular tissue is more abundant in lactating women.

The breast lies on the deep fascia of the pectoralis major muscle and other surrounding muscles. A layer of loose connective tissue (the retromammary space) separates the breast from the deep fascia and provides some degree of movement over underlying structures.

The base, or attached surface, of each breast extends vertically from ribs 2 to 6, and transversely from the sternum to as far laterally as the midaxillary line.

Arterial supply

The breast is related to the thoracic wall and to structures associated with the upper limb; therefore, vascular supply and drainage can occur by multiple routes (Fig. 3.2):

laterally, vessels from the axillary artery—superior thoracic, thoracoacromial, lateral thoracic, and subscapular arteries;

medially, branches from the internal thoracic artery; and

from the second to fourth intercostal arteries via branches that perforate the thoracic wall and overlying muscle.

Venous drainage

Veins draining the breast parallel the arteries and ultimately drain into the axillary, internal thoracic, and intercostal veins.

Innervation

Innervation of the breast is via anterior and lateral cutaneous branches of the second to sixth intercostal nerves. The nipple is innervated by the fourth intercostal nerve.

Lymphatic drainage

Lymphatic drainage of the breast is as follows:

Approximately 75% is via lymphatic vessels that drain laterally and superiorly into axillary nodes (see Fig. 3.2).

Most of the remaining drainage is into parasternal nodes deep to the anterior thoracic wall and associated with the internal thoracic artery.

Some drainage may occur via lymphatic vessels that follow the lateral branches of posterior intercostal arteries and connect with intercostal nodes situated near the heads and necks of ribs.

Axillary nodes drain into the subclavian trunks, parasternal nodes drain into the bronchomediastinal trunks, and intercostal nodes drain either into the thoracic duct or into the bronchomediastinal trunks.

Breast in men

The breast in men is rudimentary and consists only of small ducts, often composed of cords of cells, which normally do not extend beyond the areola. Breast cancer can occur in men.

Muscles of the pectoral region

Each pectoral region contains the pectoralis major, pectoralis minor, and subclavius muscles (Table 3.1, Fig. 3.4). All originate from the anterior thoracic wall and insert into bones of the upper limb.

Table 3.1 Muscles of the pectoral region

Fig. 3.4 Muscles and fascia of the pectoral region.

A continuous layer of deep fascia, clavipectoral fascia, encloses the subclavius and pectoralis minor and attaches to the clavicle above and to the floor of the axilla below.

The muscles of the pectoral region form the anterior wall of the axilla, a region between the upper limb and the neck through which all major structures pass.

THORACIC WALL

The thoracic wall consists of skeletal elements and muscles (see Fig. 3.1):

Posteriorly, it is made up of 12 thoracic vertebrae and their intervening intervertebral discs.

Laterally, the wall is formed by ribs (12 on each side) and three layers of flat muscles, which span the intercostal spaces between adjacent ribs, move the ribs, and provide support for the intercostal spaces.

Anteriorly, the wall is made up of the sternum, which consists of the manubrium of sternum, body of sternum, and xiphoid process.

The thorax wall extends between:

the superior thoracic aperture bordered by vertebra TI, rib I, and the manubrium of sternum; and

the inferior thoracic aperture bordered by vertebra TXII, rib XII, the end of rib XI, the costal margin, and the xiphoid process of the sternum.

Superior thoracic aperture

The superior thoracic aperture (Fig. 3.5; also see Fig. 3.1) consists of:

the body of vertebra TI posteriorly,

the medial margin of rib I on each side, and

the manubrium anteriorly.

Fig. 3.5 Superior thoracic aperture.

The superior margin of the manubrium is in approximately the same horizontal plane as the intervertebral disc between vertebrae TII and TIII.

The first ribs slope inferiorly from their posterior articulation with vertebra TI to their anterior attachment to the manubrium. Consequently, the plane of the superior thoracic aperture is at an oblique angle, facing somewhat anteriorly (Fig. 3.5).

At the superior thoracic aperture, the superior aspects of the pleural cavities, which surround the lungs, lie on either side of the entrance to the mediastinum (Fig. 3.6).

Fig. 3.6 Superior thoracic aperture and axillary inlet.

Structures that pass between the upper limb and thorax pass over rib I and the superior part of the pleural cavity as they enter and leave the mediastinum (Fig. 3.6). Structures that pass between the neck and head and the thorax pass more vertically through the superior thoracic aperture (Fig. 3.5).

Inferior thoracic aperture

The inferior thoracic aperture is large and expandable, and bone, cartilage, and ligaments form its margin (Fig. 3.7). It is closed by the diaphragm (3.7B), and structures passing between the abdomen and thorax pierce or pass posteriorly to this structure. Skeletal elements of the inferior thoracic aperture are:

the body of vertebra TXII posteriorly,

rib XII and the distal end of rib XI posterolaterally,

the distal cartilaginous ends of ribs VII to X, which unite to form the costal margin anterolaterally, and

the xiphoid process anteriorly.

Fig. 3.7 A. Inferior thoracic aperture. B. Diaphragm.

Joints between the costal margin and sternum lie roughly in the same horizontal plane as the intervertebral disc between vertebrae TIX and TX. Thus, the posterior margin of the inferior thoracic aperture is inferior to the anterior margin.

Skeletal framework

The skeletal elements of the thoracic wall consist of thoracic vertebrae, intervertebral discs, ribs, and sternum.

Thoracic vertebrae

There are 12 thoracic vertebrae, each of which is characterized by articulations with the ribs.

A typical thoracic vertebra

A typical thoracic vertebra has a heart-shaped vertebral body, with roughly equal dimensions in the transverse and anteroposterior directions, and a long spinous process (Fig. 3.8). The vertebral foramen is generally circular and the laminae are broad and overlap with those of the vertebra below. The superior articular processes are flat, with their articular surfaces facing almost directly posteriorly, whereas the inferior articular processes project from the laminae and their articular facets face anteriorly. The transverse processes are club shaped and project posterolaterally.

Fig. 3.8 Typical thoracic vertebra.

Articulation with ribs

A typical thoracic vertebra has three sites for articulations with ribs on each side (Fig. 3.8).

Two demifacets (i.e., partial facets) located on the superior and inferior aspects of the body for articulation with corresponding sites on the heads of adjacent ribs. The superior costal facet articulates with part of the head of its own rib, and the inferior costal facet articulates with part of the head of the rib below.

An oval facet (transverse costal facet) at the end of the transverse process articulates with the tubercle of its own rib.

Not all vertebrae articulate with ribs in the same fashion (Fig. 3.9):

The superior costal facets on the body of vertebra TI are complete and articulate with a single facet on the head of its own rib—in other words, the head of rib I does not articulate with vertebra CVII.

Similarly, vertebra TX (and often TIX) articulates only with its own ribs and therefore lacks inferior demifacets on the body.

Vertebrae TXI and TXII articulate only with the heads of their own ribs—they lack transverse costal facets and have only a single complete facet on each side of their bodies.

Fig. 3.9 Thoracic vertebrae.

Ribs

There are 12 pairs of ribs, each terminating anteriorly in a costal cartilage (Fig. 3.10).

Fig. 3.10 Ribs.

Although all ribs articulate with the vertebral column, only the costal cartilages of the upper seven ribs, known as true ribs, articulate directly with the sternum. The remaining five pairs of ribs are false ribs:

The costal cartilages of ribs VIII to X articulate anteriorly with the costal cartilages of the ribs above.

Ribs XI and XII have no anterior connection with other ribs or with the sternum and are often called floating ribs.

A typical rib consists of a curved shaft with anterior and posterior ends (Fig. 3.11). The anterior end is continuous with its costal cartilage. The posterior end articulates with the vertebral column and is characterized by a head, neck, and tubercle.

Fig. 3.11 A typical rib. A. Anterior view. B. Posterior view of proximal end of rib.

The head is somewhat expanded and typically presents two articular surfaces separated by a crest. The smaller superior surface articulates with the inferior costal facet on the body of the vertebra above, whereas the larger inferior facet articulates with the superior costal facet of its own vertebra.

The neck is a short flat region of bone that separates the head from the tubercle.

The tubercle projects posteriorly from the junction of the neck with the shaft and consists of two regions, an articular part and a nonarticular part:

The articular part is medial and has an oval facet for articulation with a corresponding facet on the transverse process of the associated vertebra.

The raised nonarticular part is roughened by ligament attachments.

The shaft is generally thin and flat with internal and external surfaces.

The superior margin is smooth and rounded, whereas the inferior margin is sharp. The shaft bends forward just laterally to the tubercle at a site termed the angle. It also has a gentle twist around its longitudinal axis so that the external surface of the anterior part of the shaft faces somewhat superiorly relative to the posterior part. The inferior margin of the internal surface is marked by a distinct costal groove.

Distinct features of upper and lower ribs

The upper and lower ribs have distinct features (Fig. 3.12).

Fig. 3.12 Atypical ribs.

Rib I

Rib I is flat in the horizontal plane and has broad superior and inferior surfaces. From its articulation with vertebra TI, it slopes inferiorly to its attachment to the manubrium of sternum. The head articulates with the body of vertebra TI and has only one articular surface. The tubercle has a facet for articulation with the transverse process. The superior surface of the rib is characterized by a distinct tubercle, the scalene tubercle, which separates two smooth grooves that cross the rib approximately midway along the shaft. The anterior groove is caused by the subclavian vein, and the posterior groove is caused by the subclavian artery. Anterior and posterior to these grooves, the shaft is roughened by muscle and ligament attachments.

Rib II

Rib II, like rib I, is flat but twice as long. It articulates with the vertebral column in a way typical of most ribs.

Rib X

The head of rib X has a single facet for articulation with its own vertebra.

Ribs XI and XII

Ribs XI and XII articulate only with the bodies of their own vertebrae and have no tubercles or necks. Both ribs are short, have little curve, and are pointed anteriorly.

Sternum

The adult sternum consists of three major elements: the broad and superiorly positioned manubrium of sternum, the narrow and longitudinally oriented body of sternum, and the small and inferiorly positioned xiphoid process (Fig. 3.13).

Fig. 3.13 Sternum.

Manubrium of sternum

The manubrium of sternum forms part of the bony framework of the neck and the thorax.

The superior surface of the manubrium is expanded laterally and bears a distinct and palpable notch, the jugular notch (suprasternal notch), in the midline (Fig. 3.13). On either side of this notch is a large oval fossa for articulation with the clavicle. Immediately inferior to this fossa, on each lateral surface of the manubrium, is a facet for the attachment of the first costal cartilage. At the lower end of the lateral border is a demifacet for articulation with the upper half of the anterior end of the second costal cartilage.

Body of the sternum

The body of the sternum is flat (Fig. 3.13).

The anterior surface of the body of the sternum is often marked by transverse ridges that represent lines of fusion between the segmental elements called sternebrae, from which this part of the sternum arises embryologically.

The lateral margins of the body of the sternum have articular facets for costal cartilages. Superiorly, each lateral margin has a demifacet for articulation with the inferior aspect of the second costal cartilage. Inferior to this demifacet are four facets for articulation with the costal cartilages of ribs III to VI.

At the inferior end of the body of the sternum is a demifacet for articulation with the upper demifacet on the seventh costal cartilage. The inferior end of the body of the sternum is attached to the xiphoid process.

Xiphoid process

The xiphoid process is the smallest part of the sternum (Fig. 3.13). Its shape is variable: it may be wide, thin, pointed, bifid, curved, or perforated. It begins as a cartilaginous structure, which becomes ossified in the adult. On each side of its upper lateral margin is a demifacet for articulation with the inferior end of the seventh costal cartilage.

Joints

Costovertebral joints

A typical rib articulates with:

the bodies of adjacent vertebrae, forming a joint with the head of the rib; and

the transverse process of its related vertebra, forming a costotransverse joint (Fig. 3.14).

Fig. 3.14 Costovertebral joints.

Together, the costovertebral joints and related ligaments allow the necks of the ribs either to rotate around their longitudinal axes, which occurs mainly in the upper ribs, or to ascend and descend relative to the vertebral column, which occurs mainly in the lower ribs. The combined movements of all of the ribs on the vertebral column are essential for altering the volume of the thoracic cavity during breathing.

Joint with head of rib

The two facets on the head of the rib articulate with the superior facet on the body of its own vertebra and with the inferior facet on the body of the vertebra above (Fig. 3.14). This joint is divided into two synovial compartments by an intra-articular ligament, which attaches the crest to the adjacent intervertebral disc and separates the two articular surfaces on the head of the rib. The two synovial compartments and the intervening ligament are surrounded by a single joint capsule attached to the outer margins of the combined articular surfaces of the head and vertebral column.

Costotransverse joints

Costotransverse joints are synovial joints between the tubercle of a rib and the transverse process of the related vertebra (Fig. 3.14). The joint is stabilized by two strong extracapsular ligaments that span the space between the transverse process and the rib on the medial and lateral sides of the joint:

The costotransverse ligament is medial to the joint and attaches the neck of the rib to the transverse process.

The lateral costotransverse ligament is lateral to the joint and attaches the tip of the transverse process to the roughened nonarticular part of the tubercle of the rib.

A third ligament, the superior costotransverse ligament, attaches the superior surface of the neck of the rib to the transverse process of the vertebra above.

Slight gliding movements occur at the costotransverse joints.

Sternocostal joints

The sternocostal joints are joints between the upper seven costal cartilages and the sternum (Fig. 3.15).

Fig. 3.15 Sternocostal joints.

The joint between rib I and the manubrium is not synovial and consists of a fibrocartilaginous connection between the manubrium and the costal cartilage. The joints between rib II through VII and the sternum are synovial and have thin capsules reinforced by surrounding sternocostal ligaments.

The joint between the second costal cartilage and the sternum is divided into two compartments by an intra-articular ligament. This ligament attaches the second costal cartilage to the junction of the manubrium and the body of the sternum.

Interchondral joints

Interchondral joints occur between the costal cartilages of adjacent ribs (Fig. 3.15), mainly between the costal cartilages of ribs VII to X, but may also involve the costal cartilages of ribs V and VI.

Interchondral joints provide indirect anchorage to the sternum and contribute to the formation of a smooth inferior costal margin. They are usually synovial, and the thin fibrous capsules are reinforced by interchondral ligaments.

Manubriosternal and xiphisternal joints

The joints between the manubrium and body of sternum and between the body of sternum and the xiphoid process are usually symphyses (Fig. 3.15). Only slight angular movements occur between the manubrium and body of sternum during respiration. The joint between the body of sternum and the xiphoid process often becomes ossified with age.

Additionally, the sternal angle is on a horizontal plane that passes through the intervertebral disc between vertebrae TIV and TV (Fig. 3.16). This plane separates the superior mediastinum from the inferior mediastinum and marks the superior border of the pericardium. It also separates the end of the ascending aorta from the beginning of the arch of the aorta, the end of the arch of the aorta from the beginning of the thoracic aorta, and passes through the bifurcation of the trachea just superior to the pulmonary trunk.

Fig. 3.16 Vertebral level TIV/V.

Intercostal spaces

Intercostal spaces lie between adjacent ribs and are filled by intercostal muscles (Fig. 3.19).

Fig. 3.19 Intercostal space. A. Anterolateral view. B. Details of an intercostal space and relationships.

Intercostal nerves and associated major arteries and veins lie in the costal groove along the inferior margin of the superior rib and pass in the plane between the inner two layers of muscles (Fig. 3.19). In each space, the vein is the most superior structure and is therefore highest in the costal groove. The artery is inferior to the vein, and the nerve is inferior to the artery and often not protected by the groove. Therefore, the nerve is the structure most at risk when objects perforate the upper aspect of an intercostal space.

Deep to the intercostal spaces and ribs, and separating these structures from the underlying pleura, is a layer of loose connective tissue called endothoracic fascia, which contains variable amounts of fat (Fig. 3.19B).

Muscles

Muscles of the thoracic wall include those that fill and support the intercostal spaces (external, internal, and innermost intercostal muscles [Table 3.2, Figs. 3.19, 3.20]); those that cross several ribs between the costal attachments (subcostal muscles [Table 3.2, Fig. 3.21A]); and those that pass between the sternum and the ribs (transversus thoracis muscles [Table 3.2, Fig. 3.21B]).

Table 3.2 Muscles of the thoracic wall

Fig. 3.20 Intercostal muscles.

Fig. 3.21 A. Subcostal muscles. B. Transversus thoracis muscles.

The muscles of the thoracic wall, together with muscles between the vertebrae and ribs posteriorly (i.e., the levatores costarum, and serratus posterior superior and serratus posterior inferior muscles), alter the position of the ribs and sternum changing the thoracic volume during breathing. They also reinforce the thoracic wall.

Intercostal muscles

The intercostal muscles are three flat muscles found in each intercostal space that pass between adjacent ribs (Figs. 3.19B, 3.20). Individual muscles in this group are named according to their positions:

The external intercostal muscles are the most superficial and their fibers pass from the rib above anteroinferiorly to the rib below (these muscles extend around the thoracic wall from the regions of the tubercles of the ribs to the costal cartilages, where each muscle continues as a thin connective tissue aponeurosis termed the external intercostal membrane).

The internal intercostal muscles are sandwiched between the external and innermost muscles, and their fibers run in the opposite direction to those of the external intercostal muscles (the internal intercostal muscles extend around the thoracic wall from the sternum to the angles of the ribs, where each muscle continues as a thin connective tissue aponeurosis termed the internal intercostal membrane).

The innermost intercostal muscles are the deepest of the three muscles and their fibers course in a similar direction as the internal intercostal muscles.

The intercostal muscles are innervated by the related intercostal nerves. As a group, the intercostal muscles provide structural support for the intercostal spaces during breathing. They can also move the ribs.

Arterial supply

Vessels that supply the thoracic wall consist mainly of posterior and anterior intercostal arteries, which pass around the wall between adjacent ribs in intercostal spaces (Fig. 3.19A). These arteries originate from the aorta and internal thoracic arteries, which in turn arise from the subclavian arteries in the root of the neck. Together, the intercostal arteries form a basket-like pattern of vascular supply around the thoracic wall.

Posterior intercostal arteries

Posterior intercostal arteries originate from vessels associated with the posterior thoracic wall. The upper two posterior intercostal arteries on each side come from the supreme intercostal artery, which descends into the thorax as a branch of the costocervical trunk in the neck. The costocervical trunk is a posterior branch of the subclavian artery (Fig. 3.22).

Fig. 3.22 Arteries of the thoracic wall.

The remaining nine pairs of posterior intercostal arteries arise from the posterior surface of the thoracic aorta. Because the aorta is on the left side of the vertebral column, those posterior intercostal vessels passing to the right side of the thoracic wall cross the midline anterior to the bodies of the vertebrae and therefore are longer than the corresponding vessels on the left.

In addition to having numerous branches that supply various components of the wall, the posterior intercostal arteries have branches that accompany lateral cutaneous branches of the intercostal nerves to the superficial regions.

Anterior intercostal arteries

The anterior intercostal arteries originate directly or indirectly as lateral branches from the internal thoracic arteries (Fig. 3.22).

Each internal thoracic artery arises as a major branch of the subclavian artery in the neck. It passes anteriorly over the cervical dome of pleura and descends vertically through the superior thoracic aperture and along the deep aspect of the anterior thoracic wall. On each side, the internal thoracic artery lies posterior to the costal cartilages of the upper six ribs and about 1 cm lateral to the sternum. At approximately the level of the sixth intercostal space, it divides into two terminal branches (Fig. 3.22):

the superior epigastric artery, which continues inferiorly into the anterior abdominal wall;

the musculophrenic artery, which passes along the costal margin, goes through the diaphragm, and ends near the last intercostal space.

Anterior intercostal arteries that supply the upper six intercostal spaces arise as lateral branches from the internal thoracic artery, whereas those supplying the lower spaces arise from the musculophrenic artery.

In each intercostal space, the anterior intercostal arteries usually have two branches:

One passes below the margin of the upper rib.

The other passes above the margin of the lower rib and meets a collateral branch of the posterior intercostal artery.

The distributions of the anterior and posterior intercostal vessels overlap and can develop anastomotic connections. The anterior intercostal arteries are generally smaller than the posterior vessels.

In addition to anterior intercostal arteries and a number of other branches, the internal thoracic arteries give rise to perforating branches that pass directly forward between the costal cartilages to supply structures external to the thoracic wall. These vessels travel with the anterior cutaneous branches of the intercostal nerves.

Venous drainage

Venous drainage from the thoracic wall generally parallels the pattern of arterial supply (Fig. 3.23).

Fig. 3.23 Veins of the thoracic wall.

Centrally, the intercostal veins ultimately drain into the azygos system of veins or into internal thoracic veins, which connect with the brachiocephalic veins in the neck.

Often the upper posterior intercostal veins on the left side come together and form the left superior intercostal vein, which empties into the left brachiocephalic vein.

Similarly, the upper posterior intercostal veins on the right side may come together and form the right superior intercostal vein, which empties into the azygos vein.

Lymphatic drainage

Lymphatic vessels of the thoracic wall drain mainly into lymph nodes associated with the internal thoracic arteries (parasternal nodes), with the heads and necks of ribs (intercostal nodes), and with the diaphragm (diaphragmatic nodes) (Fig. 3.24). Diaphragmatic nodes are posterior to the xiphoid and at sites where the phrenic nerves penetrate the diaphragm. They also occur in regions where the diaphragm is attached to the vertebral column.

Fig. 3.24 Major lymphatic vessels and nodes of the thoracic wall.

Parasternal nodes drain into bronchomediastinal trunks. Intercostal nodes in the upper thorax also drain into bronchomediastinal trunks, whereas intercostal nodes in the lower thorax drain into the thoracic duct.

Nodes associated with the diaphragm interconnect with parasternal, prevertebral, juxtaesophageal nodes, brachiocephalic (anterior to the brachiocephalic veins in the superior mediastinum), and lateral aortic/lumbar nodes (in the abdomen).

Superficial regions of the thoracic wall drain mainly into axillary lymph nodes in the axilla or parasternal nodes.

Innervation

Intercostal nerves

Innervation of the thoracic wall is mainly by the intercostal nerves, which are the anterior rami of spinal nerves T1 to T11 and lie in the intercostal spaces between adjacent ribs. The anterior ramus of spinal nerve T12 (the subcostal nerve) is inferior to rib XII (Fig. 3.25).

Fig. 3.25 Intercostal nerves.

A typical intercostal nerve passes laterally around the thoracic wall in an intercostal space. The largest of the branches is the lateral cutaneous branch, which pierces the lateral thoracic wall and divides into an anterior branch and a posterior branch that innervate the overlying skin.

The intercostal nerves end as anterior cutaneous branches, which emerge either parasternally, between adjacent costal cartilages, or laterally to the midline, on the anterior abdominal wall, to supply the skin.

In addition to these major branches, small collateral branches can be found in the intercostal space running along the superior border of the lower rib.

In the thorax, the intercostal nerves carry:

somatic motor innervation to the muscles of the thoracic wall (intercostal, subcostal, and transversus thoracis muscles),

somatic sensory innervation from the skin and parietal pleura, and

postganglionic sympathetic fibers to the periphery.

Sensory innervation from the skin overlying the upper thoracic wall is supplied by cutaneous branches (supraclavicular nerves), which descend from the cervical plexus in the neck.

In addition to innervating the thoracic wall, intercostal nerves innervate other regions:

The anterior ramus of T1 contributes to the brachial plexus.

The lateral cutaneous branch of the second intercostal nerve (the intercostobrachial nerve) contributes to cutaneous innervation of the medial surface of the upper arm.

The lower intercostal nerves supply muscles, skin, and parietal peritoneum of the abdominal wall.

DIAPHRAGM

The diaphragm is a thin musculotendinous structure that fills the inferior thoracic aperture and separates the thoracic cavity from the abdominal cavity (Figs. 3.26, 3.27, and see Chapter 4). It is attached peripherally to the:

xiphoid process of the sternum,

costal margin of the thoracic wall,

ends of ribs XI and XII,

ligaments that span across structures of the posterior abdominal wall, and

vertebrae of the lumbar region.

Fig. 3.26 Major structures passing between abdomen and thorax.

Fig. 3.27 Diaphragm.

From these peripheral attachments, muscle fibers converge to join the central tendon. The pericardium is attached to the middle part of the central tendon.

In the median sagittal plane, the diaphragm slopes inferiorly from its anterior attachment to the xiphoid, approximately at vertebral level TVIII/IX, to its posterior attachment to the median arcuate ligament, crossing anteriorly to the aorta at approximately vertebral level TXII.

Structures traveling between the thorax and abdomen pass through the diaphragm or between the diaphragm and its peripheral attachments (Figs. 3.26, 3.27):

The inferior vena cava passes through the central tendon at approximately vertebral level TVIII.

The esophagus passes through the muscular part of the diaphragm, just to the left of midline, approximately at vertebral level TX.

The vagus nerves pass through the diaphragm with the esophagus.

The aorta passes behind the posterior attachment of the diaphragm at vertebral level TXII.

The thoracic duct passes behind the diaphragm with the aorta.

The azygos and hemiazygos veins may also pass through the aortic hiatus or through the crura of the diaphragm.

Other structures outside the posterior attachments of the diaphragm lateral to the aortic hiatus include the sympathetic trunks and the least splanchnic nerves. The greater and lesser splanchnic nerves penetrate the crura.

Arterial supply

The arterial supply to the diaphragm is from vessels that arise superiorly and inferiorly to it (see Fig. 3.27). From above, pericardiacophrenic and musculophrenic arteries supply the diaphragm. These vessels are branches of the internal thoracic arteries. Superior phrenic arteries, which arise directly from lower parts of the thoracic aorta, and small branches from intercostal arteries contribute to the supply. The largest arteries supplying the diaphragm arise from below it. These arteries are the inferior phrenic arteries, which branch directly from the abdominal aorta.

Venous drainage

Venous drainage of the diaphragm is by veins that generally parallel the arteries. The veins drain into:

the brachiocephalic veins in the neck,

the azygos system of veins, or

the abdominal veins (left suprarenal vein and inferior vena cava).

Innervation

The diaphragm is innervated by the phrenic nerves (C3, C4, and C5), which penetrate the diaphragm and innervate it from its abdominal surface (Fig. 3.27).

Contraction of the domes of the diaphragm flattens the diaphragm, increasing thoracic volume. Movements of the diaphragm are essential for normal breathing.

MOVEMENTS OF THE THORACIC WALL AND DIAPHRAGM DURING BREATHING

During breathing, the dimensions of the thorax change in the vertical, lateral, and anteroposterior directions. Elevation and depression of the diaphragm significantly alter the vertical dimensions of the thorax. Depression results when the muscle fibers of the diaphragm contract. Elevation occurs when the diaphragm relaxes.

Changes in the anteroposterior and lateral dimensions result from elevation and depression of the ribs (Fig. 3.28). The posterior ends of the ribs articulate with the vertebral column, whereas the anterior ends of most ribs articulate with the sternum or adjacent ribs.

Fig. 3.28 Movement of thoracic wall during breathing. A. Pump handle movement of ribs and sternum. B. Bucket handle movement of ribs.

Because the anterior ends of the ribs are inferior to the posterior ends, when the ribs are elevated, they move the sternum upward and forward. Also, the angle between the body of the sternum and the manubrium may become slightly less acute. When the ribs are depressed, the sternum moves downward and backward. This “pump handle” movement changes the dimensions of the thorax in the anteroposterior direction (Fig. 3.28A).

As well as the anterior ends of the ribs being lower than the posterior ends, the middles of the shafts tend to be lower than the two ends. When the shafts are elevated, the middles of the shafts move laterally. This “bucket handle” movement increases the lateral dimensions of the thorax (Fig. 3.28B).

PLEURAL CAVITIES

Two pleural cavities, one on either side of the mediastinum, surround the lungs (Figs. 3.29, 3.30):

superiorly, they extend above rib I into the root of the neck;

inferiorly, they extend to a level just above the costal margin; and

the medial wall of each pleural cavity is the mediastinum (Fig. 3.31).

Fig. 3.29 Pleural cavities.

Fig. 3.30 Pleural cavities.

Fig. 3.31 Cross-section of the thorax showing the position of the mediastinum.

Pleura

Each pleural cavity is lined by a single layer of flat cells, mesothelium, and an associated layer of supporting connective tissue; together, they form the pleura.

The pleura is divided into two major types, based on location:

pleura associated with the walls of a pleural cavity is parietal pleura (Fig. 3.30); and

pleura that reflects from the medial wall and onto the surface of the lung is visceral pleura (Fig. 3.30), which adheres to and covers the lung.

Each pleural cavity is the potential space enclosed between the visceral and parietal pleurae. They normally contain only a very thin layer of serous fluid. As a result, the surface of the lung, which is covered by visceral pleura, directly opposes and freely slides over the parietal pleura attached to the wall.

Parietal pleura

The names given to the parietal pleura correspond to the parts of the wall with which they are associated (Fig. 3.32):

Pleura related to the ribs and intercostal spaces is termed the costal part.

Pleura covering the diaphragm is the diaphragmatic part.

Pleura covering the mediastinum is the mediastinal part.

The dome-shaped layer of parietal pleura lining the cervical extension of the pleural cavity is cervical pleura (dome of pleura or pleural cupola).

Fig. 3.32 Parietal pleura.

Covering the superior surface of the cervical pleura is a distinct domelike layer of fascia, the suprapleural membrane (Fig. 3.32). This connective tissue membrane is attached laterally to the medial margin of the first rib and behind to the transverse process of vertebra CVII. Superiorly, the membrane receives muscle fibers from some of the deep muscles in the neck (scalene muscles) that function to keep the membrane taught. The suprapleural membrane provides apical support for the pleural cavity in the root of the neck.

In the region of vertebrae TV to TVII, the mediastinal pleura reflects off the mediastinum as a tubular, sleevelike covering for structures (i.e., airway, vessels, nerves, lymphatics) that pass between the lung and mediastinum. This sleevelike covering, and the structures it contains, forms the root of the lung. The root joins the medial surface of the lung at an area referred to as the hilum of lung. Here, the mediastinal pleura is continuous with the visceral pleura.

Peripheral reflections

The peripheral reflections of parietal pleura mark the extent of the pleural cavities (Fig. 3.33).

Fig. 3.33 Pleural reflections.

Superiorly, the pleural cavity can project as much as 3 to 4 cm above the first costal cartilage, but does not extend above the neck of rib I. This limitation is caused by the inferior slope of rib I to its articulation with the manubrium.

Anteriorly, the pleural cavities approach each other posterior to the upper part of the sternum. However, posterior to the lower part of the sternum, the parietal pleura does not come as close to the midline on the left side as it does on the right because the middle mediastinum, containing the pericardium and heart, bulges to the left.

Inferiorly, the costal pleura reflects onto the diaphragm above the costal margin. In the midclavicular line, the pleural cavity extends inferiorly to approximately rib VIII (Fig. 3.34). In the midaxillary line, it extends to rib X. From this point, the inferior margin courses somewhat horizontally, crossing ribs XI and XII to reach vertebra TXII. From the midclavicular line to the vertebral column, the inferior boundary of the pleura can be approximated by a line that runs between the rib VIII, rib X, and vertebra TXII.

Fig. 3.34 Parietal pleural reflections and recesses.

Visceral pleura

The visceral pleura is continuous with the parietal pleura at the hilum of each lung where structures enter and leave the organ. The visceral pleura is firmly attached to the surface of the lung, including both opposed surfaces of the fissures that divide the lungs into lobes.

Costomediastinal recesses

Anteriorly, a costomediastinal recess occurs on each side where the costal pleura is opposed to the mediastinal pleura. The largest is on the left side in the region overlying the heart (Fig. 3.34).

Costodiaphragmatic recesses

The largest and clinically most important recesses are the costodiaphragmatic recesses, which occur in each pleural cavity between the costal pleura and diaphragmatic pleura (Fig. 3.34). The costodiaphragmatic recesses are the regions between the inferior margin of the lungs and inferior margin of the pleural cavities. They are deepest after forced expiration and shallowest after forced inspiration.

During quiet respiration, the inferior margin of the lung crosses rib VI in the midclavicular line, rib VIII in the midaxillary line, and then courses somewhat horizontally to reach the vertebral column at vertebral level TX. From the midclavicular line and around the thoracic wall to the vertebral column, the inferior margin of the lung can be approximated by a line running between rib VI, rib VIII, and vertebra TX. The inferior margin of the pleural cavity at the same points is rib VIII, rib X, and vertebra TXII. The costodiaphragmatic recess is the region between the two margins.

During expiration, the inferior margin of the lung rises and the costodiaphragmatic recess becomes larger.

Lungs

The two lungs are organs of respiration and lie on either side of the mediastinum surrounded by the right and left pleural cavities. Air enters and leaves the lungs via the main bronchi, which are branches of the trachea.

The pulmonary arteries deliver deoxygenated blood to the lungs from the right ventricle of the heart. Oxygenated blood returns to the left atrium via the pulmonary veins.

The right lung is normally a little larger than the left lung because the middle mediastinum, containing the heart, bulges more to the left than to the right.

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