Chapter 2. Anterior Thoracic Wall

Big Picture

The clavicle and parts of the thoracic cage provide prominent surface landmarks. Directional terms are used to help orient the reader to the thorax. The cutaneous innervation of the anterior portion of the thoracic wall is from lateral and anterior cutaneous nerves via ventral rami, forming a segmental dermatomal pattern.

Bony Landmarks

Palpable bony landmarks and reference lines are important to use for anatomic orientation as well as a guide to locate deep structures. You should become familiar with the following structures of the anterior portion of the thoracic wall (Figure 2-1A; Table 2-1):

• Clavicles. Course transversely along the superior portion of the chest wall from the manubrium to the acromion of the scapula and, therefore, are easily palpable. However, the clavicle, overly rib 1, which makes rib 1 impalpable.
• Jugular notch of the sternum. Located between the medial ends of the clavicles.
• Sternal angle (of Louis). Articulation between the manubrium and sternal body. The sternal angle serves as the location for the articulation of rib 2 with the sternum at the T4–T5 vertebral level. The sternal angle is often visible and palpable and serves as an important surface landmark for several underlying structures.
• Xiphoid process. An inferior pointed projection of the sternal body. The xiphoid process lies at the level of the T9 vertebral body.
• Costal margins. Formed by costal cartilages 7–10.

Directional Terms

The following imaginary vertical lines provide anatomic and clinical descriptions for orientation (Figure 2-1A):

• Midsternal line. Dropped through the center of the sternum. It is a component of the median plane.
• Midclavicular line. Dropped through the middle of the clavicle, just medial to the nipple.
• Anterior axillary line. Dropped through the anterior axillary fold formed by the pectoralis major muscle.
• Midaxillary line. Dropped through the middle of the axilla, along the lateral border of the thoracic wall. The midaxillary line is bounded by the anterior axillary fold (pectoralis major muscle) and the posterior axillary fold (latissimus dorsi muscle).
• Posterior axillary line. Dropped through the posterior axillary fold formed by the latissimus dorsi muscle.

Cutaneous Innervation

Lateral and anterior cutaneous branches of the intercostal nerves innervate the skin of the thorax in a segmental pattern. The following notable landmarks are helpful to identify the following dermatomes (Figure 2-1B; Table 2-2):

• Nipple–T4 dermatome
• Xiphoid process–T6 dermatome
• Umbillicus–T10 dermatome

Big Picture

The breast contains the mammary gland, a subcutaneous gland that is specialized in women for the production and secretion of milk and, as a result, enlarges during the menstrual cycle and during pregnancy. The mammary gland overlies ribs 2 to 6 between the sternum and midaxillary line. The mammary gland is located within the superficial fascia, surrounded by a variable amount of adipose tissue that is responsible for the shape and size of the breast. The breast lies on the deep fascia related to the pectoralis major and serratus anterior muscles. The retromammary space is a layer of loose connective tissue that separates the breast from the deep fascia and provides some degree of breast mobility over underlying structures. A prolongation of each breast, called the axillary tail, extends superolaterally along the inferior border of the pectoralis major into the axilla.

Mammary Gland Structure

Each gland consists of 15 to 20 radially aligned lobes of glandular tissue. A lactiferous duct drains each lobe (Figure 2-2A and B). The lactiferous ducts converge and open onto the nipple. The nipple is positioned on the anterior surface of the breast and is surrounded by a somewhat circular hyperpigmented region, the areola. Sebaceous glands within the areola enlarge to form swollen tubercles during pregnancy as the areola darkens. Small collections of smooth muscle located at the base of the nipple may cause erection of the nipple when breast feeding or sexually aroused.

Fibrous and adipose tissues occupy the spaces between the lobes of glandular tissue. To help support the weight of the breast, strong fibrous bards called suspensory (Cooper's) ligaments separate the lobes and attach between the dermis and deep layer of the superficial fascia.

Arteries, Veins, and Lymphatics of the Breast

To simplify an understanding of vascular supply and lymphatic drainage, the breast's anatomy can be best understood by division into medial and lateral regions (Figure 2-2C and D).

• Medial region of the breast. Derives its blood supply from perforating branches of the internal thoracic artery (medial mammary branches from the second through fourth intercostal spaces) and is drained by internal thoracic veins. Lymphatic vessels exiting the medial side of the breast drain into the parasternal lymph nodes, which lie beside the internal thoracic veins.
• Lateral region of the breast. Derives its blood supply from branches of the lateral thoracic artery (axillary artery origin) and mammary branches from the 2nd through the 5th posterior intercostal arteries (thoracic aorta origin) and is drained by the lateral thoracic and intercostal veins, respectively. The lymphatic vessels draining the lateral side of the breast lead primarily into the pectoral group of axillary lymph nodes and account for most of the lymph drained from the breast. Figure 2-2D shows lymph from the right breast draining into the right lymphatic duct at the junction of the right brachiocephalic vein. In contrast, the left breast (not shown) drains into the thoracic lymphatic duct at the left brachiocephalic vein junction.

Some breast lymphatics drain into the infraclavicular group of axillary nodes, as well as the supraclavicular group of deep cervical nodes.

Innervation of the Breast

The sensory innervation of the skin overlying the breast has a segmental arrangement and is supplied by branches of the 2nd through 7th intercostal nerves. However, physiologic changes in the breast are not mediated by nerves but by circulating hormones. For example, high prolactin levels result in milk production, and oxytocin causes the milk “letdown” reflex.

Big Picture

For simplicity, we will divide muscles of the anterior thoracic wall into two groups. The first group, superficial thoracic muscles, originate on the thoracic skeleton and inserts on the upper limb therefore producing motion of the upper limb. The second group, the deep thoracic muscles located within the intercostal space, act primarily with (Table 2-3).

Superficial Thoracic Muscles

The superficial muscles attached to the anterior portion of the thorax primarily act on the scapula and humerus. These muscles are shown in Figure 2-3A and include the following:

• Pectoralis major muscle. Attaches to the sternum, clavicle, and costal margins and laterally to the intertubercular groove of the humerus. The pectoralis major muscle is a prime flexor, adductor, and medial rotator of the humerus.
• Pectoralis minor muscle. Attaches anteriorly on the thoracic skeleton to ribs 3 to 5 and superiorly to the coracoid process of the scapula. The pectoralis minor muscle stabilizes the scapula against the thoracic wall.
• Serratus anterior muscle. Attaches to ribs 1 to 8 along the midaxillary line and courses posteriorly to the medial margin of the scapula. The serratus anterior muscle is a primary protractor of the scapula as well as a stabilizer of the scapula against the thoracic wall.
• Subclavius muscle. Attaches to the clavicle and rib 1 and moves the clavicle inferiorly.

These muscles also function as accessory respiratory muscles by helping to expand the thoracic cavity when inspiration is deep and forceful.

Intercostal Muscles

The area between ribs is the intercostal space (Figure 2-3B and C). The eleven intercostal spaces each contain three overlapping layers of tissue consisting of the external, internal, and innermost intercostal muscles and membranes. Intercostal nerves and vessels, coursing between the second and third layers of muscles (from superficial to deep), supply the intercostal muscles. The intercostal muscles are muscles of respiration and are named according to their positions to one another.

• Superficial muscle layer. The external intercostal muscles are the first and most superficial layer in the intercostal space and course in an oblique fashion from superior to inferior. The muscle fills the intercostal spaces posteriorly from the tubercles of the ribs to the costochondral joint anteriorly. The external intercostal muscle continues from the costochondral joint to the sternum as the external intercostal membrane. The external intercostal muscles move the ribs superiorly and, therefore, are most active during inspiration.
• Middle muscle layer. The internal intercostal muscles are the second and, therefore, the middle layer in the intercostal space and course at a right angle to the external intercostal muscles. The muscle fills each intercostal space anteriorly from the sternum to the angle of the ribs posteriorly. The internal intercostal muscle continues from the angle of the ribs to the vertebral column as the internal intercostal membrane. The internal intercostal muscles move the ribs inferiorly and, therefore, are most active during expiration.
• Deep muscle layer. Small, thin muscles form the third and the deepest layer of the intercostal spaces. The occurrence and location of these muscles is variable. In general, the muscles occur in three groups: an anterior group called the transverses thoracis muscles; a lateral group called the innermost intercostal muscles; and a posterior group called the subcostal muscles. This group of muscles most likely aids in depressing the ribs, facilitating expiration.

Big Picture

The thoracic skeleton consists of the thoracic vertebrae posteriorly, the ribs laterally, and the sternum and costal cartilages anteriorly. The costal cartilages secure the ribs to the sternum. The thoracic cage forms a protective cage around vital organs such as the heart, lungs, and great blood vessels. The thoracic skeleton provides attachment points for the muscles of the back and chest that allow support of the shoulder girdle (scapula and clavicle) and movement of the upper limbs.

Thoracic Vertebrae

The 12 thoracic vertebrae articulate with the 12 pairs of ribs. Thoracic vertebrae typically bear two costal facets on each side, one at the superior edge and the other at the inferior edge of the vertebral body, where they receive the heads of the ribs (Figure 2-4A). The bodies of T10 to T12 vary from this pattern, however, by having only a single facet for their respective ribs. In addition, the T1 to T10 transverse processes have costal facets that articulate with the tubercles of the ribs.

Ribs

Twelve pairs of ribs form the flared sides of the thoracic cage and generally extend anteriorly from the thoracic vertebrae to the sternum (Figure 2-4B). Rib pairs 1 to 7 are known as the true ribs and attach directly to the sternum by individual costal cartilages. The remaining five pairs of ribs are called false ribs because they either attach indirectly to the sternum or lack a sternal attachment entirely. Rib pairs 8 to 10 attach to the sternum indirectly by joining each other via costal cartilages immediately above. Rib pairs 11 and 12 are called floating ribs because they have no anterior attachments; instead, they are embedded in the muscles of the lateral body wall.

Ribs 2 to 10 are considered typical ribs and have the following bony landmarks (Figure 2-4C):

• Head and neck. Form costovertebral joints by articulating with the costal demifacets of adjacent thoracic vertebral bodies and intervertebral discs.
• Tubercle. Articulates with the costal facets of adjacent thoracic vertebral transverse processes (with the exception of ribs 11 and 12).
• Shaft. The long portion of the rib consisting of a smooth superior border and a sharp, thin inferior border possessing a costal groove housing the intercostal veins, arteries, and nerves. The distal end of the rib shaft articulates with the costal cartilage, forming costochondral joints.

Ribs 1, 11, and 12 are atypical ribs. Rib 1 is not palpable because it lies deep to the clavicle. It has the scalene tubercle on the upper surface for the inferior attachment of the anterior scalene muscle. The groove for the subclavian vein is anterior to the scalene tubercle; the groove for the subclavian artery is posterior to the tubercle. Ribs 11 and 12 do not articulate with the sternum and receive the name “floating ribs.”

Sternum

The sternum, or breastbone, is flat and consists of the following structures (Figure 2-4D):

• Manubrium. Along its superior border, the manubrium contains the jugular notch at the T2 vertebral level. The notch is flanked on each side by a synovial sternoclavicular joint, which provides the only bony attachment between the upper limb and axial skeleton. Although the other joints between the costal cartilages and the sternum are synovial, the first sternochondral joint with rib 1 is cartilaginous. The upper half of rib 2 attaches to the body of the sternum at the manubriosternal joint, better known as the sternal angle.
• Sternal angle. The sternal angle (of Louis) is the junction between the manubrium and the sternal body. The junction can be palpated as a prominent transverse ridge largely because the posterior facing angle between the manubrium and the body is less than 180 degrees. The sternal angle is an important clinical landmark for two reasons. First, rib 2 articulates with the sternum at the level of the sternal angle. Accordingly, palpation of the sternal angle permits bilateral identification of rib 2 and all of the lower ribs. Second, the sternal angle marks the following important clinical levels:
  • The level of the T4 and T5 vertebrae in an axial plane.
  • The start and end of the aortic arch.
  • The tracheal bifurcation into the right and left primary bronchi.
  • The azygos vein courses over the right primary bronchus to join the superior vena cava.
  • The border between the superior and middle mediastinum is demarcated.
  • The thoracic lymphatic duct transitions from the right to the left side of the thoracic cavity.
• Sternal body. The sternal body articulates with the second through seventh costal cartilages, the manubrium, and the xiphoid process.
• Xiphoid process. The shape of the xiphoid process varies. It is located at the T9 vertebral level and articulates with the sternal body. The linea alba, a thickened vertical cord of connective tissue in the midline of the abdominal wall, attaches to the xiphoid process.

Big Picture

The nerve and blood supply of the thoracic wall consists largely of the neurovascular elements that course through the intercostal spaces. The major elements in each space consist of an intercostal vein, artery, and nerve. The primary neurovascular bundle courses along the costal groove of the upper rib, between the internal and innermost intercostal muscles. The anterior and posterior intercostal arteries supply the intercostal spaces and form anastomoses with each other.

Intercostal Nerves

The intercostal nerves are derived from the ventral rami of the first 11 thoracic spinal nerves and are named accordingly (Figure 2-5A and B). For example, the right fifth intercostal nerve is derived from the ventral ramus of the fifth thoracic spinal nerve and courses through the right fifth intercostal space under the partial cover of the costal groove of the right fifth rib. The intercostal nerves give rise to the anterior and lateral cutaneous branches (sensory) and muscular branches (motor).

Arterial Supply to the Thoracic Wall

The thoracic wall receives its arterial supply from the following branches of the subclavian artery and thoracic aorta (Figure 2-5B and D):

• Superior (supreme) intercostal arteries. Branch off the costocervical trunk, a branch of the subclavian artery, giving rise to the first and second posterior intercostal arteries.
• Internal thoracic artery. Branches from the subclavian artery, where it travels along the internal surface of the ribcage just lateral to the sternum, giving rise to the anterior intercostal arteries. The internal thoracic artery branches into the musculophrenic and superior epigastric arteries.
• Anterior intercostal arteries. Arise from the internal thoracic (upper thoracic wall) and the musculophrenic arteries (lower thoracic wall) and travel posteriorly between the ribs, anastomosing with the posterior intercostal arteries.
• Anterior perforating arteries. Perforate the internal intercostal muscles in the upper intercostal spaces with the anterior cutaneous nerves. These arteries provide partial vascular supply to the pectoralis major muscle and overlying skin. The second through fourth branches provide the medial mammary branches to the breast.
• Musculophrenic artery. The lateral terminal branch of the internal thoracic artery. It follows the costal arch on the inner surface of the costal cartilages, giving rise to two anterior arteries in the seventh through ninth intercostal spaces. It perforates the diaphragm and terminates at about the 10th intercostal space, where it anastomoses with the deep circumflex iliac artery. The musculophrenic artery supplies the pericardium, the diaphragm, and the muscles of the abdominal wall.
• Superior epigastric artery. The medial terminal branch of the internal thoracic artery. It descends on the deep surface of the rectus abdominis muscle within the rectus sheath and anastomoses with the inferior epigastric artery.
• Posterior intercostal arteries. The first two posterior intercostal arteries arise from the superior (supreme) intercostal arteries, and the remaining arteries arises from the thoracic aorta. The posterior intercostal arteries course anteriorly between the internal and innermost intercostal muscles and anastomose with the anterior intercostal arteries.
• Subcostal artery. Arises from the aorta and travels anteriorly below the 12th rib.

Venous Drainage of the Thoracic Wall

Venous drainage of the thoracic wall is from the following tributaries of the subclavian and azygos system of veins (Figure 2-5C):

• Internal thoracic vein. Accompanies the internal thoracic artery along the internal surface of the rib cage just lateral to the sternum, draining into the brachiocephalic vein.
• Anterior intercostal veins. Accompany the anterior intercostal arteries between the internal and innermost intercostal muscles and are tributaries to the musculophrenic and internal thoracic veins, which in turn are tributaries of the brachiocephalic veins.
• Posterior intercostal veins. Accompany the posterior intercostal arteries between the internal and innermost intercostal muscles. The upper two posterior intercostal veins drain into the brachiocephalic veins. The nine lowest spaces are tributaries of the azygos system, which ultimately drains into the superior vena cava.

Lymphatic Drainage of the Thorax

Lymphatic drainage of the thoracic cavity is primarily through the following lymph nodes:

• Parasternal (internal thoracic) nodes. Located along the internal thoracic artery. These nodes receive lymph from the medial side of the breast, the intercostal spaces, diaphragm, and supraumbilical region of the abdominal wall. They drain into the junction of the internal jugular and subclavian veins.
• Intercostal nodes. Lie near the heads of the ribs and receive lymph from the intercostal spaces and parietal pleura. They drain into the cisternal chili or the thoracic lymphatic duct.
• Phrenic nodes. Lie on the surface of the diaphragm and receive lymph from the pericardium, diaphragm, and liver. They drain into the mediastinal nodes.

Big Picture

The diaphragm muscle divides the thoracic and abdominal cavities and serves as the principal muscle for inspiration and expiration.

Attachments

The diaphragm is a musculotendinous structure that separates the thoracic cavity from the abdominal cavity. The diaphragm is composed of a peripheral muscular portion and a central tendon (Figure 2-6A). The diaphragm is dome shaped, and upon contraction of its muscular portion, it descends (flattens). The right dome, resting atop the liver, is generally higher than the left dome, which rests atop the fundus of the stomach.

The muscular portion has three regions of origin (Figure 2-6B):

• Lumbar origin. Two crura originate from the bodies of the upper two (left crus) or three (right crus) lumbar vertebrae.
• Costal origin. From muscle fibers that arise from the inner surfaces of the lower six ribs.
• Sternal origin. From muscle fibers that arise from the inner surface of the xiphoid process.

The muscle fibers of the diaphragm extend centrally to insert into its central tendon. The central tendon is the structure that the diaphragm's muscle fibers primarily pull upon when they concentrically contract.

Innervation

The right and left phrenic nerves provide sensory and motor innervation to the diaphragm as follows:

• Sensory. Sensory innervation of the pericardium, mediastinal and diaphragmatic pleurae, and the diaphragmatic peritoneum.
• Motor. Motor innervation of the diaphragm is through the phrenic nerve, which arises from the cervical plexus via ventral rami branches of C3, C4, and C5 (“C3, C4, and C5 keep the diaphragm alive”).

The phrenic nerves course along the anterior surface of the anterior scalene muscle in the side and descend into the thoracic cavity between the subclavian vein and artery. The phrenic nerves are accompanied by the pericardiacophrenic vessels and descend anterior to the root of each lung between the mediastinal pleura and pericardium en route to the diaphragm. The vagus nerve, by contrast, courses posterior to the root of the lung.

Apertures in the Diaphragm

The diaphragm has several apertures that permit the passage of structures between the thorax and the abdomen (Figure 2-6A and B). The caval, esophageal, and aortic openings are large, and the left and right crura provide other small openings.

• Caval opening. Located at the T8 vertebral level within the central tendon of the diaphragm, just right of the midline. The caval opening allows passage for the inferior vena cava and branches of the right phrenic nerve. Branches of the left phrenic nerve pass through the diaphragm by piercing the central tendon on the left side.
• Esophageal opening. Located to the left of the midline at the T10 vertebral level. The opening usually splits the muscle fibers of the right crus. The esophageal opening allows passage for the esophagus and the left and right vagus nerves. The esophageal branches of the left gastric artery and the esophageal tributaries of the left gastric vein also passes through this opening.
• Aortic opening. Located at the T12 vertebral level, behind the two crura. Strictly speaking, the aortic opening is not an opening through the diaphragm but rather a large gap between the crura. The left and right crura form the left and right borders of the aortic opening. The aortic opening allows passage for the aorta, the azygos vein, and the thoracic lymphatic duct.
• Right and left crura. The right and left greater and lesser splanchnic nerves course from the thoracic cavity, deep to the right and left crura en route to the prevertebral ganglia of the abdomen. In addition, the left crus also allows passage for the hemiazygos vein.

Functions of the Diaphragm

• Respiration. The diaphragm is the principal muscle of inspiration. It flattens upon contraction, thus increasing the vertical dimensions of the thoracic cavity (Figure 2-6C). The roles of the diaphragm and other thoracic muscles of respiration are discussed in more detail in Chapter 3.
• Venous return. The alternating contraction and relaxation of the diaphragm causes pressure changes in the thoracic and abdominopelvic cavity that facilitate the return of venous blood to the heart.