1 Introduction

The thorax lies between the neck and abdomen, encasing the great vessels, heart, and lungs, and provides a conduit for structures passing between the head and neck superiorly and the abdomen, pelvis, and lower limbs inferiorly. Functionally, the thorax and its encased visceral structures are involved in the following:

The sternum, ribs (12 pairs), and thoracic vertebrae (12) encircle the thoracic contents and provide a stable thoracic cage that both protects the visceral structures of the thorax and offers assistance with breathing. Because of the lower extent of the rib cage, the thorax also offers protection for some of the abdominal viscera, including the liver and gallbladder on the right side, the stomach and spleen on the left side, and the adrenal (suprarenal) glands and upper poles of the kidneys on both sides.

The superior thoracic aperture (the anatomical thoracic inlet) conveys large vessels, important nerves, the thoracic lymphatic duct, the trachea, and the esophagus between the neck and thorax. Clinicians often refer to “thoracic outlet syndrome,” which describes symptoms associated with compression of the brachial plexus as it passes over the first rib (specifically, the T1 ventral ramus). Technically, this is a misnomer because these nerves are not exiting the superior thoracic aperture (thoracic inlet). The inferior thoracic aperture (the anatomical thoracic outlet) conveys the inferior vena cava (IVC), aorta, esophagus, nerves, and thoracic lymphatic duct between the thorax and the abdominal cavity. Additionally, the thorax contains two pleural cavities laterally and a central “middle space” called the mediastinum, which is divided as follows (Fig. 3-1):

Key surface landmarks for thoracic structures include the following (Fig. 3-2):

Planes of Reference

In addition to the sternal angle of Louis, physicians often use other imaginary planes of reference to assist in locating underlying visceral structures of clinical importance. Important vertical planes of reference include the following (Fig. 3-3):

• Midclavicular line

• Anterior axillary line: inferolateral margin of the pectoralis major muscle; demarcates the anterior axillary fold.

• Midaxillary line

• Posterior axillary line: margin of the latissimus dorsi and teres major muscles; demarcates the posterior axillary fold.

• Scapular line: intersects the inferior angles of the scapula.

• Midvertebral line: also called the “posterior median” line.

The thoracic cage, which is part of the axial skeleton, includes the thoracic vertebrae, the midline sternum, the 12 pairs of ribs (each with a head, neck, tubercle, and body; floating ribs 11 and 12 are short and do not have a neck or tubercle), and the costal cartilages (Fig. 3-4). This bony framework provides the scaffolding for attachment of the chest wall muscles and the pectoral girdle, which includes the clavicle and scapula and forms the attachment of the upper limb to the thoracic cage at the shoulder joint (Table 3-1).

Rib fractures can be a painful injury (we must continue to breathe) but are less common in children because their thoracic wall is still fairly elastic. The weakest part of the rib is the angle.

Joints of Thoracic Cage

Joints of the thoracic cage include articulations between the ribs and the sternum and thoracic vertebrae and between the sternum and the clavicle and are summarized in Figure 3-5 and Table 3-2.

TABLE 3-2

Joints of the Thoracic Cage

LIGAMENT	ATTACHMENT	COMMENT
Sternoclavicular (Saddle-Type Synovial) Joint with an Articular Disc
Capsule	Clavicle and manubrium	Allows elevation, depression, protraction, retraction, circumduction
Sternoclavicular	Clavicle and manubrium	Consists of anterior and posterior ligaments
Interclavicular	Between both clavicles	Connects two sternoclavicular joints
Costoclavicular	Clavicle to first rib	Anchors clavicle to first rib
Sternocostal (Primary Cartilaginous [Synchondroses]) Joints
First sternocostal	First rib to manubrium	Allows no movement at this joint
Radiate sternocostal	Ribs 2-7 with sternum	Permit some gliding or sliding movement at these synovial plane joints
Costochondral (Primary Cartilaginous) Joints
Cartilage	Costal cartilage to rib	Allow no movement at these joints
Interchondral (Synovial Plane) Joints
Interchondral	Between costal cartilages	Allow some gliding movement

Muscles of Anterior Thoracic Wall

The musculature of the anterior thoracic wall include several muscles that attach to the thoracic cage but that actually are muscles that act on the upper limb (Fig. 3-6). These muscles are as follows (for a review, see Chapter 7):

• Pectoralis major

• Pectoralis minor

• Serratus anterior

The true anterior thoracic wall muscles fill the intercostal spaces or support the ribs, act on the ribs (elevate or depress the ribs), and keep the intercostal spaces rigid, thereby preventing them from bulging out during expiration and being drawn in during inspiration (Fig. 3-6 and Table 3-3). Note that the external intercostal muscles are replaced by the anterior intercostal membrane at the costochondral junction anteriorly, and that the internal intercostal muscles extend posteriorly to the angle and then are replaced by the posterior intercostal membrane. The innermost intercostal muscles lie deep to the internal intercostals and extend from the midclavicular line to about the angles of the ribs posteriorly.

TABLE 3-3

Muscles of the Anterior Thoracic Wall

MUSCLE	SUPERIOR ATTACHMENT (ORIGIN)	INFERIOR ATTACHMENT (INSERTION)	INNERVATION	MAIN ACTIONS
External intercostal	Inferior border of rib above	Superior border of rib below	Intercostal nerves	Elevate ribs, supports intercostal space
Internal intercostal	Inferior border of rib above	Superior border of rib below	Intercostal nerves	Elevate ribs (upper four and five); others depress ribs
Innermost intercostal	Inferior border of rib above	Superior border of rib below	Intercostal nerves	Acts with internal intercostals
Transversus thoracis	Posterior surface of lower sternum	Internal surface of costal cartilages 2-6	Intercostal nerves	Depress ribs
Subcostal	Internal surface of lower rib near their angles	Superior borders of second or third ribs below	Intercostal nerves	Depress ribs
Levator costarum	Transverse processes of C7 and T1-T11	Subjacent ribs between tubercle and angle	Dorsal primary rami of C8-T11	Depress ribs and costal cartilages

Clinical Focus 3-1   Thoracic Cage Injuries

Thoracic cage injuries usually result from trauma and often involve rib fractures (ribs 1 and 2 and 11 and 12 are more protected and often escape being fractured), crush injuries with rib fractures, and penetrating chest wounds such as gunshot and stab wounds. The pain caused by rib fractures can be intense because of the expansion and contraction of the rib cage during respiration, sometimes requiring palliation by anesthetizing the intercostal nerve (nerve block).

Intercostal Vessels and Nerves

The intercostal neurovascular bundles (vein, artery, and nerve) lie inferior to each rib, running in the costal groove deep to the internal intercostal muscles (Fig. 3-7 and Table 3-4). The veins largely correspond to the arteries and drain into the azygos system of veins or the internal thoracic veins. The intercostal arteries form an anastomotic loop between the internal thoracic artery (branches of anterior intercostal arteries arise here) and the thoracic aorta posteriorly. Posterior intercostal arteries arise from the aorta, except for the first two, which arise from the supreme intercostal artery, a branch of the costocervical trunk of the subclavian artery.

TABLE 3-4

Arteries of the Internal Thoracic Wall

ARTERY	COURSE
Internal thoracic	Arises from subclavian and terminates by dividing into superior epigastric and musculophrenic arteries.
Intercostals	First two posterior branches derived from superior intercostal branch of costocervical trunk and lower nine from thoracic aorta; these anastomose with anterior branches derived from internal thoracic artery (1st-6th spaces) or its musculophrenic branch (7th-9th spaces); the lowest two spaces only have posterior branches.
Subcostal	From aorta, courses inferior to the 12th rib.
Pericardiacophrenic	From internal thoracic and accompanies phrenic nerve.

The intercostal nerves are the ventral rami of the first 11 thoracic spinal nerves. The 12th thoracic nerve gives rise to the subcostal nerve, which courses inferior to the 12th rib. The nerves give rise to lateral and anterior cutaneous branches and branches innervating the intercostal muscles (Fig. 3-7).

Female Breast

The female breast extends from approximately the second to the sixth ribs and from the sternum medially to the midaxillary line laterally. Mammary tissue is composed of compound tubuloacinar glands organized into about 15 to 20 lobes, which are supported and separated from each other by fibrous connective tissue septae (the suspensory ligaments of Cooper) and fat. Each lobe is divided in lobules of secretory acini and their ducts. Features of the breast include the following (Fig. 3-8):

• Breast: fatty tissue containing glands that produce milk; lies in the superficial fascia above the retromammary space, which lies above the deep pectoral fascia enveloping the pectoralis major muscle.

• Areola: circular pigmented skin surrounding the nipple; it contains modified sebaceous and sweat glands (glands of Montgomery) that lubricate the nipple and keep it supple.

• Nipple: site of opening for the lactiferous ducts; usually lies at about the level of the fourth intercostal space.

• Axillary tail (of Spence): extension of mammary tissue superolaterally toward the axilla.

• Lymphatic system: lymph is drained from breast tissues; about 75% of lymphatic drainage is to the axillary lymph nodes (Fig. 3-9; see also Fig. 7-11), and the remainder drains to infraclavicular, pectoral, or parasternal nodes.

The primary arterial supply to the breast includes the following:

• Anterior intercostal branches of the internal thoracic (mammary) arteries (from the subclavian artery)

• Lateral mammary branches of the lateral thoracic artery (a branch of the axillary artery)

• Thoraco-acromial artery (branch of the axillary artery)

The venous drainage (Fig. 3-9) largely parallels the arterial supply, finally draining into the internal thoracic, axillary, and adjacent intercostal veins.

Clinical Focus 3-2   Fibrocystic Breast Disease

Fibrocystic change (disease) is a general term covering a large group of benign conditions occurring in about 80% of women that are often related to cyclic changes in maturation and involution of glandular tissue. Fibroadenoma, the second most common tumor of the breast after carcinoma, is a benign neoplasm of glandular epithelium and is usually accompanied by a significant increase in connective tissue stroma. Both conditions present as palpable masses and warrant follow-up evaluation.

Clinical Focus 3-3   Breast Cancer

Breast cancer is the most common malignancy in women; approximately two thirds of all cases occur in postmenopausal women. Invasive carcinoma may involve the suspensory ligaments, causing retraction of the ligaments and dimpling of the overlying skin. Additionally, invasion and obstruction of the subcutaneous lymphatics can result in dilation and skin edema, creating an “orange peel” appearance (peau d’orange). About 50% of cancers develop in the upper outer quadrant (quadrant closest to the axilla, which includes the axillary tail). Distant sites of metastasis include the following:

• Lungs and pleura

• Liver

• Bones

• Brain

Clinical Focus 3-4   Partial Mastectomy

Several clinical options are available to treat breast cancer, including systemic approaches (chemotherapy, hormonal therapy, immunotherapy) and “local” approaches (radiation therapy, surgery). In a partial mastectomy, also called “lumpectomy” or “quadrantectomy,” the surgeon performs a breast-conserving surgery that removes the portion of the breast that harbors the tumor along with a halo of normal surrounding breast tissue. Because of the possibility of lymphatic spread, especially to the axillary nodes, an incision also may be made for a sentinel node biopsy to examine the first axillary node, which is likely to be invaded by metastatic cancer cells from the breast.

Clinical Focus 3-5   Modified Radical Mastectomy

In addition to breast-conserving surgery, several more invasive mastectomy approaches may be indicated, depending on a variety of factors, as follows:

• Total (simple) mastectomy: the whole breast is removed, with or without some axillary lymph nodes if indicated, down to the retromammary space.

• Modified radical mastectomy (illustrated here): the whole breast is removed along with most of the axillary and pectoral lymph nodes, axillary fat, and the investing fascia over the chest wall muscles. Care is taken to preserve the pectoralis, serratus anterior, and latissimus dorsi muscles and the long thoracic and thoracodorsal nerves to the latter two, respectively. Damage to the long thoracic nerve results in “winging” of the scapula, and damage to the thoracodorsal nerve weakens extension at the shoulder.

• Radical mastectomy: the whole breast is removed along with the axillary lymph nodes, fat, and chest wall muscles (pectoralis major and minor); use of the radical surgical approach is much less common now.

The thorax is divided into the following three compartments:

The lungs lie within the pleural cavity (right and left) (Fig. 3-10). This “potential space” is between the investing visceral pleura, which closely envelops each lung, and the parietal pleura, which reflects off each lung and lines the inner aspect of the thoracic wall, the superior surface of the diaphragm, and the sides of the pericardial sac (Table 3-5). Normally, the pleural cavity contains a small amount of serous fluid, which lubricates the surfaces and reduces friction during respiration. The parietal pleura is richly innervated with afferent fibers that course in the somatic intercostal nerves and, over the surface of the diaphragm, the phrenic nerve (C3-C5); the visceral pleura has few, if any, pain fibers.

Clinically, it is important for physicians to be able to “visualize” the extent of the lungs and pleural cavities topographically on the surface of their patients (Fig. 3-10). The lungs lie adjacent to the parietal pleura inferiorly to the sixth costal cartilage. (Note the presence of the cardiac notch on the left side.) Beyond this point, the lungs do not occupy the full extent of the pleural cavity during quiet respiration. These points are important to know if one needs access to the pleural cavity without injuring the lungs (Table 3-6), such as to drain inflammatory exudate (pleural effusion), hemorrhage into the cavity (hemothorax), or air (pneumothorax). In quiet respiration, the lung margins reside two ribs above the extent of the pleural cavity at the midclavicular, midaxillary, and midscapular lines.

The Lungs

The paired lungs are invested in the visceral pleura and are attached to mediastinal structures (trachea and heart) at their hilum. Each lung possesses the following surfaces:

• Apex: superior part of the upper lobe that extends into the root of the neck (above the clavicles).

• Hilum: area located on the medial aspect through which structures enter and leave the lung.

• Costal: anterior, lateral, and posterior aspects of the lung in contact with the costal elements of the internal thoracic cage.

• Diaphragmatic: inferior part of the lung in contact with the underlying diaphragm.

The right lung has three lobes and is slightly larger than the left lung, which has two lobes. Both lungs are composed of spongy and elastic tissue, which readily expands and contracts to conform to the internal contours of the thoracic cage (Fig. 3-11 and Table 3-7).

TABLE 3-7

External Features of the Lungs

STRUCTURE	CHARACTERISTICS
Lobes	Three lobes (superior, middle, inferior) in right lung; two in left lung
Horizontal fissure	Only on right lung, extends along line of fourth rib
Oblique fissure	On both lungs, extends from T2 vertebra to sixth costal cartilage
Impressions	Made by adjacent structures, in fixed lungs
Hilum	Points at which structures (bronchus, vessels, nerves, lymphatics) enter or leave lungs
Lingula	Tongue-shaped feature of left lung
Cardiac notch	Indentation for the heart, in left lung
Pulmonary ligament	Double layer of parietal pleura hanging from the hilum that marks reflection of visceral pleura to parietal pleura
Bronchopulmonary segment	10 functional segments in each lung supplied by a segmental bronchus and a segmental artery from the pulmonary artery

Clinical Focus 3-6   Chest Drainage Tubes

A chest tube provides a way to evacuate air or fluids (blood, pus) from the pleural cavity, thus reapposing the parietal and visceral pleura and enhancing the patient’s ability to breathe normally. Following administration of a local anesthetic, the tube is inserted close to the upper border of a rib to avoid the neurovascular bundle, which runs in the costal groove at the inferior margin of each rib.

The lung’s parenchyma is supplied by several small bronchial arteries that arise from the proximal portion of the descending thoracic aorta. Usually, one small right bronchial artery and a pair of left bronchial arteries (superior and inferior) can be found on the posterior aspect of the main bronchi. Although much of this blood returns to the heart via the pulmonary veins, some also collects into small bronchial veins that drain into the azygos system of veins (see Fig. 3-25).

The lymphatic drainage of both lungs is to pulmonary (intrapulmonary) and bronchopulmonary (hilar) nodes (i.e., from distal sites to the proximal hilum). Lymph then drains into tracheobronchial nodes at the tracheal bifurcation and into right and left paratracheal nodes (Fig. 3-12).

As visceral structures, the lungs are innervated by the autonomic nervous system. Sympathetic bronchodilator fibers, which relax smooth muscle, arise from upper thoracic spinal cord segments. Parasympathetic bronchoconstrictor fibers, which contract smooth muscle and increase mucus secretion, arise from the vagus nerve.

Respiration

During quiet inspiration the contraction of the diaphragm alone accounts for most of the decrease in intrapleural pressure, allowing air to expand the lungs. Active inspiration occurs when the diaphragm and intercostal muscles together increase the diameter of the thoracic wall, decreasing intrapleural pressure even more. Although the first rib is stationary, ribs 2 to 6 tend to increase the anteroposterior diameter of the chest wall, and the lower ribs mainly increase the transverse diameter. Accessory muscles of inspiration that attach to the thoracic cage may also assist in very deep inspiration.

During quiet expiration the elastic recoil of the lungs and thoracic cage expel the air. In forced expiration the abdominal muscles contract and, by compressing the abdominal viscera superiorly, raise the intraabdominal pressure and force the diaphragm upward. Having the “wind knocked out of you” shows how forceful this maneuver can be.

Clinical Focus 3-7   Pulmonary Embolism

The lungs naturally filter venous clots larger than circulating blood cells and can usually accommodate small clots because of their fibrinolytic (“clot buster”) mechanisms. However, pulmonary embolism (PE) is the cause of death in 10% to 15% of hospitalized patients. Thromboemboli originate from deep leg veins in approximately 95% of cases. Major causes are called Virchow’s triad and include the following:

• Venous stasis (e.g., caused by extended bed rest)

• Trauma (e.g., fracture, tissue injury)

• Coagulation disorders (inherited or acquired)

Other contributors to PE include postoperative and postpartum immobility and some hormone medications that increase the risk of blood clots. About 60% to 80% of PEs are “silent” because they are small; larger emboli may obstruct medium-sized vessels and lead to infarction or even obstruction of a vessel as large as the pulmonary trunk (saddle embolism). PE without infarction is common and presents as tachypnea, anxiety, dyspnea, and vague substernal pressure. Saddle embolus, on the other hand, is an emergency that can precipitate acute cor pulmonale (right-sided heart failure) and circulatory collapse.

Clinical Focus 3-8   Lung Cancer

Lung cancer is the leading cause of cancer-related death. It arises either from alveolar lining cells of the lung parenchyma or from the epithelium of the tracheobronchial tree. Although there are a number of types, squamous cell (bronchiogenic) carcinoma and adenocarcinoma (from intrapulmonary bronchi) are the most common types. Bronchiogenic carcinoma may impinge on adjacent anatomical structures. For example, in Pancoast syndrome the tumor may spread to involve the sympathetic trunk and compromise the sympathetic tone to the head. This may lead to Horner’s syndrome, characterized by the following symptoms on the affected side:

• Miosis: constricted pupil

• Ptosis: minor drooping of the upper eyelid

• Anhidrosis: lack of sweating

• Flushing: subcutaneous vasodilation

Additionally, the neurovascular components passing into the upper limb (trunks of the brachial plexus and subclavian artery) may be affected, resulting in paresthesia in the neck, head, shoulder, and limb, with 90% affecting areas of ulnar nerve distribution (C8-T1).

Clinical Focus 3-9   Chronic Obstructive Pulmonary Disease

Chronic obstructive pulmonary disease (COPD) is a broad classification of obstructive lung diseases, the most familiar being chronic bronchitis, asthma, and emphysema.

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