Chapter 3The Thorax
The thoracic cavity extends superiorly from the dome of the thoracic diaphragm to the cervical region just above the first rib. The diaphragm separates the abdominal cavity below from the thoracic cavity above.
The skeleton of the thorax consists of (1) a midline sternum, (2) 12 pairs of ribs and associated costal cartilages, and (3) 12 thoracic vertebrae (Figure 3-1). The first ribs, sternum, and first thoracic vertebra comprise the thoracic inlet.
The sternum, or breast bone, consists of three portions: (1) the manubrium; (2) a body, which joins the manubrium as a symphysis at the sternal angle; and (3) the xiphoid process, a small inferior portion (Figure 3-2). Superiorly the manubrium of the sternum presents a midline notch and two lateral notches. The midline notch is the jugular notch, or suprasternal notch, and it can be palpated at the anterior base of the neck. The lateral notches receive the clavicular heads of the upper limb girdle. The extreme lateral borders of the manubrium articulate with the costal cartilages of the first ribs.
The costal cartilages of the second ribs articulate with the sternum at the junction of the manubrium and body. Therefore the sternal angle is an important landmark in determining, from the surface, the level of the second rib and the positions of each descending rib. The costal cartilages of the upper seven ribs articulate directly with the sternum.
A typical rib consists of (1) a head, which articulates with the body of a thoracic vertebra; (2) a neck; (3) a tubercle, which articulates with the transverse process of a thoracic vertebra; (4) a shaft, or body; (5) an angle, at which the rib turns inferiorly and anteriorly; and (6) a shallow subcostal groove on the internal inferior surface, which shelters the intercostal nerve and vessels (Figure 3-3). Anteriorly the typical rib is joined to the sternum by its own costal cartilage.
In addition to all of the features attributable to a typical vertebra described in Chapter 2, the thoracic vertebrae exhibit the following unique features: (1) the body is heart-shaped, (2) the spinous processes are long and slender, and (3) the bodies and transverse processes have facets for articulation with ribs (Figures 3-5 and 3-6).
Figure 3-7 demonstrates the articulation of a rib with a thoracic vertebra. Except for ribs 1, 10, 11, and 12, the head of each rib articulates with the body of its own vertebra and that of the vertebra above. The facets on the vertebral bodies are really demifacets, and in the articulated spine the demifacet below and the one above make a complete facet. Ribs 1, 10, 11, and 12 articulate with only their own vertebrae. The tubercles of the ribs articulate with the transverse processes of each thoracic vertebra.
The joints between the thoracic vertebrae are considered in Chapter 2. The remaining thoracic joints allow movements that result in expansion of the thoracic cavity during inspiration (see Figure 3-7). The movements of respiration are discussed later in this chapter, under “Mechanics of Breathing.” There are three types of thoracic skeleton joints:
The thoracic cavity is divided into three main regions: (1) the right pleural cavity, (2) the left pleural cavity (the pleural cavities contain the lungs), and (3) the mediastinum, a midline structure that separates the right and left pleural cavities. The mediastinum is a collection of structures, including the heart and its great vessels, the trachea and esophagus, and other structures. These structures are described later in this chapter.
A dislocated rib is displaced at its sternocostal joint. A rib separation is a rib torn from its costal cartilage. A rib fracture is a break in the rib itself and often occurs at the angle of the rib. Although painful, rib fractures generally are not reduced and immobilized (as are limb fractures), unless there is evidence of lung or other internal damage.
The breast, or mammary gland, arises as modified sweat gland tissue within the superficial fascia of the anterior chest wall and is covered by skin (Figure 3-8). In postpubertal women the breast is a large organ capable of lactation. In men and children it is rudimentary.
Figure 3-8 Features of female breast and lymph drainage. A, Drainage to axillary nodes. B, Drainage to internal thoracic nodes of chest wall. C, Drainage to opposite breast. D, Drainage to nodes of anterior abdominal wall.
In women the breast overlies the pectoralis major muscles at the level of ribs 2 to 6. An axillary tail passes laterally and superiorly to the axillary region (armpit). The functional component of the breast consists of 15 to 20 lobules of glandular tissue lying within accumulated fat of the superficial fascia. The glandular ducts empty to the surface through the nipple. Surrounding the nipple is an areola containing areolar glands and ranging from light pink to dark brown. Suspensory ligaments (of Cooper) support the breast by anchoring it to underlying deep fascia. Arterial supply comes from mammary branches of the axillary artery, the internal thoracic artery, and intercostal arteries.
Knowledge of lymphatic drainage is extremely important because malignancies of the breast may spread along lymphatic routes. Lymphatics radiate out from the nipple and communicate with lymph nodes of various regions. The breast is divided into quadrants. The two lateral quadrants drain superiorly to nodes of the axilla. The two medial quadrants drain to the axillary nodes, the anterior chest wall, and the interior abdominal wall, and they may even drain across the midline to the opposite breast.
In lower mammals the multiple mammary glands develop along “milk lines” running from the axilla to the groin. Occasionally in humans of either sex, accessory nipples or breasts may be found along this line.
Breast cancer is the most common type of cancer among women and rarely occurs in men. Most cancers initially are detected by the patient as a lump or mass distinctly different from surrounding breast tissue. Others are found during routine mammography. The upper lateral quadrant is the most common site of occurrence. Diagnosis is made after fine-needle biopsy and cytological examination. More advanced lesions interfere with lymphatic drainage, causing local puffiness and an uneven dimpled skin surface termed peau d’orange (orange peel). A mastectomy is the removal of an entire breast. A radical version is the additional removal of the pectoral muscles and axillary lymph nodes. A lumpectomy is the excision of just the lesion and some surrounding tissue.
In a sequence not yet completely understood, the thoracic muscles act to stabilize the upper and lower ribs and elevate the remaining ribs to increase thoracic volume during quiet inspiration. During forced inspiration the upper ribs are elevated by the accessory muscles to further increase the thoracic volume.
The superficial muscles covering the chest wall are actually muscles belonging to other regions and are described in sections that discuss these regions. Muscles of the upper limb (see Chapter 9), which originate from the thoracic skeleton, are the pectoralis major and minor, serratus anterior, latissimus dorsi, rhomboid major and minor, levator scapu-lae, and trapezius. Muscles of the anterior abdominal wall (see Chapter 4), which attach to the thoracic skeleton, are the rectus abdominis, external oblique, internal oblique, and transversus abdominis. Posteriorly, erector spinae and muscles of the back (see Chapter 2) attach to the thoracic skeleton.
Several muscles of the neck region insert into the skeleton of the upper thorax to elevate the sternum and ribs during forced inspiration. These muscles are the scalenus anterior, scalenus medius, and scalenus posterior and are described fully in Chapter 5, page 151.
The intercostal muscles of the thorax are involved with the mechanics of breathing. They run from (1) rib to rib, (2) sternum to rib, and (3) vertebra to rib (Figure 3-10 and Table 3-1). The external intercostal muscles pass from rib to rib in an anteroinferior direction (in the same direction as the external abdominal oblique muscle) and elevate the ribs during inspiration. The internal intercostal muscle passes from rib to rib, perpendicular to the external intercostal muscle, and depresses the ribs during expiration. The innermost intercostal muscles run in same direction as the internal intercostal muscles, but the two layers are separated by the intercostal nerves and vessels. The innermost layer is subdivided into the subcostal and transversus thoracis. They likely aid the internal intercostals in depressing the ribs. Figure 3-11 shows that the muscles are not continuous sheets, and in some areas the muscle is replaced by thin membranous tendon. The intercostal muscles are supplied by intercostal nerves.
The levator costarum muscles lie on the posterior aspect of the thorax. They run from transverse processes (C7 to T11) and pass down and laterally to insert into the area between the tubercle and the angle of the rib below. They elevate the ribs during inspiration and are innervated by posterior rami of thoracic spinal nerves.
The diaphragm is the most important muscle of respiration and its attachments are described in Chapter 4, pages 96 and 97. On contraction, the diaphragm pulls the central tendon inferiorly to increase the vertical dimension of the thorax during inspiration. It is supplied by the right and left phrenic nerves (anterior rami of C3, C4, and C5).
The serratus posterior superior and inferior are thin, flat muscles on the posterior thoracic wall (see Figure 2-15). The superior muscle runs from the lower cervical and upper thoracic vertebral spines downward and laterally to the upper ribs. It elevates the ribs during inspiration. The inferior muscle arises from the upper lumbar and lower thoracic vertebral spines and passes upward and laterally to insert into the lower ribs. It depresses or stabilizes the lower ribs.
The intercostal vessels and nerves run between the ribs, under the shelter of the subcostal groove of the more superior rib (see Figures 3-10 and 3-11). The vessels and nerves run between the internal and innermost intercostal muscles.
Anterior intercostal arteries arise from the internal thoracic artery. The internal thoracic artery arises from the subclavian artery in the root of the neck, enters the thoracic inlet, and travels down the inner aspect of the chest wall. As it descends, it gives rise to anterior intercostal arteries, which supply the anterior body wall and pass laterally to anastomose with the posterior intercostal arteries.
The internal thoracic artery ends in two terminal branches as it approaches the abdomen by dividing into the superior epigastric artery of the anterior abdominal wall and the musculophrenic artery of the diaphragm.
Intercostal veins parallel the blood flow of the arteries but in a reverse direction. Anterior intercostal veins empty into the internal thoracic veins. Posterior intercostal veins empty into the azygos and hemiazygos veins. The superior intercostal veins empty into the brachiocephalic veins.
Spinal nerves arise from the spinal cord in paired segmented fashion. The anterior rami of these nerves travel laterally and anteriorly with the intercostal vessels as intercostal nerves. The lateral cutaneous branches are given off laterally, and as the nerves approach the midline, anterior cutaneous branches are given off to the anterior chest wall.
The right and left pleural cavities are completely enclosed spaces within the thorax that contain the right and left lungs (Figure 3-12). Like the peritoneal cavity, the pleural cavity is lined with serous membrane, or pleura.
Parietal pleura lines the inner aspect of the pleural cavity. It is subdivided as follows: Costal pleura lines the inner aspect of the rib cage, diaphragmatic pleura lines the superior aspect of the diaphragm, mediastinal pleura covers the mediastinum, and cervical pleura bulges up into the neck as the cupola. Where parietal pleura reflects from the mediastinum and diaphragm onto the thoracic wall is important, and the areas of reflection may be plotted based on knowledge of surface anatomy.
Visceral pleura lines the lungs, following all the contours and fissures intimately. Both visceral pleura and parietal pleura are continuous at the root of the lung. It is helpful to imagine the lung bud in the developing embryo growing into an empty pleural cavity lined with parietal pleura. As the lung invades the cavity, it carries with it parietal pleura, which eventually covers the lung as visceral pleura.
Pleural recesses are areas where there is a space between reflected layers of pleura. During quiet inspiration the recesses are not filled with lung tissue. There is a costodiaphragmatic recess laterally and inferiorly and a costomediastinal recess medially and anteriorly.
The lungs are two spongy organs resembling inverted, blunted cones cut in half. They are housed in the right and left pleural cavities. The lungs consist of small, air-filled chambers, or alveoli, where exchange of gases (oxygen [O2] and carbon dioxide [CO2] takes place with the circulatory system. In turn the alveoli are supported by elastic tissue, which tends to collapse and shrink the lung (elastic recoil) during respiratory expiration.
Each lung exhibits four surfaces, each of which takes the shape of surrounding structures (Figures 3-13 and 3-14). The surfaces are (1) the apex, which is a rounded superior aspect that bulges up through the thoracic inlet; (2) the diaphragmatic surface, or base, which rests on the dome of the diaphragm; (3) a mediastinal surface, which contacts the midline mediastinum; and (4) the costal surface, which is rounded to fit the curved ribs.
Separating the four surfaces are three borders: (1) a sharp anterior border, which separates the costal surface from the mediastinal surface; (2) a rounded posterior or vertebral border, which separates the costal and mediastinal surfaces posteriorly; and (3) the lower circumferential border, which separates the diaphragmatic surface from the costal and mediastinal surfaces.
Both the right and left lungs are divided into lobes by fissures. Both lungs have an oblique fissure, which separates each lung into an upper and a lower lobe. The right lung has a second fissure (the horizontal fissure), which creates a third middle lobe. Thus the right lung consists of three lobes, and the left lung consists of two lobes. The missing middle lobe of the left lung is represented by a deficiency, the cardiac notch; its inferior border is called the lingula because it resembles a tongue.
On the mediastinal aspect of each lung are an exit and entrance (hilum) for blood vessels and air tubes. The pulmonary artery from the right ventricle enters the hilum of the lung carrying unoxygenated blood. The pulmonary vein leaves the hilum of the lung carrying oxygenated blood.
The bronchial artery arises from the thoracic descending aorta and travels to the hilum to supply lung tissue. In addition, autonomic nerves and lymphatics enter or leave through the hilum of the lung.
The trachea consists of approximately 20 U-shaped, incomplete cartilaginous rings strung together with fibroelastic tissue (Figure 3-15). The posterior deficient portion is covered with fibrous tissue and involuntary muscle. Approximately half the course of the trachea is within the neck. It continues inferiorly from the larynx in the neck, lying anterior to and paralleling the course of the esophagus. Applied laterally to the trachea are the two lobes of the thyroid gland, connected by an isthmus that crosses the second or third tracheal ring (see Chapter 5).
The trachea descends and enters the inlet of the thorax, passing deep to the sternum, where it is covered by the thymus gland. At vertebral level T5 (sternal angle), the trachea bifurcates into a right and left primary bronchus. The last tracheal ring features the carina, a midline cartilaginous ridge that separates the lumens of the primary bronchi. It is seen during a bronchoscopic examination.
The right and left primary bronchi are approximately half the diameter of the trachea. The right bronchus differs from the left, however, in that it is slightly wider, shorter, and in a more direct line with the trachea.
The right primary bronchus divides into three secondary bronchi, or lobar bronchi, one for each of the three lobes. The secondary bronchus to the upper lobe is given off before the primary bronchus plunges through the hilus. Within the right lung the primary bronchus divides to supply secondary bronchi to the middle and lower lobes.
Each lobe of the right lung is further subdivided functionally into bronchopulmonary segments. Each secondary bronchus breaks up into tertiary or segmental bronchi to supply 10 bronchopulmonary segments. Each segment can function independently of the others, and the thoracic surgeon makes use of this fact when considering the removal of diseased portions of lung tissue without affecting the function of remaining healthy segments.
Within the bronchopulmonary segments the tertiary bronchi continue to divide, decrease in size, and lose their cartilaginous support. When this occurs, the tubes are called bronchioles and their walls are supported only by a relative increase in smooth muscle thickness. Spasm of this smooth muscle decreases airflow, a condition known as asthma.
A hazard in a dental office is an aspirated foreign body (e.g., an extracted tooth, a piece of filling material, a root canal instrument) that has fallen past the vocal folds of the larynx into the trachea. Such an object is more likely to enter the right primary bronchus because it is in a more direct line with the trachea. When possible and practicable, a rubber dam should be placed to prevent such an accident.
The bronchioles finally end as terminal bronchioles, from which ductules lead off to blind sacs, or alveoli, which are one cell thick. Surrounding each alveolus is a capillary network fed by arterioles branching from the pulm/>