Sternum

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.

Figure 3-2 Sternum. A, Anterior view. B, Lateral view.

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.

Ribs

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.

Figure 3-3 Typical right rib. A, Superior view. B, Inferior view.

There are three types of ribs:

Figure 3-4 Superior view of the first right rib.

Thoracic Vertebrae

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 3-5 A, Superior view of typical thoracic vertebra. B, Left lateral view of two articulated vertebrae.

Figure 3-6 Horizontal section through thorax to demonstrate articulation of ribs.

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.

Figure 3-7 Horizontal section through thorax to demonstrate articulation of ribs. Anterior view of surface features of thorax. A, male. B, female.

Joints

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 Breast

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.

Skeletal Landmarks

The following important landmarks in the chest are used to locate underlying structures (Figure 3-9):

Figure 3-9 Surface landmarks of anterior chest wall.

The surface anatomy of the heart and lungs is covered in the descriptions of these organs in subsequent sections.

Muscular Action

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.

Extrinsic Thoracic Muscles

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.

Accessory Extrinsic Muscles of Respiration

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.

Intercostal Muscles

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.

Figure 3-10 Typical intercostal space.

Figure 3-11 Origins of intercostal nerves and arteries. Nerves and arteries are shown separately but are bilateral structures and run together with veins as neurovascular bundles.

Back Muscles that Participate in Respiration

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.

Diaphragm

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).

Arteries and Veins

Posterior intercostal arteries arise as paired segmented branches of the thoracic aorta. These arteries run laterally and anteriorly to supply the body wall.

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.

Nerves

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.

Surfaces and Borders

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.

Figure 3-13 Anterior aspect of lungs.

Figure 3-14 Medial or mediastinal surface of lungs.

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.

Fissures and Lobes

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.

The Hilum

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 right and left bronchi arise from the midline trachea. They carry air to the hilum of the lung during inspiration and carry air from the hilum during expiration.

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.

Bronchi and Bronchioles

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.