Anyone who would attempt to operate on the heart should lose the respect of his colleagues.
Christian Albert Theodor Billroth, 1881
Many structures of the thorax are in constant motion. Although physiologic motion can make imaging difficult, a thorough knowledge of thoracic anatomy and physiology can improve diagnostic imaging of this area (Fig. 6.1). This chapter demonstrates the sectional anatomy of the structures listed in the outline.
OBJECTIVES
• Describe the structures that constitute the bony thorax.
• Define the thoracic inlet and outlet.
• Understand the function and layers of the pleura.
• Identify and describe the structures of the lungs.
• Identify the mainstem bronchi and their divisions.
• List the structures of the mediastinum, and describe their anatomic relationships to each other.
• Identify the structures of the heart, and explain the circulation of blood through the heart.
• Identify the great vessels, and describe the distribution of their associated arteries and veins.
• Differentiate between pulmonary arteries and veins by function and location.
• Identify the coronary arteries and veins.
• List the muscles involved in respiration by function and location.
• List and describe the layers of the breast.
BONY THORAX
The bony thorax protects the organs of the thorax and aids in respiration. It consists of the thoracic vertebrae, sternum, ribs, and costal cartilages (Fig. 6.2). The 12 thoracic vertebrae make up the posterior boundary of the thoracic cage. The anterior boundary is created by the sternum, located midline. The sternum has three components: manubrium, body, and xiphoid process (Figs. 6.3 and 6.4). The triangular-shaped manubrium is the most superior portion and articulates with the first two pairs of ribs and the clavicles. It articulates with the clavicles at the clavicular notch to form the sternoclavicular (SC) joints (Fig. 6.5). A common landmark, the jugular notch, is located on the superior border of the manubrium at approximately the level of T2-T3. The manubrium and body of the sternum come together at an angle to form a ridge known as the sternal angle, which is located at approximately the level of T4-T5. The slender body of the sternum has several indentations along its sides where it articulates with the cartilage of the third through seventh ribs (Figs. 6.2, 6.6, and 6.7). The small xiphoid process is located on the inferior border of the sternum and is a site for muscle attachments including the rectus abdominis and transversus abdominis muscles (Figs. 6.4 and 6.8).
Forming the lateral borders of the thoracic cage are the 12 pairs of ribs. The spaces between adjacent ribs are referred to as the intercostal spaces. All 12 pairs of ribs articulate posteriorly with the thoracic spine. The ribs consist of a head, neck, tubercle, and body (Figs. 6.7 and 6.8). The facets of the head of the rib articulate with the vertebral bodies at the costovertebral joints, whereas the facets of the tubercles articulate with the transverse processes of the vertebrae to form the costotransverse joints (Fig. 6.7). The first seven pairs of ribs (true ribs) articulate anteriorly with the sternum via costal cartilage. The lower five pairs of ribs are considered false ribs because they do not attach directly to the sternum. The costal cartilage of the 8th, 9th, and 10th ribs attach to the costal cartilage of the 7th rib. The 11th and 12th ribs are considered floating because they attach only to the thoracic vertebrae and contain no neck or tubercle, only vertebral and sternal ends (Fig. 6.2).
Thoracic Apertures
There are two openings, or apertures, associated with the bony thorax. The superior thoracic aperture (thoracic inlet) is formed by the first thoracic vertebra, the first pair of ribs and their costal cartilages, and the manubrium. This aperture allows for the passage of nerves, vessels, and viscera from the neck into the thoracic cavity. The inferior thoracic aperture (thoracic outlet) is much larger and is made up of the 12th thoracic vertebra, 12th pair of ribs and costal margins, and xiphoid sternal junction (Figs. 6.2, 6.5, and 6.8).
Thoracic Outlet Syndrome
Thoracic outlet syndrome (TOS) refers to a group of disorders causing pain and paresthesias in the neck, shoulder, arms, or hands resulting from compression of the brachial plexus and/or subclavian vessels as they pass through the thoracic outlet. The name is somewhat controversial because the location of the pathology is technically the thoracic inlet.
PLEURAL CAVITIES
Each lung lies within a single pleural cavity that is lined by a serous membrane, or pleura. The pleura can be divided into two layers. The parietal pleura, the outer layer, is continuous with the thoracic wall and diaphragm and moves with these structures during respiration. The visceral pleura is the inner layer that closely covers the outer surface of the lung and continues into the fissures to cover the individual lobes as well. Both membranes secrete a small amount of pleural fluid that provides lubrication between the surfaces during breathing (Figs. 6.9 and 6.10). Deep pockets or recesses of the pleural cavities are the costomediastinal and costodiaphragmatic recesses. The costomediastinal recesses are located at the point where the mediastinum and costal cartilages meet anteriorly, and the costodiaphragmatic recesses are located where the diaphragm and ribs connect inferiorly. These recesses serve as expansions to provide additional pleural space where parts of the lung can glide during inspiration (Figs. 6.9 and 6.10).
LUNGS
The lungs are the organs of respiration. They are composed of a spongelike material, the parenchyma, and are surrounded by the visceral pleura. The large conicalshaped lungs extend up to or slightly above the level of the first rib at their apex and down to the dome of the diaphragm at their wide concave-shaped bases or diaphragmatic surfaces (Figs. 6.11-6.13). Each lung has a mediastinal or medial surface that is apposed to the mediastinum and a costal surface that is apposed to the inner surface of the rib cage. Each lung also has inferior, anterior, and posterior borders. The inferior border extends into the costodiaphragmatic recess of the pleural cavity, and the anterior border of each lung extends into the costomediastinal recess of the pleural cavity (Fig. 6.13).
Two prominent angles can be identified at the medial and lateral edges of the lung bases. The medial angle is termed the cardiophrenic sulcus, and the lateral angle is termed the costophrenic sulcus (Figs. 6.11 and 6.12). The lungs are divided into lobes by fissures that are lined by pleura. The right lung has three lobes (superior [upper], middle, and inferior [lower]), whereas the left lung has just superior (upper) and inferior (lower) lobes (Figs. 6.11-6.15). The inferior lobe of the right lung is separated from the middle and superior lobes by the oblique (major) fissure, termed oblique because of its posterosuperior to anteroinferior course (Figs. 6.11 and 6.13). Separating the middle lobe from the superior lobe is the horizontal (minor) fissure (Figs. 6.11-6.13 and 6.15A). An oblique fissure also separates the superior and inferior lobes of the left lung (Figs. 6.11 and 6.12). The left lung has a large notch on the medial surface of its superior lobe called the cardiac notch and a tonguelike projection off its inferoanterior surface termed the lingula (Figs. 6.11 and 6.15B). Each lung has an opening on the medial surface termed the hilum. This opening acts as a passage for the mainstem bronchi, blood vessels, lymph vessels, and nerves to enter or leave the lung and is commonly referred to as the root of the lung (Figs. 6.15-6.17).
Cystic disease of the lung encompasses a wide variety of pathologic processes that are characterized by “holes” or abnormal air-containing spaces within the lung parenchyma.
BRONCHI
The trachea bifurcates into the left and right mainstem (primary) bronchi at approximately the level of T5. This location is commonly referred to as the carina (Fig. 6.11). The right mainstem bronchus is wider, shorter, and more vertical in orientation than the left. At the hilum, the mainstem bronchi enter the lungs and divide into secondary or lobar bronchi. Secondary bronchi correspond to the lobes of the lungs, with three divisions on the right (superior, middle, inferior) and two divisions on the left (superior and inferior) (Figs. 6.11 and 6.18— 6.20). There is further division of the secondary bronchi into tertiary or segmental bronchi, which extend into each segment of the lobes (bronchopulmonary segments) (Figs. 6.21 and 6.22 and Table 6.1). There are typically 10 segments within each lung. Each bronchopulmonary segment is functionally independent and can be individually removed surgically. The bronchial tree continues to divide many times into smaller bronchi, then into bronchioles. Each bronchiole continues to divide, approximately 23 times, until it reaches the terminal end as alveoli, which are the functional units of the respiratory system. Gaseous exchange between alveolar air and capillary blood occurs through the walls of the alveoli (Fig. 6.23).
The basic unit of pulmonary structure and function is called the secondary pulmonary lobule. It is the smallest component of lung tissue that is surrounded by connective tissue, and it measures approximately 1 to 2 cm. It consists of three to five acini that contain alveoli for gas exchange with a terminal bronchiole and artery located in the center of the lobule. At the periphery of the lobule is the interstitial septa formed by connective tissue, pulmonary veins, and lymphatics (Figs. 6.23 and 6.24). High-resolution computed tomography (CT) is capable of demonstrating the secondary pulmonary lobule and can help characterize interstitial lung disease based on the type of pathology present within the lobule.
Lung cancer remains the leading cause of cancer- related deaths in both men and women in the United States. Approximately 1 out of 4 cancer deaths is from lung cancer. According to the American Cancer Society more people die of lung cancer than of colon, breast, and prostate cancers combined. Lung cancer typically occurs in older individuals, with an average age at the time of diagnosis of 70 years. Overall, the chance that a man will develop lung cancer in his lifetime is about 1 in 14 while it is about 1 in 17 for women.
FIG. 6.21 Trachea and bronchopulmonary segments. Bronchopulmonary segments: 1, apical; 2, posterior; 3, anterior; 4, lateral; 5, medial; 6, superior; 7, medial basal; 8, anterior basal; 9, lateral basal; 10, posterior basal; 11, superior lingular; 12, inferior lingular.
TABLE 6.1 Bronchopulmonary Segments
|
Lobe |
Right Lung |
Left Lung |
|
Superior lobe |
Apical segment (1) Posterior segment (2) Anterior segment (3) |
Apical segment (1) Posterior segment (2) Anterior segment (3) Superior lingular segment (11) Inferior lingular segment (12) |
|
Middle lobe |
Lateral segment (4) Medial segment (5) |
|
|
Inferior lobe |
Superior segment (6) Medial basal segment (7) Anterior basal segment (8) Lateral basal segment (9) Posterior basal segment (10) |
Superior segment (6) Medial basal segment (7) Anterior basal segment (8) Lateral basal segment (9) Posterior basal segment (10) |
FIG. 6.23 Axial view of secondary pulmonary lobule.
FIG. 6.24 Axial CT of secondary pulmonary lobule.
MEDIASTINUM
The mediastinum is the midline region of the thoracic cavity located between the two pleural cavities of the lungs. It extends from the superior thoracic aperture to the diaphragm and is bordered anteriorly by the sternum and posteriorly by thoracic vertebrae. The mediastinum can be subdivided into compartments for descriptive purposes. The superior and inferior compartments are made by drawing an imaginary line between the sternal angle and the intervertebral disk of T4-T5. The superior compartment constitutes the upper portion of the mediastinum. It contains the thymus gland and acts as a conduit for structures as they enter and leave the thoracic cavity. The inferior compartment can be further divided into anterior, middle, and posterior compartments (Fig. 6.25). The anterior compartment is located anterior to the pericardial sac and posterior to the sternum. The middle compartment is the area that contains the pericardial sac, heart, and roots of the great vessels. The posterior compartment is the area lying posterior to the pericardium and anterior to the inferior eight thoracic vertebrae. Structures located within the mediastinum include the thymus gland, trachea, esophagus, lymph nodes, thoracic duct, heart and great vessels, and various nerves (Table 6.2).
TABLE 6.2 Mediastinal Compartments
|
Compartment |
Location |
Structures |
|
Superior |
Between the manubrium and T1-T4. Bounded superiorly by the thoracic inlet and inferiorly by a plane between the sternal angle and the T4-T5 disk space |
Thymus, aortic arch, superior vena cava (SVC), vagus and phrenic nerves, lymph nodes, superior trachea, esophagus, thoracic duct |
|
Inferior |
Divided into three compartments below the superior mediastinum: anterior, middle, and posterior |
|
|
Anterior |
Between sternal body and pericardium |
Inferior thymus, fat, lymph nodes, mediastinal branches of internal thoracic artery |
|
Middle |
Bounded by the fibrous pericardium |
Pericardium, heart, ascending aorta, lower half of SVC, tracheal bifurcation and main bronchi, central pulmonary and systemic vessels, lymph nodes |
|
Posterior |
Between fibrous pericardium and lower 4th—12th thoracic vertebral bodies |
Inferior esophagus, descending thoracic aorta, azygos and hemiazygos veins, thoracic duct, lymph nodes |
Thymus Gland
The thymus gland is a triangular-shaped bilobed gland of lymph tissue located in the superior portion of the mediastinum just behind the manubrium (Figs. 6.266.28). It is considered the primary lymphatic organ responsible for the development of cellular immunity. T lymphocytes within the blood reach the thymus as stem cells, where they are stored while they undergo T-cell differentiation and maturation. The thymus gland produces a hormone, thymosin, that is responsible for the development and maturation of lymphocytes. The thymus gland reaches its maximum size during puberty and gradually diminishes in size in the adult.
The thymus gland is large in children. In the newborn, it is often larger than the heart. It gradually decreases in size with increasing age and is replaced by mediastinal fat.
Trachea and Esophagus
Throughout its course in the mediastinum, the trachea runs anterior to the esophagus. Considered an elastic tube, the trachea is reinforced by approximately 16-20 C-shaped pieces of cartilage that maintain an open passageway for air. The cartilages are closed posteriorly by elastic connective tissue that allows for the passage of food through the esophagus. In crosssection, the trachea appears as a round, air-filled structure to the point at which it bifurcates at the carina (Figs. 6.10, 6.11, 6.18, and 6.25). The esophagus appears as an oval-shaped structure that descends through the mediastinum to enter the abdominal cavity at the esophageal hiatus of the diaphragm (Figs. 6.29 and 6.30).
LYMPHATIC SYSTEM
Lymph Nodes
Lymph nodes in the mediastinum are generally clustered around the great vessels, esophagus, bronchi, and carina. Mediastinal lymph nodes are classified according to their location and are grouped into 14 regional nodal stations for use in lung cancer staging (Fig. 6.31 and Table 6.3). Lymph vessels and nodes can be difficult to visualize in cross-section unless they are enlarged as a result of an abnormality (Figs. 6.28, 6.32, and 6.33).
The supraclavicular lymph nodes are commonly referred to as the sentinel lymph nodes because their enlargement alerts the medical professional to the possibility of malignant disease in the thoracic and/or abdominal cavities.
FIG. 6.31 Coronal view with distribution of mediastinal lymph nodes. L, left; R, right; 1, highest mediastinal nodes; 2, upper paratracheal nodes; 3, prevascular and retrotracheal nodes; 4, lower paratracheal (including azygos) nodes; 5, subaortic nodes; 6, para-aortic nodes; 7, subcarinal; 8, paraesophageal nodes; 9, pulmonary ligament; 10, hilar nodes; 11, interlobar nodes; 12, lobar nodes; 13, segmental nodes; 14, segmental nodes.
TABLE 6.3 International Association for the Study of Lung Cancer (IASLC) Lymph Node Map 2009
Supraclavicular Nodes 1
Station 1: Low Cervical, Supraclavicular, and Sternal Notch; Divided Into 1R and 1L by Trachea
1R—extend from lower border of cricoid cartilage to clavicles and upper margin of manubrium
1L—extend from lower border of cricoid cartilage to clavicles and upper margin of manubrium
Superior Mediastinal Nodes 2-4
Station 2: Upper Paratracheal; Divided into 2R and 2L by Lateral Border of Trachea
2R—upper border: apex of R lung to upper border of manubrium; lower border: intersection of L brachiocephalic vein with trachea
2L—upper border: apex of L lung to upper border of manubrium; lower border: superior border of aortic arch
Station 3: Prevascular (Anterior to the Vessels) and Retrotracheal (Posterior to the Esophagus)
3A—prevascular: on right side: apex of chest to level of carina and posterior sternum to anterior border of SVC. On left side: apex of chest to level of carina and posterior sternum to left carotid artery
3P—retrotracheal: apex of chest to carina
Station 4: Lower Paratracheal; Divided Into 4R and 4L by Trachea
4R—upper border: intersection of caudal margin of left brachiocephalic vein; lower border: lower border of azygos vein
4L—upper border: upper margin of aortic arch; lower border: upper margin of left main pulmonary artery
Aortic Nodes 5-6
Station 5: Subaortic; Lateral to Ligamentum Arteriosum
5— subaortic (aortopulmonary window): upper border: lower margin of aortic arch; lower border: upper rim of left main pulmonary artery
Station 6: Para-Aortic; Anterior and Lateral to Ascending Aorta and Aortic Arch
6— para-aortic (ascending aorta or phrenic): upper border: upper margin of aortic arch; lower border: lower margin of aortic arch
Inferior Mediastinal Nodes 7-9
Station 7: Subcarinal; Located Caudally to Carina
7— on right: extend caudally to lower border of bronchus intermedius; on left: extend caudally to upper margin of the lower lobe bronchus
Station 8: Paraesophageal; Below Carinal Nodes, Adjacent to Wall of Esophagus
8— on right: upper border: lower margin of bronchus intermedius; lower border: interlobar region. On left: upper margin of lower lobe bronchus; lower border: interlobar region.
Station 9: Pulmonary Ligament
9— upper border: inferior pulmonary vein; lower border: diaphragm
Hilar, Lobar, and Subsegmental Nodes 10-14
Station 10: Hilar; Adjacent to Mainstem Bronchus and Hilar Vessels
10— on right: upper border: lower rim of azygos vein; lower border: interlobar region. On left: upper border: upper rim of pulmonary artery; lower border: interlobar region.
Station 11: Interlobar; Between Origins of the Lobar Bronchi
11— superior: between upper lobe bronchus and bronchus intermedius on the right; inferior: between the middle and lower lobe bronchi on the right
Station 12: Lobar; Adjacent to Lobar Bronchi
Station 13: Segmental; Adjacent to Segmental Bronchi
Station 14: Subsegmental; Adjacent to Subsegmental Bronchi
L, Left; R, right.
Lymph Vessels
The lymphatic system consists of a network of lymphatic vessels that carry lymph fluid (excess interstitial fluid) away from the tissue and into venous circulation. Small lymph vessels (capillaries) can be found accompanying arteries and veins throughout the body. The tiny lymph vessels increase in size until they reach their terminal collecting vessels, the thoracic duct and the right lymphatic duct. The thoracic duct is the main vessel of the lymph system, draining all of the lymph fluid from tissues below the diaphragm and from the left side of the body above the diaphragm (Figs. 6.34-6.36). It begins inferior to the diaphragm at the level of L2 and passes from the abdominal cavity into the thorax through the aortic hiatus of the diaphragm. It originates in the abdomen, at the cisterna chyli, a dilated sac or confluence of lymph trunks into which lymph from the intestinal and lumbar lymphatic trunks open (Fig. 6.34). It ascends the thorax, between the azygos vein and the descending aorta, and empties into the left subclavian vein at the level of the clavicle. The smaller right lymphatic duct collects lymph from the right upper side of the body and is formed by the merging of various lymphatic trunks near the right clavicle. This duct empties into the right subclavian vein (Fig. 6.34).
HEART AND VASCULATURE
Superficial Landmarks
The heart is a hollow, four-chambered muscular organ located within the middle mediastinum. It is approximately the size of a large clenched fist and is situated obliquely in the chest with one-third of its mass lying to the right of the median plane and two-thirds to the left. The heart can be described as being roughly trapezoid shaped (Fig. 6.37). The superficial relationships of the heart include the base, apex, three surfaces (sternocostal, diaphragmatic, pulmonary), and four borders (right, inferior, left, and superior). The broad base (posterior aspect) is the most superior and posterior portion of the heart. It is formed by both atria, primarily the left atrium, and gives rise to the great vessels. The apex is formed by the left ventricle and points inferiorly, anteriorly, and to the left. It is located at the level of the fifth intercostal space, just medial to the midclavicular line. The sternocostal (anterior) surface is formed primarily by the right atrium and right ventricle with a small contribution from the left ventricle. The diaphragmatic (inferior) surface rests on the central tendon of the diaphragm and is formed by both ventricles and a small portion of the right atrium. The pulmonary (left) surface is formed mainly by the left ventricle and fills the cardiac notch of the left lung. The borders of the heart represent the external surfaces of the cardiovascular silhouette in radiographic profile. The borders include the right border, formed by the right atrium and located between the superior and inferior venae cavae; the left border, formed by the apex of the heart or the left ventricle; the superior border, formed by the right and left atria; and the inferior border, which is formed primarily by the right ventricle, with a small contribution from the left ventricle (Figs. 6.37-6.39).
Pericardium
The heart is enclosed in a pericardial sac that surrounds the heart and the proximal portions of the great vessels entering and leaving the heart (Figs. 6.40-6.42). The fibrous pericardium is attached to the central tendon of the diaphragm and is pierced by the inferior vena cava. The inner surface of the fibrous pericardium consists of a double-layered serous membrane termed the serous pericardium. The serous pericardial layers include the parietal layer, which lines the inner surface of the fibrous pericardium, and the visceral layer (epicardium), which covers the outer surface of the heart and the roots of the great vessels. Located between the two layers is a potential space (pericardial cavity) containing a thin film of serous fluid that acts as a lubricant to reduce friction to the tissues caused by heart movements. During embryonic development, the heart invaginates into the serous pericardium, which creates folds called pericardial reflections. The pericardial reflections located by the great vessels result in the formation of two potential spaces: the oblique and transverse sinuses. Within the two sinuses are potential spaces called recesses (Fig. 6.43 and Table 6.4). These spaces can be filled with fluid and may be mistaken for cystic lesions or lymphadenopathy. Located between the parietal pericardium and the heart wall is a layer of epicardial fat that is typically more prominent near the inflow and outflow of the heart, the coronary vessels, and along the grooves separating the heart chambers. Mediastinal fat is the fat present within the thoracic cavity external to the parietal pericardium (Figs. 6.40-6.42).
Pericarditis is an inflammation of the pericardium. It usually develops suddenly and may last for several months. Symptoms include sharp stabbing chest pain that may increase with coughing and swallowing as well as difficulty breathing when lying down. Most cases of pericarditis occur in men aged 20-50. Although there can be many causes of pericarditis, it is frequently due to complications of an infection (viral, bacterial, fungal, or parasitic). Left untreated, it can become a life-threatening condition called cardiac tamponade, a build-up of fluid causing a severe compression of the heart. Treatment depends on the cause but may include pain medication, antibiotics, and anti-inflammatories.
FIG. 6.43 Pericardial sinuses and recesses. (A) Anterior view of pericardial sinuses and recesses. (B) Axial view of pericardial sinuses and recesses.
TABLE 6.4 Pericardial Sinuses and Recesses
|
Transverse sinus Superior aortic recess |
Located posterior to ascending aorta and pulmonary trunk, extending to left atrium Located anterior to ascending aorta and pulmonary trunk; partially surrounds ascending aorta |
|
Inferior aortic recess |
Located between right lateral ascending aorta and right atrium; posterior to aorta and anterior to left atrium |
|
Right and left pulmonic recesses |
Right; inferior to proximal right pulmonary artery Left; inferior to left pulmonary artery, superior to left superior pulmonary vein |
|
Oblique sinus |
Located posterior to left atrium and inferior to the transverse sinus |
|
Posterior pericardial recess |
Extends superiorly behind the right pulmonary artery, medial to the intermediate bronchus |
Heart Wall
The walls of the heart consist of three layers: (1) epicardium, the thin outer layer that is consistent with the visceral layer of the pericardium; (2) myocardium, the thick middle layer consisting of strong cardiac muscle; and (3) endocardium, the thin, endothelial layer lining the inner surface (Figs. 6.39, 6.42, and 6.44). The endothelial layer also lines the valves of the heart and is continuous with the inner lining of the blood vessels. The heart is divided into four chambers: the right and left atria and the right and left ventricles. The two superior collecting chambers called atria are separated by the interatrial septum (Figs. 6.6, 6.41, and 6.44). During embryonic development, an oval opening exists within the interatrial septum called the foramen ovale. This opening allows blood flow between the right and left atria during fetal lung development. At birth, the foramen ovale closes, leaving a small depression in the septal wall called the fossa ovalis in the adult heart (Fig. 6.44). The two inferior pumping chambers called ventricles are divided by the interventricular septum (Figs. 6.39, 6.41, 6.42, and 6.44). On the external surface of the heart are grooves that separate the chambers. The atria of the heart are separated from the ventricles by the coronary groove (atrioventricular groove or sulcus). The ventricles of the heart are separated by two depressions or sulci that are located on the anterior and posterior surfaces of the heart, termed the anterior and posterior interventricular grooves (Fig. 6.45; see coronary vessels, page 339).
Chambers
The right atrium forms the right border of the heart and receives deoxygenated blood from the body via the superior and inferior venae cavae and from the coronary sinus and cardiac veins that drain the myocardium (Fig. 6.44). A small muscular embryonic appendage, the right auricle, projects upward and toward the left, covering the root of the aorta (Fig. 6.45). The right ventricle lies on the diaphragm and comprises the largest portion of the anterior surface of the heart. It receives deoxygenated blood from the right atrium and forces it into the pulmonary trunk for conveyance to the lungs. Extending from the inner surface of the ventricular walls are three conical-shaped projections of cardiac muscle called papillary muscles, which anchor the cusps of the tricuspid valve to the right ventricle (Fig. 6.44). The left atrium lies posterior to the right atrium and is the most posterior surface of the heart. It also has an embryonic appendage, the left auricle, which projects to the left of the pulmonary trunk over the superior surface of the heart (Fig. 6.45). The left atrium receives oxygenated blood directly from the lungs via the four pulmonary veins (two on each side). The left ventricle forms the apex, left border, and most of the inferior surface of the heart. It receives oxygenated blood from the left atrium and pumps it into the aorta for distribution throughout the systemic vascular circuit. The myocardium of the left ventricle is normally three times thicker than that of the right ventricle, reflecting the force necessary to pump blood to the distant sites of the systemic circulation (Figs. 6.46-6.66). Two papillary muscles project from the ventricular walls to anchor the bicuspid valve to the left ventricle (Figs. 6.44, 6.52, 6.65, and 6.66).
To see the cardiac chambers in off-axis views, refer to pages 380-388.
Cardiac Conduction System
The cardiac conduction system is the electrical system that controls the heart rate by generating electrical impulses and conducting them throughout the muscles of the heart, stimulating the heart to contract and pump blood. The electrical impulses of the myocardium travel through a specific nerve pathway in the heart beginning in the sinoatrial (SA) node, which is a mass of specialized cardiac muscle fibers that act as the “pacemaker” of the heart. The SA node lies under the epicardium in the superior aspect of the right atrium. The electrical signal generated from the SA node travels to the right and left atrium, causing them to contract and force blood into the ventricles. The electrical signal continues to the ventricles via the atrioventricular (AV) node, located in the posteroinferior region of the interatrial septum near the opening of the coronary sinus, and then to the atrioventricular (AV) bundle (bundle of His) located along the interventricular septum. The signal continues down the bundle and into the right and left bundle branches. When the signal reaches the bundle branches, it causes the ventricles to contract and force blood to the body and lungs (Fig. 6.44).
Cardiac Valves
Four valves are located in the heart that function to maintain one-way directional blood flow throughout the heart. The valves can be divided into two groups: atrioventricular and semilunar (Figs. 6.44 and 6.62).
Atrioventricular Valves. The two atrioventricular valves are found at the entrances to both ventricles and function to prevent backflow of blood between the atria and ventricles during ventricular contraction. These valves have leaflets that are attached to the papillary muscles by thin cords of fibrous tissue called chordae tendineae. The right atrioventricular valve, with three leaflets, is called the tricuspid valve, and the left atrioventricular valve, with two leaflets, is called the bicuspid (mitral) valve (Figs. 6.44, 6.52, 6.53, and 6.63).
Semilunar Valves. The semilunar valves are located at the junction where the ventricles meet the great vessels and separate the ventricles from the circulatory system. These valves are called semilunar because of their three crescentshaped cusps, and they function to prevent the flow of blood back into the ventricles during ventricular relaxation. The pulmonary semilunar valve is located at the juncture of the right ventricle and pulmonary artery, and the aortic semilunar valve lies between the left ventricle and ascending aorta (Figs. 6.62, 6.63, 6.65, and 6.66).
GREAT VESSELS
Blood travels to and from the heart through the great vessels, which include the aorta, pulmonary arteries and veins, and superior and inferior venae cavae (Figs. 6.67 and 6.68). The aorta is the largest artery in the body and can be divided into the ascending aorta, aortic arch, and descending aorta. The ascending aorta begins at the base of the left ventricle. The origin of the ascending aorta (aortic root) is divided into three dilations or protrusions that create spaces termed aortic sinuses, one left, one right, and one posterior, which correspond to the three cusps of the aortic semilunar valve. The right aortic sinus gives rise to the right coronary artery, and the left aortic sinus gives rise to the left coronary artery (Figs. 6.656.69). Because no vessels arise from the posterior aortic sinus, it is considered to be noncoronary. The ascending aorta curves superiorly and posteriorly as the aortic arch over the right pulmonary artery and left mainstem bronchus (Figs. 6.59, 6.60, 6.67, 6.68, and 6.70-6.74). The top of the aortic arch is approximately at the level of T3. The arch continues as the descending aorta posterior to the left mainstem bronchus and pulmonary trunk, on the left side of the vertebral body of T4 (Figs. 6.59, 6.60, and 6.72-6.74). The descending aorta passes slightly anterior and to the left of the vertebral column as it descends through the thoracic cavity and into the abdomen via the aortic hiatus of the diaphragm. In the thoracic cavity, the descending aorta is commonly called the thoracic aorta, and in the abdominal cavity, it is called the abdominal aorta.
The pulmonary trunk, the main pulmonary artery, lies entirely within the pericardial sac. It arises from the right ventricle and ascends in front of the ascending aorta. Then it courses posteriorly and to the left, where it bifurcates at the level of the sternal angle (T4) into the right and left pulmonary arteries (Figs. 6.67, 6.68, and 6.75-6.78). At the origin of the pulmonary trunk are slight dilations between the wall of the pulmonary trunk and cusps of the pulmonary semilunar valves, termed pulmonary sinuses (Fig. 6.69). The pulmonary trunk is attached to the aortic arch by a fibrous cord called the ligamentum arteriosum, the remnant of an important fetal blood vessel (ductus arteriosus) that links the pulmonary and systemic circuits during fetal development (Figs. 6.67 and 6.75). The right pulmonary artery courses laterally, posterior to the ascending aorta and superior vena cava, and anterior to the esophagus and right mainstem bronchus, to the hilum of the right lung (Figs. 6.15A, 6.67, and 6.75). It then divides into two branches, with the lower branch supplying the middle and inferior lobes and the upper branch supplying the superior lobe (Figs. 6.75-6.80). The left pulmonary artery, shorter and smaller than the right, is also the most superior of the pulmonary vessels. It travels horizontally, arching over the left mainstem bronchus, and enters the hilum of the left lung just superior to the left mainstem bronchus (Figs. 6.15B and 6.75-6.82). Within the lungs, each pulmonary artery descends posterolateral to the secondary bronchi and divides into lobar and segmental arteries, continuing to branch out and follow along with the smallest divisions of the bronchial tree (Figs. 6.75 and 6.76-6.82).
Located inferior to the pulmonary arteries are the four pulmonary veins, two each (superior and inferior), extending from each lung to enter the left atrium (Figs. 6.67, 6.68, 6.75, 6.76, and 6.79-6.86). They begin as a capillary network along the walls of the alveoli, where they merge with the capillaries of the pulmonary arteries. The venous capillaries successively combine to form a single trunk for each lobe: three for the right and two for the left lung. Frequently, the trunk from the middle lobe of the right lung unites with the trunk from the upper lobe, forming just two trunks on the right side before entering the left atrium. The right superior pulmonary vein collects blood from the upper-lobe segments of the right lung and passes anterior and inferior to the right pulmonary artery, behind the superior vena cava (Figs. 6.79, 6.80, 6.83, and 6.84). The right inferior pulmonary vein receives blood from the lower lobes of the right lung and crosses behind the right atrium to the left atrium (Figs. 6.75, 6.76, 6.81, 6.82, and 6.87-6.89). The left superior pulmonary vein receives blood from the left upper lobe of the left lung and courses anterior and inferior to the left main bronchus as it enters the left atrium (Figs. 6.81 and 6.90-6.92). The left inferior pulmonary vein drains the inferior lobe of the left lung and passes toward the left atrium anterior to the bronchi (Figs. 6.75, 6.76, 6.79, 6.81, 6.85, 6.86, and 6.90-6.92). The pulmonary veins course more horizontally than the pulmonary arteries and are ultimately oriented toward the left atrium. At the hilum of the lungs, the pulmonary veins are anterior and inferior to the pulmonary arteries, which are located anterior to the bronchi (Figs. 6.75, 6.78, and 6.87).
The superior and inferior venae cavae are the largest veins of the body. The superior vena cava is formed by the junction of the brachiocephalic veins, posterior to the right first costal cartilage, and carries blood from the thorax, upper limbs, head, and neck (Figs. 6.26 and 6.34). As it travels inferiorly, it is located posterior and lateral to the ascending aorta before entering the upper portion of the right atrium (Figs. 6.65, 6.67, 6.68, and 6.70-6.73).
The inferior vena cava is formed by the junction of the common iliac veins in the pelvis and ascends the abdomen to the right of the abdominal aorta and anterior to the vertebral column. It passes through the caval hiatus of the diaphragm and almost immediately enters the inferior portion of the right atrium (Figs. 6.81, 6.93, and 6.94).
Obstruction of a pulmonary artery or one of its branches is known as a pulmonary embolism. This condition prevents blood flow to the alveoli and, if left in place for several hours, will result in permanent collapse of the alveoli. It is commonly caused by thrombosis from the lower extremities.
Circulation of Blood Through the Heart
Deoxygenated blood is brought to the right atrium from the peripheral tissues of the body by the inferior and superior venae cavae. The right atrium contracts, forcing blood through the tricuspid (right atrioventricular) valve into the right ventricle. The right ventricle pumps blood through the pulmonary semilunar valve to the pulmonary arteries, which enter into the lungs for oxygen exchange. Oxygenated blood returns to the heart via the pulmonary veins, which enter the left atrium. The left atrium forces blood through the bicuspid (mitral) valve into the left ventricle, where it is then pumped through the aortic semilunar valve to the aorta and then on to the rest of the body (Fig. 6.62).
Branches of the Aortic Arch
The three main branches off of the aortic arch are the brachiocephalic trunk, left common carotid artery, and left subclavian artery (Fig. 6.95). The brachiocephalic (innominate) trunk is the first major vessel and the largest branch arising from the aortic arch. It ascends obliquely to the upper border of the right sternoclavicular joint, where it divides into the right common carotid and right subclavian arteries (Figs. 6.95-6.97). The right common carotid artery ascends the neck lateral to the trachea to the level of C4, where it divides into the right external and internal carotid arteries. The right subclavian artery curves posterior to the clavicle into the axillary region, where it becomes the right axillary artery. The left common carotid artery is the second vessel to branch from the aortic arch. It arises just behind the left sternoclavicular joint and ascends into the neck along the left side of the trachea to the level of C4, where it bifurcates into the left external and internal carotid arteries (Fig. 6.26). The left subclavian artery arises from the aortic arch posterior to the left common carotid artery and arches laterally toward the axilla in a manner similar to that of the right subclavian artery, where it continues as the left axillary artery (Figs. 6.95-6.101). The common carotid arteries supply blood to the head and neck, whereas the subclavian arteries supply blood to the upper extremities. The right and left internal thoracic (internal mammary) arteries arise from the respective subclavian artery at the base of the neck. They run deep to the ribs, just lateral to the sternum, to supply blood to the anterior portion of the thorax (Figs. 5.82 and 6.97-6.100).
The internal thoracic artery is commonly used as a graft to bypass major coronary stenosis of the heart in a procedure called a coronary artery bypass graft (CABG). Typcially, the internal thoracic artery will be grafted to the left anterior descending artery for revascularization of the heart muscle. The internal thoracic artery also creates an important collateral pathway between the aorta and external iliac vessels in the event that the descending aorta is blocked.
FIG. 6.96 MRA of aorta.
FIG. 6.97 3D CT of aortic arch.
Tributaries of the Superior Vena Cava
The superior vena cava receives blood from the head and neck via the internal and external jugular veins, as well as from the upper extremities via the subclavian veins (Figs. 6.95, 6.101, and 6.102). The subclavian veins are a continuation of the axillary veins and course posterior to the clavicles. They receive blood from the external jugular veins before uniting with the internal jugular veins behind the sternoclavicular joints, where they continue as the brachiocephalic veins. The left brachiocephalic vein courses across the midline, anterior to the branches of the aorta, to unite with the right brachiocephalic vein just posterior to the costal cartilage of the right first rib (Figs. 6.60, 6.98, and 6.99). The union of the two brachiocephalic veins forms the superior vena cava, which empties into the right atrium of the heart (Figs. 6.67, 6.68, 6.95, and 6.98-6.102).
CORONARY CIRCULATION
The cardiac muscle requires a continuous supply of oxygen and nutrients, which is supplied by coronary circulation. The coronary circulation consists of arteries that supply blood to the heart and cardiac veins that provide venous drainage. The vessels of the coronary circulation frequently vary in their development and distribution of blood to the heart.
Ten percent of the total cardiac volume of each heartbeat is required solely to supply blood to the heart muscle.
Coronary Arteries
The two main coronary arteries are the first vessels to branch off the ascending aorta (Figs. 6.103 and 6.104). The right coronary artery arises from the base or root of the aorta (right aortic sinus) and passes anteriorly between the pulmonary trunk and right atrium to descend in the coronary groove (Figs. 6.104 and 6.105). As it reaches the diaphragmatic surface, it gives off a right marginal branch that runs toward the apex of the heart. The right coronary artery then turns to the left and enters the posterior interventricular groove, where it gives off the posterior descending artery (posterior interventricular branch). The posterior descending artery continues to descend along the posterior interventricular groove toward the apex, where it commonly anastomoses with the left anterior descending artery of the left coronary artery (Fig. 6.103). The right coronary artery and its branches supply the right atrium, right ventricle, interventricular septum, and the SA and AV nodes. It also supplies a portion of the left atrium and ventricle (Figs. 6.104-6.108). The left coronary artery arises from the left aortic sinus and passes to the left between the pulmonary trunk and left atrium to reach the coronary groove (Figs. 6.103 and 6.104). Soon after reaching the coronary groove, the left coronary artery divides into the circumflex and left anterior descending arteries (Figs. 6.104 and 6.105). The circumflex artery winds around the left border of the heart to the posterior surface, where it gives off the left marginal artery that passes obliquely toward the apex of the heart. The left anterior descending artery (LAD) descends in the anterior interventricular groove toward the apex of the heart, where it reaches the diaphragmatic surface and will sometimes anastomose with the posterior descending artery. Along its course, the LAD gives off diagonal branches that supply the interventricular septum, including the AV bundles, and most of the left ventricle and atrium (Figs. 6.105-6.117).
The left anterior descending artery (LAD) is also known as the "widow maker" because many men die of blockage to this artery. The LAD can supply up to 55% of the left ventricle and is considered a key vessel for blood supply to the heart. If the artery is blocked at the beginning of its course then the rest of the vessel that supplies blood to the anterior heart wall is blocked as well. This will lead to a massive heart attack and frequently to sudden death.
Cardiac Veins
Most of the venous return from the heart is carried by the coronary sinus as it runs along the posterior section of the coronary groove and terminates in the right atrium immediately to the left of the inferior vena cava (Fig. 6.118). The coronary sinus is a wide venous channel situated in the posterior part of the coronary groove and is the main vein of the heart (Figs. 6.114, 6.115, 6.118, and 6.119). Its tributaries include the great, small, and middle cardiac veins; the left posterior ventricular vein; and the oblique vein of the left atrium (Figs. 6.67 and 6.68). The great cardiac vein, the main tributary of the coronary sinus, arises near the apex of the heart and ascends in the anterior interventricular groove adjacent to the anterior interventricular artery to the base of the ventricles (Fig. 6.118). It receives blood from the left posterior ventricular vein and the left marginal vein before emptying into the coronary sinus. The small (right) cardiac vein runs in the coronary groove between the right atrium and ventricle and joins the coronary sinus from the right side. It receives blood from the right atrium and ventricle. The middle (posterior) cardiac vein commences at the apex of the heart and ascends along the posterior interventricular groove to the base of the heart, where it drains into the coronary sinus near the drainage site of the small cardiac vein. It receives blood from the posterior surface of both ventricles. The left posterior ventricular vein carries blood from the posterior wall of the left ventricle as it runs along the diaphragmatic surface of the left ventricle to drain into either the great cardiac vein or the coronary sinus (Fig. 6.119). The small oblique vein of the left atrium descends obliquely over the posterior wall of the left atrium and enters the left end of the coronary sinus. Two small anterior cardiac veins drain directly into the right atrium (Fig. 6.118).
FIG. 6.119 3D CT of heart with coronary sinus
OFF-AXIS CARDIAC IMAGING
In an effort to standardize nomenclature for tomographic imaging of the heart, the American Heart Association (AHA) published a statement recommending that all cardiac imaging modalities use the same nomenclature for defining tomographic imaging planes. The first AHA recommendation states that “all cardiac imaging modalities should define, orient, and display the heart using the long axis of the left ventricle and selected planes oriented at 90-degree angles relative to the long axis.” The second recommendation states, “The names for the 90-degree-oriented cardiac planes used in all imaging modalities should be vertical long axis, horizontal long axis and short axis. These correspond to the short-axis, apical two-chamber, and apical four-chamber planes traditionally used in 2D echocardiography” (Fig. 6.120). We will follow these recommendations for labeling cardiac images throughout this text. In off-axis cardiac imaging, each successive acquisition provides the landmarks for planning the next acquisition (view) and provides a logical method to obtain 90-degree viewing of the heart according to its intrinsic short and long axes. Several different methods can be used to obtain views of the cardiac planes during an examination, of which we provide an example of one method. To obtain the vertical long-axis (VLA) view, an oblique coronal image can be positioned parallel to the interventricular septum, directly through the left atrium and ventricle (Figs. 6.121-6.123). This plane closely approximates the right anterior oblique projection used in cineangiography and the two-chamber view used in echocardiography. The horizontal long-axis (HLA) view can be obtained by angling an oblique coronal image to bisect the left ventricle, bicuspid valve, and left atrium (Figs. 6.124-6.126).
FIG. 6.122 Vertical long-axis CT.
FIG. 6.123 Vertical long-axis MRI.
FIG. 6.124 Vertical long-axis CT of heart for planning horizontal long-axis images.
The HLA view demonstrates the four cardiac chambers and is comparable with the four- chamber plane used in echocardiography. The short-axis (SA) view can be obtained by using the HLA image to prescribe an oblique plane through the right and left ventricles, oriented perpendicular to the interventricular septum (Figs. 6.127-6.133). The right and left ventricles both have areas called the inlet and outlet depending on the flow of blood throughout the chambers. The inlets represent the flow of blood between the atria and the ventricles, while the outlets represent the flow of blood between the ventricles and the pulmonary and systemic circulations. The inlet of the right ventricle includes the tricuspid valve, and the outlet contains the pulmonary semilunar valve. The right ventricular outflow tract (RVOT) is used to visualize the pulmonary semilunar valve and differentiate the left ventricle from the pulmonary artery (Figs. 6.134-6.136). The inlet of the left ventricle involves the bicuspid valve, and the outlet includes the aortic semilunar valve. Typically, the left ventricular outflow tract (LVOT) defines the view that provides visualization of both bicuspid and aortic semilunar valves, as well as the left atrium, left ventricle, and ascending aorta (Figs. 6.137-6.139).
FIG. 6.127 Horizontal long-axis CT of heart for planning short-axis images.
FIG. 6.138 CT of left ventricular outflow tract.
FIG. 6.139 MRI of left ventricular outflow tract.
AZYGOS VENOUS SYSTEM
The azygos venous system, which provides collateral circulation between the inferior and superior venae cavae, can be divided into the azygos and hemiazygos veins (Fig. 6.140). Together, they drain blood from most of the posterior thoracic wall and from the bronchi, pericardium, and esophagus. The larger azygos vein ascends along the right side of the vertebral column, whereas the hemiazygos vein ascends along the left side. The hemiazygos vein crosses the vertebrae to the right behind the aorta to join the azygos vein at approximately T7-T9. The azygos vein then arches over the hilum of the right lung to empty into the posterior surface of the superior vena cava (Figs. 6.35 and 6.140-6.144).
TABLE 6.5 Muscles Associated with Respiration
|
Muscle |
Origin |
Insertion |
Action |
|
Intercostal |
Inferior border of ribs |
Superior border of ribs below |
Fixes intercostal spaces during respiration and aids forced inspiration by elevating ribs |
|
Serratus posterior superior |
Spinous processes and supraspinous ligaments of C7-T2 |
Posterior aspect of 2nd—5th ribs |
Assists forced inspiration |
|
Serratus posterior inferior |
Spinous processes and supraspinous ligaments of T11-L2 |
Posterior aspect of 9th—12th ribs |
Assists in forced expiration |
|
Levatores costarum |
Transverse processes of C7 and T1-T11 |
Rib between tubercle and angle |
Elevate the ribs |
|
Diaphragm |
Xiphoid process, costal margin, fascia over the quadratus lumborum, and psoas major muscles; vertebral bodies L1-L3 |
Central tendon of the diaphragm |
Pushes the abdominal viscera inferiorly, increasing the volume of the thoracic cavity for inspiration |
MUSCLES
Muscles Associated with Respiration
Muscles associated with respiration are the intercostal, serratus posterior superior, serratus posterior inferior, levatores costarum, and diaphragm (Table 6.5). The intercostal spaces of the ribs are filled with three layers of intercostal muscles (external, internal, and innermost layer) (Figs. 6.145-6.147). These muscles act together to elevate the ribs and expand the thoracic cavity, as well as keep the intercostal spaces somewhat rigid. The serratus posterior superior muscle spans from C7-T2 to ribs 2-5 and acts to assist forced inspiration, whereas the serratus posterior inferior muscle spans from T11- L2 to ribs 9-12 and acts to assist forced expiration (Figs. 6.148-6.150). The levatores costarum muscles arise from the transverse processes of C7 and T1-T11. They extend obliquely to insert on the rib below, between the tubercle and angle (Fig. 6.148). The levatores costarum muscles act to elevate the ribs. The diaphragm is a large dome-shaped muscle that spans the entire thoracic outlet and separates the thoracic cavity from the abdominal cavity (Figs. 6.151 and 6.152). It is the chief muscle of inspiration because it enlarges the thoracic cavity vertically as the domes move inferiorly and flatten. The muscle fibers of the diaphragm converge to be inserted into a central tendon, which is situated near the center of the diaphragm immediately below the pericardium, with which it is partially blended. The diaphragm is attached to the lumbar spine via two tendinous structures termed crura (Figs. 6.151 and 6.153-6.155).
The right crus arises from the anterior surfaces of L1-L3, whereas the left crus arises from the corresponding parts of L1-L2 only. The left and right crura join together across the ventral aspect of the abdominal aorta to form the medial arcuate ligament. Three major openings, or hiatuses, of the diaphragm allow for the passage of vessels and organs from the thorax to the abdomen. The aortic hiatus allows for the passage of the descending aorta, azygos vein, and thoracic duct. The caval hiatus allows for the passage of the inferior vena cava and the right phrenic nerve. The esophageal hiatus allows for the passage of the esophagus and the vagus nerve (Figs. 6.151 and 6.152).
Muscles of the Anterior and Lateral Thoracic Walls
Muscles of the anterior and lateral thoracic region are the pectoralis major, pectoralis minor, subclavius, and serratus anterior. Muscles associated with the movement of the upper extremity, such as the pectoralis, subclavius, and serratus anterior, can also function as accessory muscles for respiration (Fig. 6.156 and Table 6.6). For example, the pectoralis muscles (major and minor), located on the anterior surface of the chest, primarily aid in the movement of the upper limb, but the pectoralis major muscle can also act to expand the thoracic cavity on deep inspiration (Figs. 6.147 and 6.156). The subclavius, a small, triangular-shaped muscle located between the clavicle and first rib, acts alone to stabilize the clavicle and depress the shoulder (Fig. 6.5). Conjointly with the pectoralis muscles, the subclavius muscles act to raise the ribs, drawing them upward and expanding the chest, thus becoming important agents in forced inspiration. Additionally, the serratus anterior muscles aid in respiration. The serratus (meaning “sawlike”) anterior muscle is visualized on the lateral border of the thorax. It extends from the medial border of the scapula to the lateral surface of the first through eighth ribs. The primary action of the serratus anterior muscle is to laterally rotate and protract the scapula. It can, however, assist in raising the ribs for inspiration (Figs. 6.152 and 6.156; see also Chapter 9, muscles and tendons).
FIG. 6.156 Anterior view of muscles associated with the thorax.
TABLE 6.6 Muscles of the Anterior and Lateral Thoracic Walls.
|
Muscle |
Origin |
Insertion |
Action |
|
Pectoralis major |
Clavicular head—medial half of clavicle Sternal head—lateral manubrium and sternum, six upper costal cartilages |
Bicipital groove of humerus and deltoid tuberosity |
Flexes, adducts, and medially rotates arm; acts as accessory for inspiration |
|
Pectoralis minor |
Anterior surface of 3rd—5th ribs |
Coracoid process of the scapula |
Elevates ribs of scapula, protracts scapula, and assists serratus anterior |
|
Subclavius |
First rib and cartilage |
Inferior surface of the clavicle |
Depresses the shoulder and assists pectoralis in inspiration |
|
Serratus anterior |
Angles of superior 8th or 9th ribs |
Medial border of scapula |
Laterally rotates and protracts scapula |
BREAST
The female breast, or mammary gland, lies within the subcutaneous tissue overlying the pectoralis major muscle. Typically, the breast extends laterally from the sternum to the axilla and inferiorly from the second to the seventh ribs. For examination purposes, the breast can be divided into four quadrants (upper inner, upper outer, lower outer, lower inner) and the tail of Spence (Fig. 6.157). The breast consists of three layers of tissue: subcutaneous layer, mammary layer, and retromammary layer (Fig. 6.158). The subcutaneous layer contains the skin and all of the subcutaneous fat. The mammary layer consists of glandular tissue, excretory (lactiferous) ducts, and connective tissues. The glandular tissue consists of 15 to 20 lobes arranged radially around a centrally located nipple. The glandular lobes are embedded in connective tissue and fat, which give the breast its size and shape. Excretory (lactiferous) ducts extend from each lobe to the nipple, where they terminate as small openings. Cords of connective tissue coursing throughout the mammary layer, from the dermis to the thoracic fascia, are known as the suspensory ligaments of the breast, or Cooper’s ligaments. These ligaments provide support for the breasts. The retromammary layer contains muscle, deep connective tissue, and retromammary fat (Figs. 6.159 and 6.160).
Axillary lymph nodes drain the lymphatics from the breast, arm, and walls of the thorax. They are frequently clustered around the axillary vessels, the borders of the pectoralis muscles, and the posterior margin of the axilla.
FIG. 6.159 Sagittal, T1-weighted MRI of female breast.
FIG. 6.160 Axial, T1-weighted MRI of female breast.
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