Sternum.

The sternum is an elongated, flat bone in the anterior midline of the thorax. Anteriorly, it is covered only by skin, superficial fascia, and periosteum. It consists of three parts: the manubrium, the body, and the xiphoid process. The manubrium is the most superior of the three parts. Its upper border is indented by a midline jugular notch, sometimes called the suprasternal notch, which is easily palpable. The jugular notch is at the level of the disc between the second and third thoracic vertebrae. Clavicular notches, located at the superolateral margins of the manubrium on either side of the jugular notch, form articulating surfaces for the clavicle.

Inferiorly, the manubrium is joined to the body of the sternum by fibrocartilage and ligaments. The manubrium and body of the sternum do not articulate in a straight line. Instead, their line of junction projects forward, forming the sternal angle (angle of Louis). This reliable landmark generally is 5 cm below the jugular notch and locates the sternal end of the second rib. It also marks the level of the intervertebral disc between the fourth and fifth thoracic vertebrae. The trachea bifurcates into the two bronchi and the aortic arch begins at the level of the sternal angle. The body, forming the bulk of the sternum, articulates with the second through seventh costal cartilages.

The smallest and most inferior part of the sternum is the xiphoid process. It consists of hyaline cartilage in youth but gradually ossifies during adulthood, so that by 40 years of age, it generally is bony in nature and is fused with the body of the sternum at the xiphisternal junction. The xiphisternal junction generally is at the level of the ninth thoracic vertebra.

Ribs.

The lateral boundaries of the chest are formed by the 12 pairs of ribs. These flat, elongated, curved, and slightly twisted bones, along with their costal cartilages, extend from the thoracic vertebrae posteriorly to the sternum anteriorly. The ribs make up the major portion of the thoracic skeleton. The first seven ribs are considered vertebrosternal ribs, or true ribs, because they articulate directly with the sternum by way of their costal cartilages. The remaining five pairs are called false ribs. The costal cartilage of the eighth, ninth, and tenth ribs (the first three pairs of false ribs) is attached to the cartilage of the preceding rib rather than directly to the sternum. These are called vertebrochondral ribs. The last two pairs of false ribs are vertebral ribs, or floating ribs, because they have no anterior attachment to the sternum. The costal cartilages of the vertebrochondral ribs join such that their inferior edges form a continuous costal margin. As the costal margins diverge from the xiphisternal junction, they delineate the infrasternal angle, or costal arch.

Thoracic Vertebrae.

The posterior median skeleton of the thoracic cage is formed by the 12 thoracic vertebrae. Features of these vertebrae that are specifically related to the thorax include facets on the transverse processes and vertebral bodies for articulation with the ribs and the long spinous processes. Features of the thoracic vertebrae are illustrated in Fig. 2-2. When the vertebral column is flexed, the most prominent spinous process usually is that of the seventh cervical vertebra, although sometimes the first thoracic spinous process may be just as evident. When the arms are at the sides, a line drawn through the tip of the third thoracic spinous process indicates the level of the base of the scapular spine. The inferior angle of the scapula is at the same level as the middle of the seventh thoracic spinous process. The position of these landmarks changes when the arms are raised.

Diaphragm.

The diaphragm covers the thoracic outlet, forming a muscular, movable partition between the thoracic and abdominal cavities. Contraction of the diaphragm enlarges the thoracic cavity in the vertical dimension during inspiration. Because the diaphragm is visualized to a greater extent in abdominal sections than in thoracic sections, it is described in greater detail in the Chapter 3.

Intercostal Muscles.

Three layers of intercostal muscles fill the spaces between the ribs. These are the external, internal, and innermost intercostal muscles, all of which receive motor impulses from the intercostal nerves. The external intercostal muscles arise from the lower border of one rib and insert on the upper limit of the next rib below. The fibers are directed inferiorly and anteriorly. The internal intercostal muscles occupy the intercostal spaces deep to the external intercostals. Also originating from the lower border of one rib and inserting on the upper limit of the next one below, the fibers are directed inferiorly and posteriorly, at right angles to the external intercostal fibers. The innermost intercostal muscles appear similar to the internal intercostals but are separated from them by a neurovascular bundle containing an intercostal nerve, artery, and vein.

Levator Costarum Muscles.

Twelve fan-shaped levator costarum muscles originate on the transverse processes of the thoracic vertebrae and insert on the rib immediately below. These are visualized on transverse sections as extending from the vertebral column to a rib. The action of the levator costarum muscles increases both the transverse and anteroposterior dimensions.

Muscles of the Pectoral Region.

Numerous muscles span the back and pectoral regions of the thorax but are functionally associated with the upper extremity. Many of these muscles anchor the arm to the trunk, as well as function in movement. Sections of the thorax also show the humerus and bones of the pectoral girdle. The pectoral girdle consists of the clavicle, or collar bone, anteriorly and the scapula, or shoulder blade, posteriorly. Because these skeletal and muscle components are clearly evident on thoracic sections, they are included here. (A more thorough discussion of the upper extremity and its associated articulations is presented in Chapter 7.)

The pectoral region is located on the anterior thoracic wall. Four muscles are associated with this region. These muscles help attach the upper limb to the thoracic skeleton. All are associated with movements of the arm either by acting directly on the humerus or by acting on the bones of the pectoral girdle. These muscles are summarized in Table 2-2 and are illustrated in Fig. 2-4.

Muscles of the Back and Shoulder Region.

The muscles of the back and shoulder region may be divided into three groups: superficial back muscles, deep back muscles, and muscles associated with the scapula. Several of these muscles are illustrated in Fig. 2-4, and they are summarized in Table 2-3.

Pleural Cavities.

The two pleural cavities are completely closed and separated from each other. They are lined by a serous membrane called the pleura. The pleura is essentially a continuous sheet in each cavity; however, for descriptive purposes, it is divided into the visceral layer and the parietal layer. You can visualize this as a balloon indented by your fist. The balloon is a single sheet of material, but when you stick your fist into it, you have two layers, the outside, or parietal, layer and the layer next to your hand, the visceral layer. The visceral pleura is intimately adherent to the lung, covering its entire surface and continuing deeply into its fissures. The parietal pleura lines the thoracic wall and is divided into four regions, as illustrated in Fig. 2-5. The costal parietal pleura is applied to the ribs, costal cartilages, intercostal muscles, and sternum. The diaphragmatic parietal pleura is fused with the diaphragm and is continuous with the mediastinal parietal pleura, which is adjacent to the mediastinum. The cervical parietal pleura projects into the thoracic inlet to cover the apex of the lung.

The space between the parietal and visceral pleural layers is the pleural cavity, or pleural space. In reality, this is a potential space filled with only a capillary layer of serous lubricating fluid. This fluid reduces friction to allow the two surfaces to glide easily over each other during respiratory movements. The visceral pleura is insensitive to pain, but the parietal layer is very sensitive.

The most inferior portion of the pleural space is located in the posterolateral region where the diaphragm attaches to the ribs. This space is called the costodiaphragmatic recess. Fluid tends to accumulate in this space when a person is in an erect position.

Lungs.

Each lung is an elongated structure shaped roughly like a half cone. The apex, or cervical dome, lies posterior to the middle third of the clavicle. It extends slightly above the first rib to project through the superior thoracic aperture. The slightly concave medial (mediastinal) surface has an opening, called the hilus, where the bronchi, blood vessels, lymph vessels, and nerves enter and leave the lung. The structures that traverse the hilus are collectively called the root of the lung. Each lung is freely movable within its own pleural cavity except at the root or hilus, where it is attached. The right and left lungs are separated from each other by the mediastinum. The base of each lung is concave as it conforms to the dome of the diaphragm. The right lung, although shorter because of the volume of the liver on that side, is wider and has a greater volume than the left lung. The heart makes an indentation, called the cardiac notch, in the left lung. Each lung is partially transected by an oblique fissure that separates the lung into superior and inferior lobes. The right lung is further subdivided by the horizontal fissure to form a wedge-shaped middle lobe. Within the lung each primary bronchus divides into secondary bronchi, two on the left side and three on the right side, providing a secondary bronchus for each lobe of the lung. Each secondary bronchus further divides into tertiary segmental bronchi that supply specific regions. A tertiary segmental bronchus with the specific sector of lung it supplies is called a bronchopulmonary segment.

Pulmonary arteries transport deoxygenated blood from the heart to the lungs, where carbon dioxide diffuses into the alveoli and ultimately is exhaled. At the same time, oxygen diffuses from the alveoli into the blood. This freshly oxygenated blood is returned to the left atrium of the heart by the pulmonary veins. In general, the blood in the pulmonary arteries and veins does not provide for the vascular needs of the lung parenchyma. Bronchial arteries, which are branches of the descending thoracic aorta, supply blood for the nutrition and maintenance of the lung tissue. Bronchial veins drain the parenchyma of the lung. On the right side, these vessels terminate in the azygos vein. On the left side, the bronchial veins drain into either the intercostal veins or the hemiazygos vein.

Afferent vessels of the tracheobronchial lymph nodes drain lymph from the lungs and bronchi. The tracheobronchial lymph nodes are divided into five groups: pulmonary lymph nodes in the substance of the lungs; bronchopulmonary lymph nodes in the hilus of each lung; inferior tracheobronchial lymph nodes and superior tracheobronchial lymph nodes at the tracheal bifurcation, in the region of the carina; and the paratracheal lymph nodes on each side of the trachea. Lymph vessels leaving these nodes join with efferent vessels of other nodes to form right and left bronchomediastinal trunks. These trunks may drain into the right lymphatic duct on the right side and the thoracic duct on the left, or they may open independently into the junction of the subclavian vein and internal jugular vein on the respective side. Pollutants from the air enter the tracheobronchial lymph nodes. These nodes usually are affected in tuberculosis and carcinoma of the lung.

Mediastinum.

The lungs and pleura occupy the lateral portions of the thoracic cavity. All other thoracic structures are crowded into a central space called the mediastinum. The mediastinum is divided into four regions, as illustrated in Fig. 2-6.

Pericardium.

A fibroserous sac, the pericardium, surrounds the heart and proximal portions of the great vessels that enter and leave the heart. There are essentially two types of pericardium, fibrous and serous. The external, strong fibrous pericardium is composed of tough fibrous connective tissue. Superiorly, the fibrous pericardium blends with the tunica externa, the fibrous connective tissue outer layer of the great vessels. Inferiorly, the fibrous layer fuses with the central tendon of the diaphragm, and respiratory movements thus influence the movement of the pericardial sac. In the anterior midline the fibrous pericardium is attached to the posterior surface of the sternum by a strong sternopericardial ligament.

The double-layered serous pericardium is composed of a thin, transparent serous membrane. The outer, or parietal, layer of serous pericardium, sometimes called parietal pericardium, forms a smooth, moist lining for the fibrous pericardium. The fibrous and parietal serous pericardia are closely adherent and difficult to separate, and together they make up the pericardial sac. The inner, or visceral, layer of serous pericardium covers the cardiac muscle of the heart wall. Because it forms the outer layer of the heart wall, the visceral serous pericardium often is called epicardium.

The two layers of serous pericardium, parietal and visceral, form a continuous closed sac around the heart in the same way that the pleura surrounds the lungs. Between the parietal and visceral pericardia is a potential space, the pericardial cavity, which contains a small amount of serous fluid that is distributed as a capillary film on the opposing surfaces. The lubricating action of this fluid keeps the surfaces moist and reduces friction so that the surfaces glide easily over each other during heart movements.

Heart Wall.

The heart wall consists of three layers. The outermost layer is the epicardium, which is the visceral layer of serous pericardium. Heart muscle, called cardiac muscle, makes up the middle layer, the myocardium. This layer makes up the bulk of the heart wall and is the layer that contracts to perform the pumping action of the heart. The thickness of the myocardium in the heart wall varies. The harder a particular chamber has to work to pump blood, the thicker the wall. The atria have relatively thin walls because they are primarily “receiving” chambers rather than “pumping” chambers. Ventricles have thick walls because they forcefully eject blood from the heart. In an adult, the left ventricle has the thickest wall because it pumps blood into systemic circulation throughout the body. The endocardium is the innermost layer. This is a thin, smooth layer of simple squamous epithelium called the endothelium. The endothelial lining of the heart also covers the heart valves and is continuous with the endothelial lining of the blood vessels.

Superficial Features of the Heart.

Superficial features of the heart include an apex, base, three surfaces, and four borders. The apex, formed entirely by the left ventricle, points downward and to the left. Located in the fifth intercostal space, at the level of the eighth thoracic vertebra, it is the most inferior region of the heart, and it is to the left of midline. These positions vary and depend on the phase of respiration.

The base of the heart is the broad superior portion of the heart that is opposite the apex. This means that the base projects superiorly, posteriorly, and to the right. It extends between the fifth and eighth thoracic vertebrae. The two atria are the primary components of the base, and the posteriorly positioned left atrium is the predominate portion of the base. The ascending aorta, pulmonary trunk, and superior vena cava emerge from the base. The base sometimes is referred to as the posterior surface.

In addition to the posterior surface, or base, the heart has two other surfaces. The anterior sternocostal surface is created primarily by the right atrium and right ventricle, although the left auricular appendage and left ventricle contribute a small portion. The two ventricles resting on the diaphragm comprise the diaphragmatic surface.

The right atrium, in line with the superior and inferior venae cavae, forms the right border. The left border is more convex and is outlined by the left ventricle. The right ventricle, with a small contribution from the left ventricle near the apex, forms the horizontal inferior border. The superior border, where the great vessels enter and leave the heart, is formed by both atria.

Chambers and Valves.

The heart is divided into four chambers: the right and left atria, and the right and left ventricles. On the surface a groove that encircles the heart separates the atria from the ventricles. This is the coronary sulcus, or atrioventricular sulcus. Similarly, an interventricular sulcus marks the division between the right and left ventricles.

Valves.

A system of valves is required to keep blood flowing through the heart in the appropriate direction. The heart has two basic types of valves. Both types consist of cusps, or flaps of fibrous tissue covered with endothelium. Semilunar valves are found at the exit ports of the ventricles. Atrioventricular valves function as inflow valves where the blood flows from the atria into the ventricles.

Semilunar valves consist of three cusps that balloon out from the vessel wall to prevent backflow of blood from the aorta and pulmonary trunk into the ventricles. When the ventricles contract, the increased pressure forces the valve cusps flat against the vessel wall, opening the valve to allow ejection of blood. After contraction, as ventricular pressure decreases, the cusps are caught in a passive backflow and balloon out from the walls to close the orifice. This prevents blood from flowing backward into the ventricles.

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1. Introduction to Sectional Anatomy
2. The Vertebral Column, Spinal Cord, and Neck
3. The Thorax
4. The Abdomen

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