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Body Surface

The primary surface landmarks of the thorax are associated with the sternum (jugular [suprasternal] notch, sternoclavicular joint, manubrium, sternal angle and body), clavicle, ribs (2nd and 5th intercostal spaces), nipple and mid-clavicular line. The skin overlying the thorax is innervated by the anterior and lateral cutaneous branches of the intercostal nerves (ventral rami of the thoracic spinal nerves).

Skeleton and Joints

The bones and joints associated with the thorax are the sternum and ribs articulating at the sternocostal joints, and the ribs and thoracic vertebrae articulating at the costovertebral joints. The sternocostal joints are the plane (gliding) articulations between the sternum and costal cartilages supported by fibrous joint capsules and sternocostal ligaments. The costovertebral (costocorporeal) joints are the plane (gliding) articulations between the costal heads and vertebral bodies. The costotransverse joints are the plane articulations between the costal tubercles and vertebral transverse processes. The joints are supported by fibrous joint capsules and costovertebral ligaments.

Organization

The thorax includes the chest wall, pleural cavities and lungs, and mediastinum, including the heart and pericardium, separated from the abdomen by the diaphragm.

The chest wall is comprised of the sternum, ribs, and external and internal intercostal and transversus thoracis muscles. The external intercostal muscles pass from rib to rib and are oriented from superior-lateral to inferior-medial. The internal (and innermost) intercostal muscles pass from rib to rib and are oriented from superior-medial to inferior-lateral. The transversus thoracis muscles pass from the posterior sternum to the costal cartilages. The intercostal and transversus thoracis muscles elevate and depress the ribs during respiration. The diaphragm passes from the xiphoid process, lower ribs and costal cartilages, medial and lateral arcuate ligaments and lumbar vertebrae to the central tendon. The diaphragm contracts during inspiration. Endothoracic (deep) fascia covers the inner surface of the chest wall and diaphragm.

The lungs are invaginated into the closed pleural sacs. The visceral pleura is a serous membrane closely applied to the lung surface and its interlobar fissures. The parietal pleura lines the chest wall, vertebral bodies and lateral mediastinal structures. The visceral and parietal pleurae are continuous with each other where they overlie the hilar structures of the lung.

The mediastinum is positioned between the two lungs (or pleural sacs). The mediastinum is divided into superior and inferior divisions by an imaginary line extending from the sternal angle to the T4-5 intervertebral disc. The superior mediastinum contains vascular (aortic arch, brachiocephalic, left common carotid, left subclavian and internal thoracic arteries, and brachiocephalic and internal thoracic veins), visceral (esophagus, trachea, thoracic duct, thymic remnant and lymph nodes) and nervous (left recurrent laryngeal, vagus and phrenic nerves) structures.

The inferior mediastinum is further subdivided into an anterior, middle and posterior mediastinum. The anterior mediastinum is positioned between the sternal body and the pericardium, and may contain remnants of the thymus along with lymph nodes. The middle mediastinum contains the pericardium and heart along with vascular (ascending aorta, pulmonary trunk and pulmonary arteries, superior vena cava and pulmonary veins), visceral (tracheal bifurcation, main bronchi and lymph nodes) and nervous (left recurrent laryngeal, vagus and phrenic nerves) structures. The posterior mediastinum is positioned between the tracheal bifurcation and pericardium, and the vertebral column. It contains vascular (descending aorta, and azygos and hemiazygos veins), visceral (esophagus, thoracic duct and lymph nodes) and nervous (vagus and splanchnic nerves) structures.

Viscera

The right lung has three (superior, middle and inferior) and the left lung two (superior and inferior) lobes each with a branch of the bronchial tree (lobar bronchus). The oblique fissure separates the superior and inferior lobes of the left lung, and the superior and middle lobes from the inferior lobe of the right lung. The horizontal fissure separates the right superior from the right middle lobe. The lobes of the lungs are subdivided into bronchopulmonary segments. Each bronchopulmonary segment is associated with a single segmental bronchus and adjacent segmental branch of the pulmonary artery.

The heart has four chambers, right and left atria and ventricles. Blood draining from the body enters the right atrium through the superior and inferior vena cavae. From the right atrium blood flows through the right atrioventricular (tricuspid) valve into the right ventricle. Blood flows from the right ventricle to the lungs by passing through the pulmonary valve and pulmonary trunk. Oxygenated blood retuns to the left atrium through the pulmonary veins. Blood passes from the left atrium to the left ventricle through the left atrioventricular (mitral or bicuspid) valve. Blood is ejected from the left ventricle into the ascending aorta through the aortic valve.

The surface of the heart is characterized by the auricles (ear-shaped appendages) of the atria, and two grooves or sulci separating the atria and ventricles. The coronary sulcus separates the atria from the ventricles. The interventricular sulcus separates the two ventricles.

The inside of the right atrium has a rough-walled section, the pectinate muscles. A distinct ridge, the crista terminalis, borders the edge of the pectinate muscles extending between the orifices of the inferior and superior vena cavae. The smooth-walled portion of the atrial wall (sinus venarum) has the opening of the coronary sinus and a shallow depression, the fossa ovalis (oval fossa). The thick-walled right and left ventricles have columns of muscle, the trabeculae carneae, and papillary muscles and chordae tendineae associated with the cusps of the tricuspid and bicuspid valves.

Innervation

The intercostal muscles and adjacent parietal pleura are innervated by the intercostal (motor and sensory) nerves, and the diaphragm and adjacent parietal pleura are innervated by the phrenic (motor and sensory) nerves. The thoracic viscera are innervated by the vagus (sensory and parasympathetic) and cardiopulmonary splanchnic (sensory and sympathetic) nerves.

Blood Supply

The anterior and posterior intercostal arteries supply the chest wall. The anterior intercostal arteries are branches of the internal thoracic and musculophrenic arteries. The posterior intercostal arteries are branches of the costocervical trunk and thoracic aorta. The musculophrenic, lower posterior intercostal and inferior phrenic arteries supply the diaphragm. Tributaries of the internal thoracic and azygos veins drain the chest wall. The musculophrenic and inferior phrenic veins drain the diaphragm.

The segmental branches of the pulmonary arteries supply the lungs. The pulmonary veins drain the lungs into the left atrium of the heart.

Branches of the right and left coronary arteries supply the atria and ventricles of the heart. The great, middle and small cardiac veins drain the heart into the coronary sinus.

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