Anatomy of the Thorax

Thorax

• The thorax (or chest) is the body region between the neck and abdomen.
• The bony framework of the thorax is formed by the vertebral column, ribs, and costal cartilages and the sternum.
• It connects to the neck superiorly and the abdomen inferiorly, separated by the diaphragm.
• The thorax protects the lungs and heart and provides attachments for muscles of the thorax, upper extremities, abdomen, and back.

Structure of the Thoracic Cage

• The thoracic cage is composed of the thoracic vertebrae posteriorly, the sternum and costal cartilages anteriorly, and the ribs and intercostal spaces laterally.
• It is covered by skin and muscles attaching the shoulder girdle to the trunk.
• The internal lining of the thoracic wall is the parietal pleura.
• Superiorly, the thoracic wall is formed by the suprapleural membrane, and inferiorly, by the diaphragm, which separates the thoracic cavity from the abdominal cavity.

Bony Framework of the Thorax

• The framework consists of the sternum, ribs, and thoracic vertebrae.
• The sternum has three parts: manubrium, body, and xiphoid process.
• Ribs are classified as true (first through seventh), false (eight through tenth), and floating (eleventh and twelfth).

Thoracic Openings

• The superior thoracic aperture is the anatomical thoracic inlet.
• It's bounded posteriorly by vertebra T1, laterally by the first pair of ribs and their costal cartilages, and anteriorly by the superior border of the manubrium.
• The inferior thoracic aperture is the anatomical thoracic outlet.
• It's bounded posteriorly by the twelfth thoracic vertebra, posterolaterally by the eleventh and twelfth pairs of ribs, anterolaterally by the joined costal cartilages of ribs 7 and 10 (forming the costal margins), and anteriorly by the xiphisternal joint.

The Ribs

• Ribs are curved, flat bones forming the anterolateral walls of the thoracic cage.
• They are remarkably light but resilient.
• Each rib has a spongy interior containing bone marrow, which produces blood cells.
• Ribs are classified as true, false, and floating depending on their connection to the sternum.

Structure of a Typical Rib

• Ribs typically have a head, neck, tubercle, and body (shaft).
• The head is wedge-shaped with two facets, separated by the crest of the head, for articulation with the vertebral bodies.
• The neck connects the head to the body, and the tubercle is at the junction of the neck and body, with a smooth articular part for transverse process articulation and a rough non-articular part for costotransverse ligament attachments.
• The body is thin, flat, and curved, with a costal groove along its inferior border for intercostal nerve and vessel protection.

Atypical Ribs

• The first rib is the broadest and nearly horizontal, shortest, and most sharply curved of the true ribs.
• It has a single facet on its head articulating with T1 vertebra and two transverse grooves for subclavian vessels, separated by a scalene tubercle and ridge.
• The second rib has no costal groove and a rough area on its upper surface (tuberosity) for serratus anterior origin.
• The 10th and 12th ribs have only one facet on their heads articulating with a single vertebra.
• The 11th and 12th ribs are short and have no neck or tubercle.

Intercostal Spaces

• Intercostal spaces are the areas between adjacent ribs and their costal cartilages.
• There are 11 intercostal spaces and 11 intercostal nerves.
• The spaces contain intercostal muscles, membranes, and two sets of intercostal blood vessels and nerves.
• The space below the 12th rib is the subcostal space, containing the subcostal nerve.

Intercostal Nerves

• Intercostal nerves are the anterior rami of the upper 11 thoracic spinal nerves.
• They are typically divided into anterior and posterior branches.
• The first two thoracic nerves and the last contribute to the brachial plexus supplying the upper limbs.

Muscles of Respiration

• The main inspiratory muscle is the diaphragm.
• Accessory muscles of inspiration are the sternocleidomastoid, scalenes, and pectoralis minor.
• The expiratory muscles are the internal intercostals and abdominal muscles.

Blood Supply to the Thoracic Wall

• Blood supply to the thoracic wall is primarily from two sources: anterior intercostal arteries (formed by branches from internal thoracic artery and musculophrenic artery (lower ribs)) and posterior intercostal arteries (branches from descending aorta).

The Sternum

• The sternum is a flat bone located in the midline of the anterior chest wall.
• It's divided into three parts: the manubrium, body, and xiphoid process.
• It articulates with the first seven pairs of ribs.

The Diaphragm

• The diaphragm is a dome-shaped muscle that separates the thoracic and abdominal cavities.
• It has openings for the vena cava, esophagus, and aorta.

Development of the Diaphragm

• The diaphragm develops from four embryonic structures: the septum transversum, the pleuroperitoneal membranes, and the dorsal mesentery of the esophagus.

Diaphragmatic Hernia

• Congenital diaphragmatic hernia occurs due to incomplete fusion of the septum transversum, dorsal mesentery, and pleuroperitoneal membranes.
• Acquired hernias, common in middle-aged individuals, are caused by weak musculature around the esophageal opening in the diaphragm, typically presenting as sliding or paraesophageal hernias.

Pleural Cavity

• A costodiaphragmatic recess is the space between the costal and diaphragmatic pleurae.
• Costomediastinal recesses are slit-like spaces where the costal pleurae reflect to become mediastinal pleura.

Nerve Supply of the Pleura

• Parietal pleura is sensitive to pain, temperature, touch, and pressure and receives segmental supply from intercostal nerves.
• Mediastinal pleura is supplied by the phrenic nerves.
• Diaphragmatic pleura is supplied by phrenic nerves and lower six intercostal nerves.
• Visceral pleura is insensitive to pain but sensitive to stretch and pressure. It receives autonomic nerve supply from the pulmonary plexus.

Clinical Notes

• Air in the pleural cavity with serous fluid is known as hydropneumothorax, with pus as pyopneumothorax, and with blood as hemopneumothorax.
• A collection of pus in the pleural cavity (without air) is called an empyema

The Trachea

• The trachea is a continuation of the larynx, running from the cricoid cartilage to the carina.
• It bifurcates into the right and left main bronchi at the sternal angle.
• The superior mediastinum's trachea is related to the sternum, thymus, left brachiocephalic vein, left and right common carotid arteries, and the arch of the aorta posteriorly.
• The right side of the trachea relates to the azygos vein, right vagus nerve, and pleura.
• The left side of the trachea relates to the arch of the aorta, left common carotid and subclavian arteries, left vagus and phrenic nerves, and pleura.

Blood Supply and Lymph Drainage of the Trachea

• The upper two-thirds of the trachea are supplied by inferior thyroid arteries, while the lower third is supplied by bronchial arteries.
• Lymph drainage goes to pretracheal, paratracheal lymph, and deep cervical nodes.
• Sensory nerve supply originates from vagus and recurrent laryngeal nerves.
• Sympathetic nerves supply the trachealis muscle.

The Principal Bronchi

• The right main bronchus is wider, shorter, and more vertical than the left.
• It divides into three lobar bronchi (upper, middle, lower).
• The left main bronchus is narrower, longer, and more horizontal than the right.
• It divides into two lobar bronchi (upper, lower).

The Lungs

• The lungs are conical, covered by visceral pleura, suspended freely in their own pleural cavities, and connected to the mediastinum only by their roots.
• Bronchopulmonary segments are the functional and surgical units of the lungs, formed by segmental bronchi and surrounded by connective tissue.
• The right lung has 10 segments, while the left lung has 8-9.

The Bronchopulmonary Segments

• The right lung's segments are apical, posterior, anterior (superior lobe), lateral, medial (middle lobe), superior (apical), medial basal, anterior basal, lateral basal, and posterior basal (inferior lobe).
• The left lung's segments are apical, posterior, anterior (superior lobe), superior lingular, inferior lingular, and superior (apical), medial basal, anterior basal, lateral basal, and posterior basal (inferior lobe).

The Pericardium

• The pericardium is a fibroserous sac surrounding the heart and the roots of the great vessels.
• It acts as a container, restricting heart movements and providing lubrication.
• It is located within the middle mediastinum, posterior to the body of the sternum and second to sixth costal cartilages, anterior to the fifth to eighth thoracic vertebrae.
• The pericardium is composed of a fibrous and serous layer, with both visceral and parietal components.

Cardiac Conducting System

. The cardiac conducting system involves the sinoatrial node, internodal pathways, atrioventricular node, atrioventricular bundle, right and left bundle branches, and Purkinje fibers.

Blood Supply to the Heart

• The coronary arteries, originating from the aorta, supply blood to the heart, branching into anterior and circumflex branches to the right and left sides.
• Cardiac veins drain blood from the cardiac muscle, ultimately joining the coronary sinus, which drains into the right atrium.

Surface Marking of the Heart

• The cardiac apex is located in the fifth left intercostal space, approximately 3.5 inches (9cm) from the midline.
• The superior border is formed by the great vessels, extending from the second left costal cartilage to the third right costal cartilage, both 0.5 inches (1.3cm) from the edge of the sternum.
• The right border extends downward from the 3rd right costal cartilage to the 6th.
• The left border extends from the second left costal cartilage to the apex beat of the heart.
• The inferior border is formed by the right ventricle and a portion of the left ventricle, extending from the sixth right costal cartilage to the apex beat.

Surface Anatomy of Heart Valves

• The tricuspid valve lies behind the right half of the sternum, at the fourth intercostal space.
• The mitral valve is positioned behind the left half of the sternum, at the fourth costal cartilage.
• The pulmonary valve is located behind the medial end of the third left costal cartilage.
• The aortic valve is behind the left half of the sternum, at the third intercostal space..

Auscultation of Heart Valves

• These valves can be auscultated according to their position; the tricuspid valve is found in the lower end of the sternum's body, the mitral valve found in the fifth left intercostal space, the aortic valve found in the second right intercostal space, and the pulmonary valve found in the second left intercostal space.

Pleural Effusion

• Pleural effusion is the abnormal accumulation of fluid in the pleural cavity, which can cause various symptoms, such as diminished breath sounds, absent breath sounds, and the trachea being displaced.

Other

• Content of Anterior Mediastinum: Thymus gland, sternal pericardial ligaments, pericardial branches of pericardiacophrenic artery, areolar connective tissue, lymph nodes

• Content of Middle Mediastinum: Heart, great vessels (aorta, pulmonary trunk, superior/inferior vena cava), trachea and bronchi, pericardial and phrenic nerves, pericardiophrenic arteries and veins

• Content of Posterior Mediastinum: Esophagus, vagus nerves (left and right), splanchnic nerves (greater, lesser, least), thoracic duct(s), and sympathetic chain

• The objectives of studying the thoracic cavity are to understand the arrangement of thoracic viscera and their relationship to each other and to the chest wall, and to define the mediastinum and arrangement of the pleura relative to the lungs as functional, essential knowledge for understanding lung function and disease.

• Surface Marking for Parietal Pleura: Both the right and left parietal pleura have characteristic anterior, lower, and cervical borders; the specific points are described for each side.