Thoracic Cavity Boundaries, Compartments, and Viscera

The study of the thorax begins with a focus on the thoracic boundaries and the viscera contained within the cavity.
Key objectives include identifying the specific boundaries of the thoracic cavity.
The thoracic cavity is compartmentalized into several distinct regions, each housing specific organs and structures.

The thoracic cavity is situated within the thorax and is defined by two major openings: the thoracic inlet (superiorly) and the thoracic outlet (inferiorly).
Thoracic Inlet (Superior Boundary):
- This opening allows for communication between the thoracic cavity and the head, neck, and upper limbs.
- Anterior Boundary: The superior border of the manubrium, which is the most superior portion of the sternum.
- Lateral Boundary: The medial borders of the first rib ( $1^{st}$ rib).
- Posterior Boundary: The body of the $T1$ vertebra.
Thoracic Outlet (Inferior Boundary):
- This opening allows for communication with the abdominopelvic cavity.
- Anterior Boundary: The xiphisternal joint, located at the junction between the body of the sternum and the xiphoid process.
- Anterolateral Boundary: The costal margin, which consists of cartilage attaching ribs $7$ through $10$ to the sternum.
- Posterolateral Boundary: The lowermost two ribs, specifically rib $11$ and rib $12$ .
- Posterior Boundary: The body of the $T12$ vertebra.
- Despite its name as an "outlet," the entirety of this opening is covered by the diaphragm.

The thoracic cavity is divided into three primary compartments:
- Pulmonary Cavities: Two lateral cavities that house the lungs and the pleural membranes covers them.
- Mediastinum: A central compartment located medially between the two pulmonary cavities. It contains all other thoracic organs except the lungs.

The pleura consists of two continuous layers of serous membrane:
- Visceral Pleura: The innermost layer that attaches directly to the surface of the lungs.
- Parietal Pleura: The outer layer that attaches to the body wall and surrounding structures.
Subdivisions of the Parietal Pleura:
- Cervical Pleura: Extends into the neck, covering the apices (superior portions) of the lungs as they pass through the thoracic inlet.
- Costal Pleura: Surrounds and attaches to the inner surfaces of the ribs.
- Diaphragmatic Pleura: Covers the superior border of the diaphragm.
- Mediastinal Pleura: The medial-most portion adjacent to the mediastinum.
The Hilum of the Lung:
- The hilum is the entry and exit point for structures such as the bronchi, pulmonary arteries, and pulmonary veins.
- At this location, the visceral pleura becomes continuous with the parietal pleura (specifically the mediastinal part).
Pleural Cavity (Pleural Space):
- This is a potential space existing between the visceral and parietal layers.
- It normally contains a very small amount of viscous fluid that reduces friction, allowing the membranes to glide smoothly against each other during inspiration and expiration.
- It is termed a "potential space" because it is normally empty (save for the fluid), but can become a "real space" when occupied by air, fluid, or other materials due to pathology.

Pathological accumulation of substances within the pleural cavity transforms the potential space into a real space:
- Pneumothorax: Accumulation of air within the cavity, which may originate from the outside (e.g., a stab wound to the chest) or from the inside (due to lung disease).
- Hydrothorax: Accumulation of water, typically a clear fluid known as transudate leaking from blood vessels.
- Hemothorax: Accumulation of blood, usually resulting from the rupture of blood vessels.
- Empyema: Accumulation of pus, which contains dead bacteria, debris, and white blood cells known as leukocytes, typically resulting from infection.

The mediastinum is divided by the Transverse Thoracic Plane.
This imaginary horizontal line passes from the manubriosternal junction (sternal angle) anteriorly to the intervertebral disc between the $T4$ and $T5$ vertebrae posteriorly.
Superior Mediastinum: The region located above the transverse thoracic plane.
Inferior Mediastinum: The region located below the transverse thoracic plane, further subdivided into three parts:
- Anterior Mediastinum: Located posterior to the sternum and anterior to the pericardium.
- Middle Mediastinum: Coincides with the layers of the pericardium; contains the heart.
- Posterior Mediastinum: Located posterior to the pericardium and anterior to the vertebral column.

Located superior to the transverse thoracic plane, this compartment contains several critical structures:
Thymus Gland (Superior Portion):
- A lymphatic organ where T lymphocytes undergo maturation and selection.
- Functional in children and adolescents, but it undergoes involution and atrophy (atrophies/degenerates) as a person ages.
- Due to this degeneration, the gland is typically not visible in cadaveric specimens, which have a median age of approximately $70$ years.
Great Vessels:
- Aortic Arch.
- Superior Vena Cava (SVC).
- Brachiocephalic Veins (which join to form the SVC).
- Pulmonary Arteries (entering the lungs posterior to the ascending aorta and arch).
Nervous Structures:
- Vagus Nerve: Descends from the neck adjacent to the common carotid arteries.
- Phrenic Nerve: Descends from the neck, running lateral to the heart but medial to the lungs.
- Left Recurrent Laryngeal Nerve: A branch of the vagus nerve that hooks around the arch of the aorta and ascends back to the neck to supply the larynx.
Airway and Digestive Tract:
- Trachea: The point where the trachea bifurcates into the main bronchi is located at the level of the sternal angle.
- Esophagus: Located posterior to the trachea.

Anterior Mediastinum:
- Contains the inferior portion of the thymus gland (in young individuals).
- Adipose tissue (fat) for heart cushioning.
- Sternopericardial ligaments, which anchor the pericardium to the anterior thoracic wall.
Middle Mediastinum:
- The heart and the surrounding pericardium.
- Origins of great vessels: Superior Vena Cava (bringing deoxygenated blood to the heart), Ascending Aorta (pumping oxygenated blood from the left ventricle), and the Pulmonary Trunk (pumping deoxygenated blood from the right ventricle toward the lungs).
- Main bronchi (positioned posterior to the vascular structures).
Posterior Mediastinum:
- Esophagus (continuing its route toward the abdomen).
- Descending Aorta (continuation of the aortic arch).
- Vagus Nerves (left and right), which travel alongside the esophagus.
- Azygos Vein: Positioned posteriorly against the vertebral column, it drains venous blood from the thoracic cage and delivers it to the superior vena cava.