The mediastinum is divided into superior and inferior compartments by an imaginary plane. Which of the following anatomical landmarks defines this plane?

Explanation

## Mediastinal Divisions: The Transverse Plane ### Anatomical Landmark **Key Point:** The mediastinum is divided into superior and inferior compartments by a transverse plane that passes through the **sternal angle (angle of Louis) at the level of the 2nd costal cartilage/rib** anteriorly and the **T4–T5 intervertebral disc** posteriorly. ### Why the 2nd Rib — Not the 4th The sternal angle (manubriosternal junction) is the articulation point of the **2nd costal cartilage** with the sternum — this is the universally accepted anatomical teaching in Gray's Anatomy, Moore's Clinically Oriented Anatomy, and Snell's Clinical Anatomy. The 2nd rib is the standard landmark used to count ribs clinically starting from the sternal angle. The 4th rib lies considerably lower and does NOT correspond to the sternal angle. ### Clinical Significance This plane is a critical anatomical reference point in thoracic anatomy: - Marks the junction between the manubrium and body of the sternum (manubriosternal joint) - Corresponds to the level of the aortic arch (beginning and end) - Marks the level of the tracheal bifurcation (carina) at T4–T5 - Defines the upper boundary of the inferior mediastinum - Azygos vein arches over the right main bronchus to enter the SVC at this level ### Mnemonic **STERNAL ANGLE = 2nd RIB:** The sternal angle is palpable clinically and always corresponds to the **2nd costal cartilage** (2nd rib). This is the standard teaching in all major anatomy textbooks and is the basis for rib counting in clinical examination. **High-Yield:** The sternal angle (angle of Louis) is the most reliable surface landmark for identifying the 2nd rib and therefore the mediastinal plane. Option A (4th rib) is incorrect — the 4th rib is at a lower level, near the inferior border of the superior mediastinum only if one miscounts from the sternal angle. ### Related Structures at This Level (T4–T5 / Sternal Angle) - Aortic arch (begins and ends) - Tracheal bifurcation (carina) - Azygos vein entry into superior vena cava - Ligamentum arteriosum attachment - Thoracic duct crosses midline *Reference: Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy, 8th ed. Chapter 1 — Thorax; Gray's Anatomy, 41st ed.*