Материал: Kaplan USMLE-1 (2013) - Anatomy

Chapter 2 • Thorax

Anterior Mediastinum

The anterior mediastinum is the small interval between the sternum and the anterior surface of the pericardium. It contains fat and areolar tissue and the infe­ rior part ofthe thymus gland. A tumor ofthe thymus (thymoma) can develop in the anterior or superior mediastina

Posterior Mediastinum

Theposteriormediastinumis located between the posterior surface ofthe pericar­ dium and the T5-Tl2 thoracic vertebrae. Inferiorly, it is closed by the diaphragm.

There are 4 vertically oriented structures coursing within the posterior medias­ tinum:

•Thoracic (descending) aorta

-Important branches are the bronchial, esophageal, and posterior inter­ costal arteries

-Passes through the aortic hiatus (with the thoracic duct) at the T12 vertebral level to become the abdominal aorta

•Esophagus

-Lies immediately posterior to the left primary bronchus and the left atrium, forming an important radiological relationship

-Is constricted (1) at its origin from the pharynx, (2) posterior to the arch of the aorta, (3) posterior to the left primary bronchus, and (4) at the esophageal hiatus of the diaphragm.

•Thoracic duct

-Lies posterior to the esophagus and between the thoracic aorta and azygos vein

-Ascends the posterior and superior mediastina and drains into the junction of the left subclavian and internal jugular veins

-Arises from the cisterna chyli in the abdomen (at vertebral level L1) and enters the mediastinum through the aortic hiatus of the diaphragm

•Azygos system ofveins

-Drains the posterior and thoracic lateral wall

-Communicates with the inferior vena cava in the abdomen and ter- minates by arching over the root of the right lung to empty into the superiorvena cava above the pericardium

-Forms a collateralvenous circulation between the inferior and superior vena cava

Middle Mediastinum

The middle mediastinum contains the heart and great vessels and pericardium and willbe discussed later.

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Chapter 2 • Thorax

The relationships ofthese structures in the superior mediastinum are best visual­ ized in a ventral to dorsal orientation between the sternum anteriorly and the vertebrae posteriorly:

• Thymus: Located posterior to the manubrium, usually atrophies in the adult and remains as fatty tissue

• Rightandleftbrachiocephalicveins: Rightvein descends almostvertically and the left vein obliquely crosses the superiormediastinumposterior to the thyrnic remnants.

The 2 veins join to form the superiorvenacava posterior to the right first costalcartilage.

The superiorvena cava descends and drains into the right atrium deep to the right third costal cartilage.

• Aortic arch and its 3 branches: Aortic arch begins and ends at the plane ofthe sternal angle and is located just inferior to the left brachiocephalic vein. As a very important radiological landmark, the origins ofthe

3 branches ofthe aortic arch (brachiocephalic,left common carotid,and left subclavian) are directly posterior to the left brachiocephalic vein.

• Trachea: Lies posterior to the aortic arch and bifurcates at the level of

T4 vertebra to form the right and left primary bronchi. The carina is an internal projection of cartilage at the bifurcation.

• Esophagus: Lies posterior to the trachea and courses posterior to the left primary bronchus to enter the posterior mediastinum

ln addition to these structures, the superior mediastinum also contains the right and leftvagus and phrenic nerves and the superior end ofthe thoracic duct.

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