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Mediastinum - Complete Anatomy

Source: Gray's Anatomy for Students

Overview and Definition

The mediastinum is a thick midline partition within the thoracic cavity. It extends:

Anteriorly: from the sternum
Posteriorly: to the thoracic vertebrae
Superiorly: from the superior thoracic aperture
Inferiorly: to the inferior thoracic aperture (diaphragm)

The pleural cavities lie on either side of it, one on the right and one on the left.

Subdivisions of the Mediastinum - lateral view showing Superior, Anterior, Middle, and Posterior compartments with rib numbering

Fig. 3.59 - Subdivisions of the Mediastinum (Gray's Anatomy for Students)

Cross-section and lateral bony landmarks of the thorax showing mediastinum position, sternal angle, and diaphragm

Fig. 3.57/3.58 - Cross-Section of the Thorax and lateral view

Subdivisions

A horizontal plane through the sternal angle (manubriosternal joint) and the intervertebral disc between TIV and TV divides the mediastinum into two major parts:

Division	Location
Superior mediastinum	Above the sternal angle plane
Inferior mediastinum	Below the sternal angle plane

The inferior mediastinum is further divided by the pericardial sac into:

Subdivision	Position
Anterior mediastinum	Between sternum and pericardium (in front)
Middle mediastinum	The pericardium + heart themselves
Posterior mediastinum	Between pericardium and thoracic vertebrae (behind)

1. Superior Mediastinum

Boundaries

Anterior: manubrium of the sternum
Posterior: bodies of T1-T4 vertebrae
Superior: oblique plane from the jugular notch upward and posteriorly to the superior border of T1 (communicates with the neck)
Inferior: horizontal plane at the sternal angle / TIV-TV disc
Lateral: mediastinal parietal pleura on each side

Contents (in order from anterior to posterior)

1. Thymus

Most anterior structure in the superior mediastinum
Lies immediately posterior to the manubrium
Bilobed, asymmetrical; may extend into the neck up to the thyroid gland, and inferiorly into the anterior mediastinum over the pericardial sac
Vital in early immune development; involutes after puberty and is largely replaced by fat in adults
Blood supply: small branches from internal thoracic arteries
Venous drainage: mainly into the left brachiocephalic vein
Lymphatics drain to parasternal nodes, tracheobronchial nodes, and neck nodes

2. Brachiocephalic Veins

Posterior to the thymus
Both form at the junction of the internal jugular and subclavian veins
Right brachiocephalic vein: short, passes vertically downward
Left brachiocephalic vein: longer, crosses the midline posterior to the manubrium (lies above the manubrium in children - clinically important for procedures)
The two unite to form the superior vena cava (SVC) posterior to the right first costal cartilage

3. Superior Vena Cava (SVC)

Formed by the union of the two brachiocephalic veins
Descends on the right side to enter the right atrium
The azygos vein arches over the root of the right lung to drain into the SVC

4. Arch of the Aorta Three major branches arise from the aortic arch:

Brachiocephalic trunk (right side) - divides into right common carotid + right subclavian arteries
Left common carotid artery
Left subclavian artery - major blood supply to the left upper limb

Ligamentum arteriosum: connects the pulmonary trunk to the arch of the aorta; remnant of the fetal ductus arteriosus (which shunted blood past the unexpanded lungs in fetal life)

5. Trachea

Descends in the midline, slightly to the right of center
Bifurcates at the level of the sternal angle (TIV/TV) into the right and left main bronchi - the carina is the internal ridge at bifurcation
Right main bronchus is wider, shorter, and more vertical than the left (hence foreign bodies preferentially go right)

6. Esophagus

Passes through the posterior part of the superior mediastinum
Lies posterior to the trachea
Continues into the posterior mediastinum

7. Nerves

Phrenic nerves (C3, C4, C5): pass through the superior mediastinum and descend into the thorax
Vagus nerves (CN X): descend through the superior mediastinum; the left recurrent laryngeal nerve loops around the arch of the aorta (at the ligamentum arteriosum) and ascends back to the larynx - this is why left-sided mediastinal masses can cause hoarseness
Left superior intercostal vein also present

8. Thoracic Duct

The main lymphatic channel of the body
Runs through the superior mediastinum

Structures in the Superior Mediastinum - major vessels and their relationships

Fig. 3.89 - Structures in the Superior Mediastinum (Gray's Anatomy for Students)

2. Anterior Mediastinum

Boundaries

Anterior: posterior surface of the body of the sternum
Posterior: anterior surface of the pericardial sac
Superior: the TIV/TV plane (the sternal angle level) - separates it from the superior mediastinum
Inferior: diaphragm
Lateral: mediastinal parietal pleura

Inferior extension of the thymus gland (major structure)
Fatty connective tissue
Lymph nodes
Mediastinal branches of the internal thoracic vessels
Sternopericardial ligaments: pass from the posterior surface of the sternum to the fibrous pericardium, helping hold the heart in position

This is the smallest of the four compartments. Clinically, anterior mediastinal masses are classically recalled as the "4 T's": Thymoma, Teratoma (germ cell tumour), Terrible lymphoma, and Thyroid (retrosternal).

3. Middle Mediastinum

The middle mediastinum is the centrally located compartment. It contains:

Pericardium

The pericardium is a fibroserous sac enclosing the heart and roots of the great vessels. It has two layers:

Fibrous pericardium (outer): tough connective tissue, defines the middle mediastinum boundaries; attached to the diaphragm inferiorly, and continuous with the adventitia of the great vessels superiorly
Serous pericardium (inner): thin, two-layered:
- Parietal layer: lines the inner fibrous pericardium
- Visceral layer (epicardium): adheres to the heart surface

The pericardial cavity is the narrow potential space between parietal and visceral serous layers. It contains a small amount of serous fluid (15-50 mL) that lubricates cardiac movement.

Pericardial sinuses (clinically important):

Transverse pericardial sinus: passage behind the ascending aorta and pulmonary trunk, and in front of the SVC and pulmonary veins. Surgeons can place a finger through it to control aortic and pulmonary trunk bleeding.
Oblique pericardial sinus: a blind recess posterior to the left atrium, between the pulmonary veins. It is accessed from below. Fluid can accumulate here and not be detected by standard pericardiocentesis.

The Heart

The heart lies obliquely in the middle mediastinum. Its surfaces are:

Anterior (sternocostal) surface: mainly the right ventricle
Posterior (base): mainly the left atrium, receiving the four pulmonary veins
Inferior (diaphragmatic) surface: right and left ventricles
Right border: right atrium
Left border: left ventricle and auricle

Great Vessels (roots)

Ascending aorta (exits left ventricle)
Pulmonary trunk (exits right ventricle)
Superior vena cava (enters right atrium)
Pulmonary veins (enter left atrium)
Inferior vena cava (enters right atrium - short intrapericardial course)

Nerves in the Middle Mediastinum

Phrenic nerves (from C3, C4, C5): descend between the pericardium and the mediastinal pleura, providing motor supply to the diaphragm and sensory supply to the pericardium, mediastinal pleura, and diaphragm

4. Posterior Mediastinum

Boundaries

Anterior: pericardial sac and diaphragm
Posterior: mid-to-lower thoracic vertebral bodies (T5-T12)
Superior: TIV/TV disc level
Inferior: diaphragm
Lateral: mediastinal parietal pleura

1. Esophagus

Muscular tube from pharynx (CVI) to stomach (TXI gastro-esophageal junction)
Descends through the thorax in a midline position, then deviates anteriorly and to the left as it approaches the diaphragm
Passes through the esophageal hiatus at vertebral level TX
Relations in the posterior mediastinum:
- Right side: covered by mediastinal pleura
- Left side: related to the thoracic aorta
- Anterior: right pulmonary artery, left main bronchus, then the left atrium (important - atrial enlargement causes dysphagia)
- Posterior: thoracic duct (crosses from right to left), hemizygos veins, posterior intercostal vessels
Surrounded by the esophageal nerve plexus (from vagus nerves); just above the diaphragm these converge into the anterior vagal trunk (mainly left vagus) and posterior vagal trunk (mainly right vagus)

2. Thoracic Aorta

Begins at the lower border of TIV (continuation of the aortic arch)
Ends at TXII, passing through the aortic hiatus (posterior to the diaphragm, between the crura) to become the abdominal aorta
Initially lies to the left of the vertebral column, then moves to the midline inferiorly

Branches of the thoracic aorta:

Branch	Description
Posterior intercostal arteries (×9 pairs)	Supply lower 9 intercostal spaces (first 2 from supreme intercostal artery)
Bronchial arteries	2 left (from thoracic aorta), 1 right (usually from 3rd intercostal)
Esophageal arteries (4-5)	Form a continuous anastomotic chain on the esophagus
Pericardial branches	Small vessels to posterior pericardium
Mediastinal branches	Supply lymph nodes, nerves, areolar tissue
Superior phrenic arteries	Supply posterior superior diaphragm
Subcostal artery	Below rib XII

3. Azygos System of Veins

A series of longitudinal vessels on each side of the vertebral column that drain the body wall and thoracic viscera into the SVC
Serves as an important collateral pathway if the inferior vena cava is obstructed (blood from lower limbs can drain via ascending lumbar veins into the azygos system and reach the SVC)
Azygos vein (right side): ascends to arch over the root of the right lung at T4, draining into the SVC
Hemi-azygos vein (left side): runs along the left side of lower thoracic vertebrae
Accessory hemi-azygos vein: above the hemi-azygos, on the left
Cross-connections exist between all three vessels

4. Thoracic Duct

The main lymphatic vessel of the body; drains all lymph below the diaphragm and from the left side of the body above
Begins in the abdomen as the cisterna chyli at the level of L1-L2
Enters the thorax through the aortic hiatus
Ascends on the right side of the vertebral column in the lower posterior mediastinum, posterior to the esophagus
Crosses to the left at around T4-T5
Exits the thorax by passing into the neck and empties into the junction of the left subclavian and left internal jugular veins
Injury (e.g., during esophagectomy or chest trauma) causes a chylothorax

5. Sympathetic Trunks

Two chains of ganglia lying lateral to the vertebral bodies
Each thoracic sympathetic trunk has 10-12 ganglia
Provide sympathetic innervation to the thoracic viscera and body wall

6. Thoracic Splanchnic Nerves

Arise from the sympathetic trunks
Greater splanchnic nerve (T5-T9): passes through the crura of the diaphragm to the celiac ganglion
Lesser splanchnic nerve (T10-T11): to the aorticorenal ganglion
Least splanchnic nerve (T12): to the renal plexus
Convey pre-ganglionic sympathetic fibers to the abdominal viscera and visceral afferent (pain) fibers

Clinical Correlations

Structure	Clinical Relevance
Left recurrent laryngeal nerve (loops around aortic arch)	Hoarseness from aortic aneurysm, left lung cancer, or lymph node enlargement
Left brachiocephalic vein (above manubrium in children)	Risk of injury during midline neck procedures in pediatric patients
Esophagus adjacent to left atrium	Left atrial enlargement (e.g., mitral stenosis) causes dysphagia
Esophageal hiatus at TX	Hiatus hernia - stomach herniates into chest
Phrenic nerve	Injury causes ipsilateral diaphragm paralysis (elevated hemidiaphragm on CXR)
SVC obstruction	SVC syndrome: facial oedema, arm swelling, dilated neck veins
Azygos vein	Collateral for IVC obstruction; azygos lobe (anatomical variant)
Aortic dissection	Fixed points (aortic valve ring, ligamentum arteriosum, median arcuate ligament) are sites of trauma
Coarctation of the aorta	Narrowing just distal to the left subclavian artery; rib notching on CXR from intercostal collaterals
Thoracic duct injury	Chylothorax (milky pleural fluid rich in chylomicrons)
Transverse pericardial sinus	Cardiac surgeons pass ligature here to clamp aorta/pulmonary trunk

Summary Table: Four Compartments at a Glance

Compartment	Key Boundary	Major Contents
Superior	Above sternal angle (TIV/TV)	Thymus, SVC, brachiocephalic veins, aortic arch + 3 branches, trachea, esophagus, thoracic duct, phrenic nerves, vagus nerves, left recurrent laryngeal nerve
Anterior	Behind sternum, in front of pericardium	Thymus (inferior extension), fat, lymph nodes, sternopericardial ligaments, internal thoracic vessels
Middle	The pericardial sac itself	Pericardium, heart, ascending aorta (root), pulmonary trunk (root), SVC, pulmonary veins, IVC (short segment), phrenic nerves
Posterior	Behind pericardium, in front of T5-T12	Esophagus, thoracic aorta, azygos/hemiazygos veins, thoracic duct, sympathetic trunks, thoracic splanchnic nerves

Gray's Anatomy for Students, pp. 165-166, 220, 252-268

Genarte mcqs

Here are 25 high-yield MCQs on Mediastinum Anatomy — exam-style, with answers and explanations:

🧠 Mediastinum Anatomy - MCQs

Q1. The mediastinum is divided into superior and inferior parts by a plane passing through which of the following?

A) Jugular notch and T1
B) Sternal angle and TIV/TV intervertebral disc
C) Xiphoid process and TVIII
D) Manubriosternal joint and TIII

✅ Answer: B The sternal angle (angle of Louis) corresponds to the TIV/TV disc level. This horizontal plane is the key landmark dividing the mediastinum into superior and inferior parts.

Q2. Which of the following structures is the MOST anterior in the superior mediastinum?

A) Trachea
B) Arch of aorta
C) Thymus
D) Esophagus

✅ Answer: C The thymus is the most anterior structure, lying immediately posterior to the manubrium. The order front to back is: Thymus → Brachiocephalic veins → Aortic arch → Trachea → Esophagus.

Q3. A 65-year-old patient develops sudden hoarseness after being diagnosed with a left-sided lung mass. Which nerve is most likely compressed?

A) Right recurrent laryngeal nerve
B) Left phrenic nerve
C) Left recurrent laryngeal nerve
D) Left vagus nerve at the jugular foramen

✅ Answer: C The left recurrent laryngeal nerve loops under the arch of the aorta at the ligamentum arteriosum, making it vulnerable to compression by left-sided mediastinal and lung masses. The right RLN loops around the subclavian artery in the neck and is not in the mediastinum.

Q4. Which structure passes through the esophageal hiatus of the diaphragm?

A) Thoracic duct
B) Azygos vein
C) Thoracic aorta
D) Esophagus, anterior and posterior vagal trunks

✅ Answer: D The esophageal hiatus (at TX) transmits the esophagus plus the anterior and posterior vagal trunks. The aorta passes through the aortic hiatus (TXII), and the IVC through the caval opening (TVIII).

Q5. The left brachiocephalic vein is clinically important in children because it:

A) Drains directly into the azygos vein
B) Passes above the superior border of the manubrium
C) Contains valves that can obstruct blood flow
D) Receives the thoracic duct

✅ Answer: B In infants and children, the left brachiocephalic vein rises above the manubrium and is therefore less protected by bone, making it vulnerable during midline neck procedures. In adults, it passes posterior to the manubrium.

Q6. Which of the following is NOT a content of the posterior mediastinum?

A) Thoracic duct
B) Azygos vein
C) Phrenic nerve
D) Thoracic aorta

✅ Answer: C The phrenic nerves (C3, C4, C5) descend between the pericardium and the mediastinal pleura, making them contents of the middle mediastinum (and superior mediastinum). They are NOT in the posterior mediastinum.

Q7. A patient with mitral stenosis and marked left atrial enlargement presents with difficulty swallowing. Which structure is being compressed?

A) Trachea
B) Esophagus
C) Superior vena cava
D) Left main bronchus

✅ Answer: B The esophagus lies immediately posterior to the left atrium in the posterior mediastinum, separated only by pericardium. Significant left atrial enlargement can compress the esophagus, causing dysphagia.

Q8. The thoracic duct crosses from the right side to the left side of the vertebral column at which vertebral level?

A) T1-T2
B) T4-T5
C) T8-T9
D) T12-L1

✅ Answer: B The thoracic duct ascends on the right side in the lower posterior mediastinum, then crosses the midline to the left at approximately T4-T5. It then ascends on the left side and exits the thorax to drain into the left venous angle (junction of left subclavian and internal jugular veins).

Q9. Which compartment of the mediastinum contains the sternopericardial ligaments?

A) Superior mediastinum
B) Posterior mediastinum
C) Anterior mediastinum
D) Middle mediastinum

✅ Answer: C Sternopericardial ligaments pass from the posterior surface of the sternum to the fibrous pericardium. They are contents of the anterior mediastinum along with the thymus extension, fat, lymph nodes, and internal thoracic vessel branches.

Q10. The transverse pericardial sinus lies between which structures?

A) Posterior to the left atrium, between the pulmonary veins
B) Behind the ascending aorta/pulmonary trunk and in front of the SVC/pulmonary veins
C) Between the fibrous and serous layers of pericardium
D) Between the right and left atria

✅ Answer: B The transverse pericardial sinus is a passage posterior to the ascending aorta and pulmonary trunk, and anterior to the SVC and pulmonary veins. Cardiac surgeons use it to place clamps on these vessels. (The oblique sinus is posterior to the left atrium — option A.)

Q11. Which of the following correctly describes the aortic hiatus of the diaphragm?

A) Located at TX, transmits esophagus
B) Located at TVIII, transmits IVC
C) Located at TXII, posterior to the diaphragm between the crura
D) Located at TIX, transmits thoracic duct

✅ Answer: C The aortic hiatus is at TXII and lies posterior to the diaphragm (between the two crura), so it is technically behind the diaphragm, not a true hole through it. It transmits the thoracic aorta and the thoracic duct. Mnemonic: I 8 (ate) 10 eggs at 12 — IVC T8, Esophagus T10, Aorta T12.

Q12. The azygos vein drains into which structure?

A) Inferior vena cava
B) Right atrium directly
C) Superior vena cava
D) Left brachiocephalic vein

✅ Answer: C The azygos vein arches over the root of the right lung at approximately T4 to drain into the posterior aspect of the superior vena cava. It is an important collateral when the IVC is obstructed.

Q13. A foreign body is aspirated and lodges in a bronchus. In which bronchus is it most likely to go?

A) Left main bronchus, because it is wider
B) Right main bronchus, because it is wider, shorter, and more vertical
C) Either side equally
D) Left main bronchus, because it is longer

✅ Answer: B The right main bronchus is wider, shorter, and more vertical than the left. Foreign bodies preferentially enter the right side. Within the right lung, objects most commonly lodge in the right lower lobe bronchus.

Q14. The ligamentum arteriosum connects which two structures?

A) Ascending aorta to the SVC
B) Pulmonary trunk to the arch of the aorta
C) Ductus venosus to the portal vein
D) Left subclavian artery to the left pulmonary artery

✅ Answer: B The ligamentum arteriosum is the fibrous remnant of the fetal ductus arteriosus, connecting the pulmonary trunk to the arch of the aorta. It closes soon after birth. The left recurrent laryngeal nerve loops around it.

Q15. Which of the following is the vertebral level at which the trachea bifurcates?

A) TII/TIII
B) TIV/TV (sternal angle level)
C) TVII
D) CI/CII

✅ Answer: B The trachea bifurcates at the carina at the level of the sternal angle, which corresponds to the TIV/TV intervertebral disc. This is the same landmark that separates the superior from the inferior mediastinum.

Q16. In which mediastinal compartment would you find the oblique pericardial sinus?

A) Anterior mediastinum
B) Superior mediastinum
C) Middle mediastinum
D) Posterior mediastinum

✅ Answer: C Both pericardial sinuses (transverse and oblique) are within the pericardial sac, which is the middle mediastinum. The oblique sinus is a blind recess posterior to the left atrium between the pulmonary veins.

Q17. Injury to the thoracic duct (e.g., during esophagectomy) results in:

A) Hemothorax
B) Pneumothorax
C) Chylothorax
D) Empyema

✅ Answer: C Thoracic duct injury causes a chylothorax — accumulation of milky, chylomicron-rich lymph in the pleural cavity. It typically appears on the left side because the thoracic duct runs on the left side of the upper posterior mediastinum.

Q18. The right brachiocephalic vein receives all of the following tributaries EXCEPT:

A) Right internal thoracic vein
B) Right vertebral vein
C) Left superior intercostal vein
D) First posterior intercostal vein (right)

✅ Answer: C The left superior intercostal vein drains into the left brachiocephalic vein, not the right. All the other listed veins are tributaries of the right brachiocephalic vein.

Q19. In coarctation of the aorta, the narrowing typically occurs:

A) Proximal to the origin of the left subclavian artery
B) Just distal to the origin of the left subclavian artery
C) At the aortic valve
D) At the level of the ligamentum arteriosum proximally, before the left subclavian

✅ Answer: B Coarctation of the aorta is a congenital narrowing just distal to the origin of the left subclavian artery, near the ligamentum arteriosum. Blood supply to the lower body is reduced and collaterals develop via the intercostal arteries, causing rib notching on chest X-ray.

Q20. Which nerve supplies motor fibers to the diaphragm AND provides sensory fibers to the pericardium?

A) Vagus nerve
B) Intercostal nerves (T7-T12)
C) Phrenic nerve
D) Greater splanchnic nerve

✅ Answer: C The phrenic nerve (C3, C4, C5 - "C3, 4, 5 keeps the diaphragm alive") provides motor supply to the diaphragm and sensory fibers to the central diaphragm, pericardium, and mediastinal pleura. Referred pain from the diaphragm goes to the shoulder (C4 distribution).

Q21. Which of the following structures is anterior to the esophagus at the level below the tracheal bifurcation?

A) Thoracic aorta
B) Thoracic duct
C) Left atrium
D) Azygos vein

✅ Answer: C Below the tracheal bifurcation, the esophagus is immediately posterior to the left atrium (separated only by pericardium). This is why atrial enlargement causes dysphagia and why transesophageal echocardiography gives excellent views of the left atrium.

Q22. The greater splanchnic nerve arises from which thoracic sympathetic ganglia?

A) T1-T4
B) T5-T9
C) T10-T11
D) T12

✅ Answer: B The greater splanchnic nerve arises from T5-T9 ganglia and passes through the diaphragmatic crura to the celiac ganglion. The lesser splanchnic is T10-T11 (to aorticorenal ganglion), and the least splanchnic is T12 (to the renal plexus).

Q23. Which statement about the thymus is CORRECT?

A) It is largest in the elderly adult
B) Venous drainage is mainly into the right brachiocephalic vein
C) It develops from the third pharyngeal pouch
D) It is the most posterior structure in the superior mediastinum

✅ Answer: C The thymus develops from the third pharyngeal pouch (same as the inferior parathyroid glands), making it a common site for ectopic parathyroid tissue. It is the most anterior structure (not posterior), venous drainage is into the left brachiocephalic vein, and it is largest in childhood.

Q24. A trauma patient sustains a deceleration injury in a road accident. Aortic trauma is most likely at which location?

A) At the origin of the brachiocephalic trunk
B) At the ligamentum arteriosum
C) At the origin of the left subclavian artery
D) At the root of the aorta in the aortic sinus

✅ Answer: B The aorta has three fixed points: the aortic valve ring, the ligamentum arteriosum, and the point where it passes behind the median arcuate ligament. Deceleration injuries most commonly tear the aorta at the ligamentum arteriosum (the junction of the arch and descending thoracic aorta). This is seen in high-speed MVAs.

Q25. Which of the following correctly pairs the structure with the vertebral level at which it crosses the diaphragm?

A) IVC - T10, Esophagus - T8, Aorta - T12
B) IVC - T8, Esophagus - T10, Aorta - T12
C) IVC - T12, Esophagus - T8, Aorta - T10
D) IVC - T8, Esophagus - T12, Aorta - T10

✅ Answer: B Classic mnemonic: "I 8 (ate) 10 eggs at 12"

IVC = T8
Esophagus = T10
Aorta = T12

Score Yourself

Score	Level
23-25	Excellent - exam ready
18-22	Good - review weak areas
13-17	Fair - re-read the anatomy
< 13	Needs focused revision

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