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The mediastinum is the central compartment of the thoracic cavity, situated between the two pleural sacs. It acts as a partition separating the right and left pleural cavities and contains nearly all thoracic organs except the lungs and pleurae.
Superiorly: Thoracic inlet (plane of 1st rib and manubrium).
Inferiorly: Diaphragm.
Anteriorly: Sternum.
Posteriorly: Bodies of thoracic vertebrae (T1–T12).
Laterally: Mediastinal pleura of both lungs.
The mediastinum is divided into superior and inferior parts by an imaginary plane passing from the sternal angle (Angle of Louis) to the lower border of T4 vertebra (the transverse thoracic plane).
Superior Mediastinum — above this plane.
Inferior Mediastinum — below this plane, further divided into:
Anterior mediastinum (in front of pericardium)
Middle mediastinum (containing heart and pericardium)
Posterior mediastinum (behind pericardium)
(You requested not to go into details of each mediastinum, so substructures are omitted.)
The mediastinum contains:
Heart and great vessels
Trachea and primary bronchi
Oesophagus
Thymus
Thoracic duct
Lymph nodes and nerves (vagus, phrenic, sympathetic trunks)
During dissection:
The mediastinum is approached by removing the sternum (median sternotomy).
The pericardium enclosing the heart is exposed centrally.
The great vessels, trachea, and oesophagus are identified in relation to it.
The phrenic nerves lie on each side of the pericardium, while vagus nerves pass posteriorly.
The thoracic duct runs upward in the posterior mediastinum, between the aorta and azygos vein.
Mediastinitis: infection following oesophageal rupture or surgery.
Mediastinal masses: may compress trachea, superior vena cava, or heart.
Mediastinal shift: occurs due to pleural effusion, pneumothorax, or lung collapse.
Mediastinoscopy: used for biopsy and diagnostic visualization of mediastinal structures.
Anteriorly: Manubrium sterni.
Posteriorly: Upper four thoracic vertebrae (T1–T4).
Superiorly: Thoracic inlet (plane of 1st rib).
Inferiorly: Transverse thoracic plane — from the sternal angle to the lower border of T4 vertebra.
Laterally: Mediastinal pleura of the right and left lungs.
1. Thymus (or its remnants in adults)
Lies just behind the manubrium.
Large and active in children; undergoes fatty degeneration in adults.
Supplied by internal thoracic and inferior thyroid arteries.
2. Large Veins
Brachiocephalic veins (right and left): formed by union of internal jugular and subclavian veins behind sternoclavicular joints.
Superior vena cava (SVC): formed by union of brachiocephalic veins and descends vertically to enter the right atrium.
Left superior intercostal vein: drains upper intercostal spaces into left brachiocephalic vein.
3. Large Arteries
Arch of the aorta and its three branches:
Brachiocephalic trunk → divides into right common carotid and right subclavian arteries.
Left common carotid artery.
Left subclavian artery.
Arch gives rise to aortic knuckle on chest X-ray.
Ligamentum arteriosum connects arch to pulmonary trunk — remnant of fetal ductus arteriosus.
4. Nerves
Vagus nerves (right and left):
Right → passes behind root of right lung.
Left → gives off left recurrent laryngeal nerve that hooks under aortic arch.
Phrenic nerves: descend anterior to root of lungs, supplying diaphragm.
Cardiac nerves: from cervical sympathetic chain and vagus; form cardiac plexuses.
5. Trachea and Esophagus
Trachea: lies anterior to esophagus, begins at lower border of cricoid cartilage (C6) and ends at sternal angle (T4) where it bifurcates.
Esophagus: flattened muscular tube, posterior to trachea, entering thorax behind the aortic arch.
6. Thoracic Duct
Lies on the left side of esophagus, between aorta and azygos vein.
Crosses to the left at T5 and opens into the junction of the left subclavian and internal jugular veins.
From anterior to posterior, remember the mnemonic:
“Thymus – Veins – Arteries – Trachea – Esophagus – Duct – Spine”
Mediastinal widening on chest X-ray may indicate aortic aneurysm, lymphadenopathy, or mediastinal tumors.
Compression of trachea or SVC by enlarged thymus, aortic aneurysm, or malignancy causes dyspnea and venous congestion.
Aortic aneurysm may cause hoarseness of voice by pressing on the left recurrent laryngeal nerve.
Thymoma (tumor of thymus) is associated with myasthenia gravis.
The superior mediastinum is thus the highway of the thorax, through which nearly every major vascular, nervous, and aerodigestive structure passes before continuing to the neck, lungs, or abdomen.
The inferior mediastinum is the portion of the mediastinum below the transverse thoracic plane (from the sternal angle to the lower border of T4 vertebra) and above the diaphragm.
It is divided into three parts — anterior, middle, and posterior — depending on their relation to the pericardium.
Superior: Transverse thoracic plane.
Inferior: Diaphragm.
Anterior: Sternum and costal cartilages.
Posterior: Bodies of T5–T12 vertebrae.
Lateral: Mediastinal pleura of lungs.
(Lies between the sternum and the pericardium)
Contents:
Loose areolar connective tissue and fat.
Remnants of thymus (in adults).
Lymph nodes and small branches of internal thoracic vessels.
Sternopericardial ligaments attaching pericardium to sternum.
Clinical Note:
Site for retrosternal goitre or thymic cysts/tumors.
Can be approached surgically through a sternal incision.
(Lies between the anterior and posterior mediastina — essentially occupied by the pericardium and its contents.)
Contents:
Pericardium and heart.
Ascending aorta, pulmonary trunk, and SVC.
Main bronchi and pulmonary vessels at the roots of the lungs.
Phrenic nerves with pericardiacophrenic vessels running anterior to the root of each lung.
Lymph nodes (tracheobronchial and cardiac).
Clinical Note:
Pericardial effusion (fluid accumulation) may compress the heart, causing cardiac tamponade.
Pericardiocentesis is done through the left 5th intercostal space near sternum.
(Lies behind the pericardium and in front of the vertebral column.)
Contents:
Descending thoracic aorta and its branches (posterior intercostal, esophageal, bronchial arteries).
Azygos vein (on right) and hemiazygos/accessory hemiazygos veins (on left).
Thoracic duct — lies between aorta and azygos vein.
Esophagus and esophageal nerve plexus (from vagus).
Sympathetic trunks and splanchnic nerves (greater, lesser, least).
Clinical Note:
Common site for aortic aneurysm, esophageal carcinoma, or lymph node enlargement.
Posterior mediastinal tumors may compress the esophagus or spinal nerves.
A-M-P:
A → Anterior: Areolar tissue, remnants of thymus.
M → Middle: Heart and pericardium.
P → Posterior: Pipes (aorta, esophagus, thoracic duct) and nerves (vagus, splanchnic, sympathetic).
The middle mediastinum forms the cardiac silhouette on chest X-ray.
Knowledge of mediastinal subdivisions is essential for locating masses or lymph nodes in radiological imaging (CT, MRI).
Mediastinal shift toward one side indicates lung collapse; shift away suggests pleural effusion or pneumothorax.
The inferior mediastinum thus acts as a corridor of circulation and innervation, housing the heart, major vessels, esophagus, and autonomic pathways that link the thoracic and abdominal cavities.
The anterior mediastinum is the smallest subdivision of the inferior mediastinum.
It lies in front of the pericardium and behind the sternum.
Anteriorly: Body of sternum and left costal cartilages (4th–7th).
Posteriorly: Fibrous pericardium.
Superiorly: Continuous with superior mediastinum at the level of the sternal angle.
Inferiorly: Diaphragm.
Laterally: Mediastinal pleura of both lungs.
The space contains mainly connective and lymphatic structures, along with remnants of embryonic tissues.
Loose areolar connective tissue and fat
Remnants of thymus (more prominent in children, replaced by fat in adults)
Lymph nodes and small lymph vessels (draining anterior part of pericardium and diaphragm)
Small branches of internal thoracic arteries and veins
Sternopericardial ligaments, which anchor the fibrous pericardium to the posterior surface of the sternum
Anteriorly: Sternum and anterior thoracic wall.
Posteriorly: Pericardium enclosing the heart.
Superiorly: Continuous with superior mediastinum.
Inferiorly: In contact with upper surface of diaphragm.
The thymus originates from the third pharyngeal pouch and descends into the anterior mediastinum.
It is large in childhood, aiding in immune function, and later involutes into fatty tissue in adulthood.
Thymic enlargement (thymoma) may cause pressure symptoms on the trachea or great veins.
Retrosternal goitre or anterior mediastinal tumors may project into this space and cause dyspnea, dysphagia, or venous congestion.
Pericardiocentesis may be performed through this region — a needle is inserted through the 5th or 6th left intercostal space near the sternum to drain fluid from the pericardial cavity.
Anterior mediastinal masses often include thymic, teratomatous, thyroid, and lymphoid origins — remembered by the mnemonic “4 T’s”:
Thymoma, Teratoma, Thyroid (ectopic), Terrible lymphoma.
“Fat Thymic Lymph Tissue Small Vessels Ligaments” →
Fat, Thymus remnants, Lymph nodes, Small vessels, Sternopericardial ligaments.
The anterior mediastinum, though narrow, has great diagnostic and surgical relevance — being the site of important thymic and lymphatic structures, and a key approach area for pericardial and thoracic interventions.
The middle mediastinum is the largest and most central part of the inferior mediastinum.
It contains the heart enclosed in the pericardium and structures closely related to it.
Anteriorly: Anterior mediastinum (behind the sternum).
Posteriorly: Posterior mediastinum (in front of vertebral column).
Superiorly: Continuous with superior mediastinum (at the level of sternal angle).
Inferiorly: Diaphragm.
Laterally: Mediastinal pleura and roots of the lungs.
The middle mediastinum houses the heart and its major vascular and neural connections.
Pericardium (fibrous and serous layers) enclosing the heart.
Heart itself — with atria, ventricles, and coronary vessels.
Roots of the great vessels:
Ascending aorta (from left ventricle, giving coronary arteries).
Pulmonary trunk (dividing into right and left pulmonary arteries).
Superior vena cava (receiving blood from head, neck, and upper limbs).
Inferior vena cava (opening into right atrium).
Pulmonary veins (two from each lung entering left atrium).
Phrenic nerves — descend on each side between pleura and pericardium to the diaphragm.
Pericardiacophrenic vessels — accompany phrenic nerves.
Tracheal bifurcation and main bronchi.
Lymph nodes — tracheobronchial and cardiac nodes.
Small portions of the vagus nerves near the roots of lungs.
Anteriorly: Sternum and anterior mediastinum.
Posteriorly: Posterior mediastinum (containing esophagus and descending aorta).
Superiorly: Continuous with great vessels of superior mediastinum.
Inferiorly: Fibrous pericardium rests on the diaphragm.
Fibrous pericardium: Tough outer layer attached to the diaphragm and sternum (via sternopericardial ligaments).
Serous pericardium: Has parietal and visceral (epicardium) layers enclosing pericardial cavity containing a thin film of lubricating fluid.
Pericardial effusion: Accumulation of fluid in the pericardial cavity compresses the heart, leading to cardiac tamponade — impaired filling of ventricles and circulatory collapse.
Pericardiocentesis: Fluid is aspirated via a needle inserted in the left 5th intercostal space close to the sternum, avoiding injury to coronary vessels.
Mediastinal widening or shift: Seen in cases of enlarged heart, pericardial effusion, or tumors.
Phrenic nerve palsy: May lead to paralysis of diaphragm on the affected side.
Trauma or surgery in this area requires care to preserve phrenic and vagus nerves.
“HAPPIE Nerves”
H → Heart
A → Ascending aorta
P → Pulmonary trunk and veins
P → Pericardium
I → Inferior and superior vena cava
E → Epicardium
Nerves → Phrenic and vagus
The middle mediastinum forms the central hub of thoracic circulation, housing the heart and major vessels, the motor nerves to the diaphragm, and the primary bronchi — the vital crossroads of the cardiovascular and respiratory systems.
The posterior mediastinum is the rearmost part of the inferior mediastinum.
It lies behind the pericardium and in front of the vertebral column, forming a deep vertical compartment.
Anteriorly: Fibrous pericardium and diaphragm.
Posteriorly: Bodies of mid and lower thoracic vertebrae (T5–T12).
Superiorly: Continuous with superior mediastinum (at T4 level).
Inferiorly: Diaphragm.
Laterally: Mediastinal pleura of both lungs.
The posterior mediastinum serves as a conduit between the thorax and abdomen, transmitting several major structures.
Descending thoracic aorta
Continuation of aortic arch beginning at T4 level.
Descends on the left side of the vertebral column.
Branches:
Posterior intercostal arteries (3rd–11th spaces)
Subcostal arteries
Bronchial arteries (to bronchi and lungs)
Esophageal arteries
Pericardial and mediastinal branches
Azygos venous system
Provides venous drainage of thoracic wall to superior vena cava (SVC).
Azygos vein: on right side, ascends and arches over right lung root to open into SVC.
Hemiazygos vein: on left, crosses at T9 to join azygos.
Accessory hemiazygos vein: drains upper left thoracic wall and joins azygos around T7–T8.
Thoracic duct
Largest lymphatic channel of the body.
Begins at cisterna chyli (L1–L2), passes through aortic opening of diaphragm, and ascends between aorta and azygos vein, behind the esophagus.
Crosses to the left side at T5 and opens into the junction of left subclavian and internal jugular veins.
Function: drains lymph from entire body except right upper quadrant.
Esophagus
Muscular tube connecting pharynx to stomach.
Lies posterior to trachea in upper mediastinum, then anterior to vertebral column here.
Crossed by aortic arch and left bronchus in upper part, descends behind the heart, and passes through the esophageal hiatus (T10) of the diaphragm.
Supplied by esophageal branches of aorta and left gastric artery.
Vagus nerves and esophageal plexus
Right and left vagus nerves form a plexus around the esophagus, then reform below as anterior and posterior vagal trunks, which pass through the diaphragm with the esophagus.
Provide parasympathetic supply to thoracic and abdominal viscera.
Sympathetic trunks and splanchnic nerves
Sympathetic chain lies along the heads of ribs on either side of vertebral column.
Gives off three splanchnic nerves that pierce the diaphragm to reach abdominal ganglia:
Greater splanchnic (T5–T9): to celiac ganglion.
Lesser splanchnic (T10–T11): to aorticorenal ganglion.
Least splanchnic (T12): to renal plexus.
“DATE VSS”
D → Descending aorta
A → Azygos and hemiazygos veins
T → Thoracic duct
E → Esophagus
V → Vagus nerves (forming esophageal plexus)
S → Sympathetic trunk
S → Splanchnic nerves
Pericardium → Esophagus → Thoracic duct → Aorta → Vertebral column.
Aortic aneurysm: Dilation of descending aorta may compress esophagus, thoracic duct, or left recurrent laryngeal nerve, causing dysphagia or hoarseness.
Esophageal carcinoma: Can invade mediastinal structures, leading to tracheoesophageal fistula or dysphagia.
Azygos vein enlargement: May occur in SVC obstruction, forming collateral pathway.
Posterior mediastinal tumors: Often arise from nerve sheaths or lymph nodes; can compress sympathetic chain or aorta.
Hiatus hernia: Protrusion of stomach into thoracic cavity through esophageal opening at T10.
The posterior mediastinum thus forms the communication corridor of the thorax — transmitting vital vascular, lymphatic, and autonomic structures between the neck, thoracic organs, and upper abdomen, and serving as a key region for both diagnostic imaging and surgical access.
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