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Subdivisions of the mediastinum (Fig. 3.1)
• Upper thirdainferior thyroid veins.
The mediastinum is the space located between the two pleural sacs. For
• Middle thirdaazygos system.
descriptive purposes it is divided into superior and inferior mediastinal
• Lower thirdaboth the azygos (systemic system) and left gastric
regions by a line drawn backwards horizontally from the angle of Louis Similarly the venous drainage varies throughout its length:
(manubriosternal joint) to the vertebral column (T4/5 intervertebral disc). veins (portal system).
The superior mediastinum communicates with the root of the neck The dual drainage of the lower third forms a site of portal-systemic
through the ‘thoracic inlet’. The latter opening is bounded anteriorly by anastomosis. In advanced liver cirrhosis, portal pressure rises result-
the manubrium, posteriorly by T1 vertebra and laterally by the 1st rib. ing in back-pressure on the left gastric tributaries at the lower oesoph-
The inferior mediastinum is further subdivided into the: agus. These veins become distended and fragile (oesophageal varices).
• Anterior mediastinum: the region in front of the pericardium. They are predisposed to rupture, causing potentially life-threatening
• Middle mediastinum: consists of the pericardium and heart. haemorrhage.
• Posterior mediastinum: the region between the pericardium and • Lymphatic drainage: this is to a peri-oesophageal lymph plexus and
vertebrae. then to the posterior mediastinal nodes. From here lymph drains into
supraclavicular nodes. The lower oesophagus also drains into the nodes
The contents of the mediastinum (Figs 3.1 and 3.2) around the left gastric vessels.
The oesophagus Carcinoma of the oesophagus carries an extremely poor prognosis.
• Course: the oesophagus commences as a cervical structure at the Two main histological typesbsquamous and adenocarcinomab
level of the cricoid cartilage at C6 in the neck. In the thorax the oesoph- account for the majority of tumours. The incidence of adenocarcinoma of
agus passes initially through the superior and then the posterior medi- the lower third of the oesophagus is currently increasing for unknown
astina. Having deviated slightly to the left in the neck the oesophagus reasons. Most tumours are unresectable at the time of diagnosis. The
returns to the midline in the thorax at the level of T5. From here, it insertion of stents and use of lasers to pass through tumour obstruction
passes downwards and forwards to reach the oesophageal opening in have become the principal methods of palliation.
the diaphragm (T10).
• Structure: the oesophagus is composed of four layers: The thoracic duct (Fig. 3.3)
• An inner mucosa of stratified squamous epithelium. • The cisterna chyli is a lymphatic sac that receives lymph from the
• A submucous layer. abdomen and lower half of the body. It is situated between the abdom-
• A double muscular layeralongitudinal outer layer and circular inal aorta and the right crus of the diaphragm.
inner layer. The muscle is striated in the upper two-thirds and • The thoracic duct carries lymph from the cisterna chyli through the
smooth in the lower third. thorax to drain into the left brachiocephalic vein. It usually receives
• An outer layer of areolar tissue. tributaries from the left jugular, subclavian and mediastinal lymph
• Relations: the relations of the oesophagus are shown in Fig. 3.2. On trunks, although they may open into the large neck veins directly.
the right side the oesophagus is crossed only by the azygos vein and the • On the right side the main lymph trunks from the right upper body
right vagus nerve and hence this forms the least hazardous surgical usually join and drain directly through a common tributary, the right
approach. lymph duct, into the right brachiocephalic vein.
• Arterial supply and venous drainage: owing to the length of this
structure (25 cm), the oesophagus receives arterial blood from varied The thymus gland
sources throughout its course: • This is an important component of the lymphatic system. It usually
• Upper thirdainferior thyroid artery. lies behind the manubrium (in the superior mediastinum) but can
• Middle thirdaoesophageal branches of thoracic aorta. extend to about the 4th costal cartilage in the anterior mediastinum.
• Lower thirdaleft gastric branch of coeliac artery. After puberty the thymus is gradually replaced by fat.

The mediastinum Ibthe contents of the mediastinum 11