Subdivisions of the mediastinum (Fig. 3.1)

The mediastinum is the space located between the two pleural sacs. Fordescriptive purposes it is divided into superior and inferior mediastinalregions by a line drawn backwards horizontally from the angle of Louis(Manubriosternal joint) to the vertebral column (T4 / 5 intervertebral disc).The superior mediastinum communicates with the root of the neckthrough the 'thoracic inlet'. The latter opening is bounded anteriorly bythe manubrium, posteriorly by T1 vertebra and laterally by the 1st rib.The inferior mediastinum is further subdivided into the:• Anterior mediastinum: the region in front of the pericardium.• Middle mediastinum: consists of the pericardium and heart.• Posterior mediastinum: the region between the pericardium andvertebrae.The contents of the mediastinum (Figs 3.1 and 3.2)The oesophagus• Course: the oesophagus commences as a cervical structure at thelevel of the cricoid cartilage at C6 in the neck. In the thorax the oesophaguspasses initially through the superior and then the posterior mediastina.Having deviated slightly to the left in the neck the oesophagusreturns to the midline in the thorax at the level of T5. From here, itpasses downwards and forwards to reach the oesophageal opening inthe diaphragm (T10).• Structure: the oesophagus is composed of four layers:• An inner mucosa of stratified squamous epithelium.• A submucous layer.• A double muscular layeralongitudinal outer layer and circularinner layer. The muscle is striated in the upper two-thirds andsmooth in the lower third.• An outer layer of areolar tissue.• Relations: the relations of the oesophagus are shown in Fig. 3.2. Onthe right side the oesophagus is crossed only by the azygos vein and theright vagus nerve and hence this forms the least hazardous surgicalapproach.• Arterial supply and venous drainage: owing to the length of thisstructure (25 cm), the oesophagus receives arterial blood from variedsources throughout its course:• Upper thirdainferior thyroid artery.• Middle thirdaoesophageal branches of thoracic aorta.• Lower thirdaleft gastric branch of coeliac artery.Similarly the venous drainage varies throughout its length:• Upper thirdainferior thyroid veins.• Middle thirdaazygos system.• Lower thirdaboth the azygos (systemic system) and left gastricveins (portal system).The dual drainage of the lower third forms a site of portal-systemicanastomosis. In advanced liver cirrhosis, portal pressure rises resultingin back-pressure on the left gastric tributaries at the lower oesophagus.These veins become distended and fragile (oesophageal varices).They are predisposed to rupture, causing potentially life-threateninghaemorrhage.• Lymphatic drainage: this is to a peri-oesophageal lymph plexus andthen to the posterior mediastinal nodes. From here lymph drains intosupraclavicular nodes. The lower oesophagus also drains into the nodesaround the left gastric vessels.Carcinoma of the oesophagus carries an extremely poor prognosis.Two main histological typesbsquamous and adenocarcinomabaccount for the majority of tumours. The incidence of adenocarcinoma ofthe lower third of the oesophagus is currently increasing for unknownreasons. Most tumours are unresectable at the time of diagnosis. Theinsertion of stents and use of lasers to pass through tumour obstructionhave become the principal methods of palliation.The thoracic duct (Fig. 3.3)• The cisterna chyli is a lymphatic sac that receives lymph from theabdomen and lower half of the body. It is situated between the abdominalaorta and the right crus of the diaphragm.• The thoracic duct carries lymph from the cisterna chyli through thethorax to drain into the left brachiocephalic vein. It usually receivestributaries from the left jugular, subclavian and mediastinal lymphtrunks, although they may open into the large neck veins directly.• On the right side the main lymph trunks from the right upper bodyusually join and drain directly through a common tributary, the rightlymph duct, into the right brachiocephalic vein.The thymus gland• This is an important component of the lymphatic system. It usuallylies behind the manubrium (in the superior mediastinum) but canextend to about the 4th costal cartilage in the anterior mediastinum.After puberty the thymus is gradually replaced by fat.The