892 THE ORGANS OF DIGESTION. 



In the thorax, it is at first situated a little to the left of the median line ; it 

 then passes behind the aortic arch, separated from it by the trachea, and descends 

 in the posterior mediastinum, along the right side of the aorta, nearly to the 

 Diaphragm, where it passes in front and a little to the left of the artery, previous 

 to entering the abdomen. It is in relation, in front, with the trachea, the arch of 

 the aorta,the left carotid and left subclavian arteries, which incline toward its left 

 side, the left bronchus, the pericardium, and the Diaphragm ; behind, it rests upon 

 the vertebral column, the Longi colli muscles, the right intercostal arteries, and the 

 vena azygos minor ; and below, near the Diaphragm, upon the front of the aorta ; 

 laterally^ comes in contact with both pleurae, especially with the left pleura above 

 and the right pleura below : it overlaps the vena azygos major, which lies on its 

 right side, while the descending aorta is placed on its lelt side. The pneumogastric 

 nerves descend in close contact with it, the right nerve passing down behind, and 

 the left nerve in front of it ; the two nerves uniting to form a plexus (the plexus 

 gulce) around the tube. 



In the lower part of the posterior mediastinum the thoracic duct lies to 

 the right side of the oesophagus ; higher up, it is placed behind it, and, cross- 

 ing about the level of the fourth dorsal vertebra, is continued upward on its 

 left side. 



Structure. The oesophagus has three coats an external or muscular; a 

 middle or areolar ; and an internal or mucous coat. 



The muscular coat is composed of two planes of fibres of considerable thickness, 

 an external longitudinal and an internal circular. 



The longitudinal fibres are arranged, at the commencement of the tube, in three 

 fasciculi : one in front, which is attached to the vertical ridge on the posterior 

 surface of the cricoid cartilage ; and one at each side, which is continuous with 

 the fibres of the Inferior constrictor : as they descend they blend together and 

 form a uniform layer, which covers the outer surface of the tube. 



Accessory slips of muscular fibres are described by Cunningham as passing 

 between the oesophagus and the left pleura, where it covers the thoracic aorta 

 (almost always), or the root of the left bronchus (usually), or the back of the 

 pericardium, as well as other still more rare accessory fibres. In Fig. 485, taken 

 from a dissection in the Museum of the Royal College of Surgeons of England, 

 sereral of these accessory slips may be seen passing from the oesophagus to the 

 pleura, and two slips to the back of the trachea just above its bifurcation. 



The circular fibres are continuous above with the Inferior constrictor ; their 

 direction is transverse at the upper and lower parts of the tube, but oblique in the 

 central part. 



The muscular fibres in the upper part of the oesophagus are of a red color, and 

 consist chiefly of the striped variety, but below they consist for the most part of 

 involuntary muscular fibre. 



The areolar coat connects loosely the mucous and muscular coats. 



The mucous coat is thick, of a reddish color above and pale below. It is 

 disposed in longitudinal folds, which disappear on distension of the tube. Its 

 surface is studded with minute papillae, and it is covered throughout with a thick 

 layer of stratified pavement epithelium. Beneath the mucous membrane, between 

 it and the areolar coat, is a layer of longitudinally arranged non-striped 

 muscular fibres. This is the muscularis mucosce. At the commencement it is 

 absent, or only represented by a few scattered bundles ; lower down it forms a 

 considerable stratum. 



The oesophageal glands are numerous small compound racemose glands 

 scattered throughout the tube ; they are lodged in the submucous tissue, and open 

 upon the surface by a long excretory duct. They are most numerous at the lower 

 part of the tube, where they form a ring round the cardiac orifice. 



Vessels of the (Esophagus. The arteries supplying the oesophagus are de- 

 rived from the inferior thyroid branch of the thyroid axis of the subclavian, 

 from the descending thoracic aorta, and from the gastric branch of the coeliac 



