THE THORAX 



I02? 



coat is provided with numerous papillae, and it is covered by stratified 

 squamous epithelium, which is thrown into elevations by the papillae. 



The mucous glands are racemose, and are situated in the submucous coat. 

 Their ducts are large and long, and on their way to the free surface some of 

 them traverse small collections of lymphoid tissue. 



Development. — The oesophagus is developed from that part of the fore-gut 

 which succeeds to the portion from which the pharynx is developed. At first 

 it is very short, on account of the imperfect development of the neck. As, 

 however, the neck becomes formed, and as the stomach descends, the oesoph- 

 agus becomes elongated. 



Stratified Epithelium— 



Mucosa _ 

 Muscularis Mucosae - 



Sabmucosa 



Circular Muscular Fibres, 



Longitudinal Muscular 

 Fibres 



Mucous Gland 



.Fibrous Sheath 



Fig. 421. — Transverse Section of the Wall of the CEsophagus. 



Descending Aorta. — ^The descending aorta, which is the con- 

 tinuation of the aortic arch, commences on the left side of the body 

 of the fourth thoracic vertebra on a level with its lower border, 

 and it terminates at the lower border of the body of the twelfth 

 thoracic vertebra. At the latter level it passes through the aortic 

 opening of the dia^^hragm, and enters upon the abdominal part of 

 its course. The vessel lies in the posterior mediastinmn, and its 

 course is downwards and inwards, so that, though at first on the 



