NOKMAL HISTOLOGY. Ill 



CHAPTEE XV. 



THE ALIMENTARY CANAL. 



The alimentary canal is derived from the hypoderm and meso- 

 derm. The squamous epithelium which lines the oesophagus is of 

 epidermic origin. The columnar epithelium which lines the ali- 

 mentary tract from the cardiac orifice onward is derived from the 

 hypoderm. The connective tissue and muscles of the tract are of 

 mesodermic origin. A careful study of the subjoined outlines will 

 indicate to the student the very close analogy between the different 

 structures of the canal. The chief differences will be found in the 

 pharynx and oesophagus. The upper half of the pharynx is lined 

 with ciliated epithelium, its lower half and the whole course of the 

 oesophagus being provided with stratified squamous epithelium. The 

 stomach and intestine have, instead, columnar epithelium. The 

 pharynx and oesophagus are provided with striated muscle fibre, 

 whereas the stomach and intestine have only smooth muscle. The 

 four coats which are present in all these structures are the mucosa, 

 sub-mucosa, muscular coat, and serous, or fibrous, coat. The mouth 

 is lined with stratified squamous epithelium, beneath which is the 

 tunica propria, or connective tissue stroma, which consists of inter- 

 lacing bundles of fibrous connective tissue containing elastic fibres. 

 There are numerous papillae in the tunica propria, and a large sup- 

 ply of small mucous racemose glands. 



ALIMENTARY CANAL. 



f Squamous epithelium. 

 ,., ! Membrana propria. 



Mucosa Stroma. 



Muscularis mucosse. 



Fibrous connective tissue. 



Sub-mucosa <! Elastic fibres. 



Glands. 

 (Esophagus. 



f Circular muscle layer. 



Muscular coat 



\ Longitudinal muscle layer. 

 Fibrous connective tissue. 



Serous coat. 



Elastic fibres. 



