CHAPTER XVI 

 THE DIGESTIVE SYSTEM 



The digestive system consists of the alimentary tract and certain 

 associated structures such as glands, teeth, etc. 



The alimentary tract'is a tube extending from lips to anus. Dif- 

 ferent parts of the tube present modifications both as to caHbre and 

 as to structure of wall. 



The embryological subdivision of the canal into headgut, foregut, 

 midgut, and endgut admits of further subdivision upon an anatomical 

 basis as follows: 



I. Headgut: (a) Mouth, including the tongue and teeth. 



The entire canal is lined by mucous membrane, the modifications 

 of which constitute the most essential difference in structure of its 

 several subdivisions. 



Beneath the mucosa is usually more or less connective tissue, 

 which in a large portion of the canal forms a definite submucosa. 



Muscular tissue is present beneath the submucosa throughout the 

 greater part of the canal. In most regions it forms a definite, con- 

 tinuous, muscular tunic. 



The upper and lower ends of the tube — mouth, pharynx, oesoph- 

 agus, and rectum — are quite firmly attached by fibrous tisue to the 

 surrounding structures. The remainder of the tube is less firmly 

 attached, lying coiled in the abdominal cavity, its surface covered, 

 except along its attached border, by a serous membrane, the visceral 

 peritoneum. 



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