ALIMENTAEY CANAL. 1105 



following parts in order : mouth, pharynx, oesophagus, stomach, small and large 

 intestines. The term tubus digestorius is applied to the whole of the canal below 

 the lower end of the pharynx. The mouth cavity is the first division of the tube. 

 It is separated from the nasal cavities above by the palate, and opens posteriorly 

 into the pharynx. This latter is an expanded portion of the canal lying posterior 

 to the mouth, nasal cavity and larynx, the mouth opening into it through the 

 isthmus of the fauces, the nasal cavity through the choanae (O.T. posterior nares) ; 

 whilst lower down, immediately below the base of the tongue, the aperture of the 

 larynx is found in its anterior wall. Opposite the lower border of the larynx, 

 the pharynx is continued into the oesophagus, a long and comparatively straight 

 portion of the digestive tube, passing through the neck and thorax to the abdomen, 

 which it reaches by piercing the diaphragm. Immediately after entering the 

 abdomen the tube expands into a pear-shaped dilated chamber, the stomach. This 

 is followed by over 20 feet of small intestine, the junction of the two being 

 marked by a constriction, the pylorus. The small intestine presents three more 

 or less arbitrary divisions namely, (a) the duodenum, a part about 10 inches in 

 length, curved somewhat like a horse-shoe, and closely united to the posterior 

 abdominal wall ; (6) the jejunum, which includes the upper two-fifths, and (c) the 

 ileum, the lower three-fifths of the small intestine beyond the duodenum. The 

 jejunum and ileum are connected to the posterior abdominal wall by the mesentery, 

 a fan- shaped fold of connective tissue covered by the peritoneum, or lining mem- 

 brane of the abdominal cavity. 



The terminal part of the ileum opens into the side of the large intestine, 

 a few inches (2) from the blind commencement of the latter. There is thus 

 formed at the beginning of the great intestine a cul-de-sac, called the caecum, in 

 connexion with which there is a small worm-shaped diverticulum, the vermiform 

 process. 



The orifice through which the ileum opens into the large intestine is guarded 

 by the valve of the colon (O.T. ileo-csecal valve), which prevents the return of its 

 contents from the large into the small bowel. After the caecum comes the 

 ascending colon, which runs up on the right side of the abdomen. This is succeeded, 

 in order, by the transverse colon, crossing from right to left, the descending colon, 

 running down on the left side of the abdomen, and the iliac colon, lying in the left 

 iliac fossa. Beyond this are the pelvic colon, which lies in part or entirely within 

 the pelvis minor (O.T. true pelvis), the rectum, and the anal canal. 



The rectum lies within the pelvis minor, and the anal canal, the terminal part 

 of the intestine, is a short channel passing between the muscles which form the 

 pelvic floor, to open on the surface at the anal orifice. 



The B.N.A. term colon sigmoideum includes the portion named above as pelvic colon, and 

 the term colon descendens includes the descending and iliac colon. 



II. Special Organs found in the Wall of the Tube. In the mouth are 

 found the teeth, gums, tongue, and behind them, in the pharynx, are the 

 palatine tonsils. The teeth, 32 in number in the adult, are portions of .the 

 mucous membrane of the mouth and of the subjacent tissue, calcified on the 

 surface, and specially formed for mastication, that is, the division and triturition 

 of the food which take place in the mouth before the bolus, as the resulting mass 

 is termed, can be swallowed. They are rooted in the jaws and are partly surrounded 

 by the gums. 



The tongue is a muscular organ, useful alike in mastication, deglutition, and 

 speech. It is covered with epithelium, which in places is modified so as to form 

 taste corpuscles, which are the end organs of the gustatory sense. 



The roof of the mouth is formed by the palate, which separates the mouth 

 from the nose. It consists of a bony part in front called the hard palate, and 

 a movable sheet, called the soft palate, behind. 



The palatine tonsils are two large masses of lymph tissue, found one on each 

 side of the wall of the pharynx, just posterior to the mouth. They form the 

 most prominent portions of an almost complete ring of lymph tissue placed 

 around the circumference of the tube at this level. 



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