DIGESTION. 



137 



FIG. 20. 



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re, 



respect, it is about six times the length of the body. Its principal 

 divisions, enumerated from above downward, are : the mouth, the 

 pharynx, the 03sophagus, the 

 stomach, the small intestine, 

 and the large intestine. At 

 its commencement (Fig. 20) 

 is the cavity of the mouth, 

 which communicates, imme- 

 diately beyond the fauces, 

 with the pharynx. From the 

 pharynx, a straight tubular 

 canal, the oesophagus (a), 

 leads directly to the stomach 

 (b], a flask-shaped expansion, 

 surrounded at its cardiac and 

 pyloric orifices (c, d) by 

 special bands of muscular 

 fibres. Then follows the 

 small intestine (e), different 

 parts of which, owing to cer- 

 tain differences in size, struc- 

 ture, or convolution, bear the 

 names of duodenum, jejunum, 

 and ileum. In the uppermost 

 division, or duodenum, are the 

 orifices of the biliary and pan- 

 creatic ducts (f, g) Lastly, 

 comes the large intestine (h, 

 t, j, k) separated from the pre- 

 ceding by the ileo-caacal valve, 

 and terminating at the anus, 

 where it is provided with a 

 double sphincter muscle guard- 

 ing its orifice. Everywhere 

 the alimentary canal is com- 

 posed of a mucous membrane 

 and a muscular coat, with a 

 layer of connective tissue be- 

 tween the two. The muscular 

 coat consists of a double layer 

 of longitudinal and transverse 

 fibres, by the alternate con- 

 traction and relaxation of which the food is carried through the canal 

 from above downward. The mucous membrane presents a different 

 structure in different parts. That of the mouth and oesophagus is 

 smooth, with a hard, white, tessellated epithelium, which terminates 

 abruptly at the cardiac orifice of the stomach. The mucous membrane 



HUMAN* AI.IMEXTAKY < AXAL. a. (Esophagus. 6. 

 Stomach, c. Cardiac orifice, d. Pylorus, e. Small 

 intestine. /. Biliary duct. g. Pancreatic duct. ft. As- 

 cending colon, i. Transverse colon, j. Descending 

 colon. A-. Rectum. 



