304 DIGESTION. 



at the period when the largest amount of acid is flowing into 

 the stomach, and the greatest protection is required, then, is 

 the provision afforded in its highest state of efficiency." 



Dr. Pavy's theory is the best and most ingenious hitherto 

 framed in connexion with this subject ; but the experiments 

 adduced in its favour are open to many objections, and 

 afford only a negative support to the conclusions they are 

 intended to prove. The matter, therefore, can scarcely be 

 considered finally settled. 



DIGESTION IN THE INTESTINES. 



The intestinal canal is divided into two chief portions, 

 named, from their differences in diameter, the small and 

 large intestine. These are continuous with each other, and 

 communicate by means of an opening guarded by a valve, 

 the ileo-cacal valve, which allows the passage of the pro- 

 ducts of digestion from the small into the large bowel, but 

 not, under ordinary circumstances, in the opposite direction. 



The structure and functions of each organ or tissue con- 

 cerned in intestinal digestion will be first described in 

 detail, and afterwards a summary will be given of the 

 changes which the food undergoes in its passage through 

 the intestines, ist, from the pylorus to the ileo-csecal valve; 

 and, 2nd, from the ileo-csecal valve to the anus. 



Structure and Secretions of the Small Intestine. 



The small intestine, the average length of which in an 

 adult is about twenty feet, has been divided, for conve- 

 nience of description, into three portions, viz., the duo- 

 denum, which extends for eight or ten inches beyond the 

 pylorus ; the jejunum, which occupies two-fifths, and the 

 ileum, which occupies three-fifths of the rest of the canal. 



The small intestine, like the stomach, is constructed of 

 three principal coats, viz., the serous, muscular, and 

 mucous. The serous coat, formed by the visceral layer of 

 the peritoneum, need not be here specially described. The 



