238 DIGESTION. 



ach, is sufficient to neutralize the acidity of the gastric juice, 

 were it, so to speak, to make an attempt at digesting parts 

 with which it has no business ; and as may be gathered from 

 what has been previously said (p. 228), the neutralization of 

 the acidity of the gastric secretion is quite sufficient to destroy 

 its digestive powers. He also very ingeniously argues that 

 this very alkalinity must, from the conditions of the circula- 

 tion naturally existing in the walls of the stomach, be in- 

 creased in proportion to the need of its protective influence. 

 " In the arrangement of the vascular supply," he remarks, " a 

 doubly effective barrier is, as it were, provided. The vessels 

 pass from below upwards towards the surface : capillaries 

 having this direction ramify between the tubules by which 

 the acid of the gastric juice is secreted ; and being separated 

 by secretion below, must leave the blood that is proceeding 

 upwards correspondingly increased in alkalinity ; and thus, 

 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 connection with this subject; but the experiments 

 adduced in its favor 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 com- 

 municate by means of an opening guarded by a valve, the 

 ileo-ccecal valve, which allows the passage of the products 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, 1st, 

 from the pylorus to the ileo-csecal valve ; and, 2d, from the 

 ileo-csecal valve to the anus. 



Structure arid Secretions of the Small Intestine. 



The small intestine, the average length of which in an 

 adult is about twenty feet, has been divided, for convenience 



