130 



DIGESTION AND ABSORPTION 



The salivary digestion of starch continues in the stomach until 

 the acid permeates all parts ot the food mass. Proteins are the 



nutrients chiefly affected 

 in gastric digestion, but 

 even their digestion is 

 incomplete. Many bac- 

 teria that enter with the 

 food are killed by the 

 germicidal action of the 

 hvdrochloric acid. 



The small intestine. 

 The small intestine is a 

 narrow, tubular organ 

 about one inch in diam- 

 eter and from twenty to 

 thirty feet in length. It 

 is so coiled that it easilv 

 fits into the central part 

 of the abdominal cavity. It is continuous with the pyloric end of 

 the stomach. A membrane, the mesentery, attaches the coils of 

 the intestine to each other and to the backbone. 



The small intestine has muscles which permit two kinds of 

 movement, one called rhythmic segrnentation and the other peri- 

 stalsis. The rhythmic segmentation is caused by local constric- 

 tions of the intestinal wall at regular intervals. These constrictions 

 divide the chyme into a number of equal parts. Within a few 

 seconds, each of these portions is halved and unites with the cor- 

 responding halves of the adjacent segments. In the next con- 

 striction the segments are restored to their original position. This 

 enables the mass to be thoroughly mixed with the digestive juices. 

 Then a peristaltic movement, a succession of waves of contraction 

 and relaxation, beginning at the anterior end of the small intestine, 

 sweeps along, forcing the food onward through the tube. 



Rhythmic constrictions of the walls of the small in- 

 testine break the food into equal segments. Each of 

 these segments is halved and unites with the adjacent 

 half of the next segment. Segmentation continues, in- 

 suring the complete mixture of digestive juices and food. 



