THE INTESTINAL FLUID 



157 



the blood to the tissue which requires it, and there used for this 

 purpose. Glycogen is also stored in the muscles, where it is oxi- 

 dized to release energy when the muscles are exercised. 



The Intestinal Fluid. — Within the waU of the small intestine 

 are numerous glands which pour out a fluid known as the succus 

 enter'icus. Its functions are not well known, but it contains at 

 least one hormone and several enzymes with which it assists the 

 pancreatic fluid to do its work. The remaining peptones are broken 

 up into amino-acids by the enzyme erep'- 

 sin, while maVtase, su'crase, and lac'tase 

 change malt sugar, cane sugar, and milk 

 sugar to the simplest or single sugars (so 

 called because of the molecule C6H12O6). 



Structure of the Small Intestine. — 

 The small intestine in man is a slender 

 tube nearly twenty feet in length and 

 about one inch in diameter. As one of 

 the chief functions of the small intestine 

 is that of absorption, we must look for 

 adaptations which increase the absorbing 

 surface of the tube. This end is gained 

 in part by the inner surface of the tube 

 being thrown into transverse folds which 

 not only retard the rapidity with which 

 food passes down the intestine, but also 

 give more absorbing surface. But far 

 more important for absorption are mil- 

 lions of little projections which cover the 

 inner surface of the small intestine. 



The Villi. — So numerous are these projections that the whole 

 surface presents a velvety appearance. Collectively, these struc- 

 tures are called the villi (sing, villus) . They form the chief organs 

 of absorption in the intestine, several thousand being distributed 

 over every square inch of surface. By means of the folds and 

 the villi the small intestine is estimated to have an absorbing sur- 

 face equal to twice that of the surface of the body. Between the 

 villi are found the openings of the intestinal glands which secrete 

 the succus entericus. 



Diagram of wall of small 

 intestine, greatly magnified. 



a, mouths of intestinal glands ; 



b, villus cut lengthwise to 

 show blood vessels and lacteal 

 (in center) ; e, lacteal sending 

 branches to other villi ; i, in- 

 testinal glands ; m, artery ; 

 V, vein ; I, t, muscular coats 

 of intestine wall. 



