DIGESTION AND ABSORPTION IN THE INTESTINES 97 



peritoneum running from one side of the great abdominal 

 vessels, enveloping the gut, and returning to the parietal wall 

 on the opposite side of the vessels. The fold thus attaching 

 the intestine to the abdominal wall is the mesentery. The 

 distance along the mesentery from this parietal region to the 

 gut is three or four inches, except at the beginning and end of 

 the small intestine, where it is shorter, to bind the tube more 

 firmly in place. The upper eight or ten inches of the small 

 gut is called the duodenum, the next eight feet the jejunum, 

 and the remainder the ileum. No anatomical peculiarity sep- 

 arates these parts. The average diameter is about one and a 

 quarter inches. 



Histology. The wall of the intestine is in four layers, 

 serous, muscular, submucous and mucous. The serous layer 

 consists of the enveloping fold of peritoneum and needs no 

 description, except that, like serous membranes elsewhere, it 

 furnishes a lubricating secretion to provide for the easy glid- 

 ing of the intestines over each other and over the other vis- 

 cera. The muscular coat has its muscular fibers disposed in 

 two layers, an external longitudinal and an internal circular. 

 The latter is the stronger. Between the two muscular layers 

 is the nervous plexus of Auerbach. Between the circular 

 layer and the mucous coat is the submucous layer which con- 

 tains the nerve plexus of Meissner. These communicate 

 with others by fibers of extension. The mucous coat pre- 

 sents several points deserving mention. These are (i) val- 

 vulse conniventes; (2) villi ; (3) secreting glands, (a) of 

 Brunner and (b) of Lieberkuhn; (4) solitary and agminate 

 glands. 



i . The valvulae conniventes are simply tpansverse folds or 

 tucks of the entire mucous membrane, each of which extends 

 from one-third to one-half around the circumference of the 

 tube and projects by its middle portion sometimes to the cen- 

 ter of the lumen. The small folds, 800 to 1,000 in number, 

 extend from about the middle of the duodenum to the begin- 

 ning of the last third of the ileum and greatly increase the 



