DIGESTION AND ABSORPTION IN THE INTESTINES 97 



numerous coils which are held loosely in place by a fold of perito- 

 neum 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 intes- 

 tine, 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 separates these parts. 

 Their 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 gliding 

 of the intestines over each other and over the other viscera. 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 contains the nerve plexus of 

 Meissner. These communicate with others by fibers of extension. 

 The mucous coat presents several points deserving mention. 

 These are (i) valvulae conniventes; (2) villi; (3) secreting glands, 

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

 glands. 



i. The valvulae conniventes are simply transverse 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 center of the 

 lumen. These small folds, 800 to 1,000 in number, extend from 

 about the middle of the duodenum to the beginning of the last 

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