THE SMALL INTESTINE 



1173 



The mucous membrane (tunica mucosa) is thick and highly vascular at the upper part of 

 the small intestine, but somewhat paler and thinner below. It consists of the following structures: 



Capillaries 

 ic vessel 



Capillaries 

 -- Lymphatic vessel 





Small artery Lymphatic plexus 

 FIG. 1061 Villi of small intestine, showing bloodvessels and lymphatic vessels. (Cadiat.) 



next the areolar or submucous coat is a double layer of unstriped muscular fibers, outer longi- 

 tudinal and inner circular, the muscularis mucosae; internal to this is a quantity of retiform 

 tissue, enclosing in its meshes lymph corpuscles, and in this the 

 bloodvessels and nerves ramify; lastly, a basement membrane, 

 supporting a single layer of epithelial cells, which throughout the 

 intestine are columnar in character. The cells are granular in 

 appearance, and each possesses a clear oval nucleus. At their 

 superficial or unattached ends they present a distinct layer of 

 highly refracting material, marked by vertical -striae, the striated 

 border. 



The mucous membrane presents for examination the following 

 structures, contained within it or belonging to it: 



Circular folds. Duodenal glands. 



Villi. Solitary lymphatic nodules. 



Intestinal glands. Aggregated lymphatic nodules. 



The circular folds (plicae circular es [Kerkringi] ; valvulce conni- 

 ventes; valves of Kerkring) are large valvular flaps projecting into 

 the lumen of the bowel. They are composed of reduplications of the 

 mucous membrane, the two layers of the fold being bound together 

 by submucous tissue; unlike the folds in the stomach, they are per- 

 manent, and are not obliterated when the intestine is distended. 

 The majority extend transversely around the cylinder of the intes- 

 tine for about one-half or two-thirds of its circumference, but some 

 form complete circles, and others have a spiral direction; the latter 

 usually extend a little more than once around the bowel, but occa- 

 sionally two or three times. The larger folds are about 8 mm. in 

 depth at their broadest part; but the greater number are of smaller 

 size. The larger and smaller folds alternate with each other. They 

 are not found at the commencement of the duodenum, but begin 

 to appear about 2.5 or 5 cm. beyond the pylorus. In the lower 



part of the descending portion, below the point where the bile and pancreatic ducts enter the 

 intestine, they are very large and closely approximated. In the horizontal and ascending 

 portions of the duodenum and uooer half of the jejunum they are large and numerous, but 



FIG. 1062. An intestinal gland 

 from the human intestine. (Flem- 

 ming.) 



