1290 THE ORGANS OF DIGESTION 



portion of the intestine by a fibrous band. It represents the remains of the 

 vitelline or omphalomesenteric duct, the duct of communication between the 

 umbilical vesicle and the alimentary canal in early fetal life. 



Structure of the Small Intestine, Including the Duodenum. The wall of the small 

 intestine is composed of four coats serous, muscular, submucous, and mucous. 



The Serous Coat (tunica serosd). The relation of the peritoneum to the duodenum has been 

 described. The remaining portion of the small intestine is surrounded by the peritoneum, 

 excepting along its attached or mesenteric border; here a space is left for the vessels and nerves 

 to pass to the gut. 



The muscular coat (tunica muscularis) consists of two layers of fibres, an external or longi- 

 tudinal layer and an internal or circular layer. 



The longitudinal fibres (stratum longitudinale) are thinly scattered over the surface of the 

 intestine, and are more distinct along its free border. 



The circular fibres (stratum drculare) form a thick, uniform layer; they surround the cylinder 

 of the intestine in the greater part of its circumference, and are composed of smooth muscle cells 

 of considerable length. The muscular coat is thicker at the upper than at the lower part of the 

 small intestine. 



The submucous coat (tda submucosa) connects the mucous and muscular layers. It con- 

 sists of loose, filamentous areolar tissue, which forms a bed for the subdivisions of the nutrient 

 vessels, previous to their distribution to the mucous surface, also for the lymph channels and 

 nerves. 



The submucous coat contains lymph nodules (noduli lymphatic?). Each nodule is pyramidal 

 or pear-shaped, and the apex lies in the mucous membrane and forms a rounded elevation. 

 These rounded elevations mark the solitary follicles and Peyer's patches (Figs. 1017 and 

 1024), and nowise resemble villi. In the duodenum the submucous tissue contains the duo- 

 denal glands. The submucous tissue is prolonged into the valvulae conniventes. It contains 

 bloodvessels, Meissner's plexus of nerves, and lymph vessels. 



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: 

 next the areolar or submucous coat is a layer of unstriped muscle tissue, the muscularis mucosae ; 

 internal to this is a quantity ofjetiform tissue, enclosing in its meshes lymph corpuscles (diffuse 

 lymphoid tissue), and in which the bloodvessels and nerves ramify. Lastly, a basement mem- 

 brane, supporting a single layer of columnar epithelial and goblet cells. They are granular 

 in appearance, and each possesses a clear, oval nucleus. At their superficial or unattached 

 end they present a distinct layer of highly refracting material, the cuticular border, marked by 

 vertical striae. 



The mucous membrane presents for examination the following structures contained within it 

 or belonging to it: 



Valvulae conniventes. 



Intestinal glands. Lymphatic nodules { p^r? or "agminated follicles. 



The valvulae conniventes or the valves of Kerkring (plicae circulares [Kerkringi]) (Fig. 1016) 

 are large folds or valvular flaps projecting into the lumen of the bowel. They are composed of 

 reduplications or folds of the mucous membrane, the two layers of the fold being bound together 



by submucous tissue; they contain no muscular 

 fibres, and, unlike the folds in the stomach, they 

 are permanent, and are not obliterated when 

 the intestine is distended. The majority ex- 

 tend transversely across the cylinder of the 

 intestine 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 occasionally two or three 

 times. The spiral arrangement is the charac- 

 teristic one of the shark family of fishes. The 

 larger folds are about one-third of an inch in 

 depth at the 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 

 FIG. 1016. Valvulae oonniventes in the upper part , , , 



of the small intestine. (Poirier and Charpy ) duodenum, but begin to appear about one or 



