THE STOMACH AND INTESTINE. 



279 



may be regarded as a differentiation of the latter. Toward the 

 muscularis mucosae the mucosa is terminated by a reticulated elastic 

 membrane (F. P. Mall, in the dog), containing openings for the 

 entrance of vessels, nerves, and muscle-fibers. 



The muscularis mucosce consists of two layers of unstriped 

 muscular fibers arranged in a manner similar to that in the external 

 muscular tunic i. e., having an inner circular and an outer longi- 

 tudinal layer. The fibers are frequently gathered into bundles, 

 which appear to be separated from each other by connective tissue. 

 From both layers, but more especially from the inner, muscle-fibers 

 are given off at right angles, which enter the tunica propria and 

 pass between the glands of Lieberkiihn, and even into the villi. 

 In the latter these muscle-fibers are arranged in bundles, and lie 



Leucocyte 



inepithe- f/Ts^'- 

 Hum. 



Epithelium. -'/-/ 



Intermedi- . 

 ary zone. 



Submucosa. - 



Fig. 219. Section of solitary lymph-nodule from vermiform appendix of guinea- 

 pig, showing crypt ; X about 400 (Flemming's fluid). 



near their axes around the lacteal vessels. The contraction of these 

 fibers causes a contraction of the entire villus. 



Lymph-nodules are distributed throughout the mucosa of the 

 small intestine, occurring either singly, as solitary follicles, or 

 aggregated, as Peyer's patches. At the points where they occur, 

 the villi are absent and a lateral displacement of the glands of 

 Lieberkiihn is observed. The lymph-nodule is usually pyriform in 

 shape. The thinner portion protrudes somewhat in the direction 

 of the lumen of the intestine, while the thicker portion extends 

 outward to the muscularis mucosae, the latter being frequently in- 

 dented or even perforated if the lymph-nodules be markedly devel- 

 oped. Their structure is similar to that of the lymph-follicles (see 

 under these), and consists of reticular adenoid tissue, supporting 



