138 



NORMAL HISTOLOGY. 



The valvulse conniventes begin a short distance below the pylorus, 

 and are very numerous and prominent in the duodenum, but become 

 progressively less frequent and pronounced in the lower portions of 

 the small intestine. 



The absorbent surface of the small intestine is still further in- 



FIG. 119. 





Diagram representing the structure of the human small intestine. (Bohm, Davidoff, and 

 Mall, slightly modified.) Two villi are represented. In the one on the left the blood- 

 vessels are shown ; in the one on the right, the lymphatics. The line S indicates the sur- 

 face of the mucous membrane between the villi. a, central lacteal vessel ; b, smooth 

 muscular fibres extending into the villus from the muscularis mucosse : c, lymphadenoid 

 tissue beneath the epithelial covering of the villus ; d, crypt of Lieberkiihn ; e, tunica 

 propria of lymphadenoid tissue, and continuous with that of the villus ; /, muscularis 

 mucosae, forming the deepest portion of the mucous membrane ; g, submucosa containing 

 the larger bloodvessels and the lymphatic plexus h; i, encircling layer of the muscular 

 coat ; j, longitudinal layer ; k, lymphatic plexus within the muscular coat ; I, serous coat ; 

 TO, vein. The crypts are lined, and the villi covered, with columnar epithelium. 



creased by the presence of innumerable minute, finger-like projec- 

 tions from the surface of the mucous membrane, the villi. These 

 are just discernible by the unaided eye, and give the internal surface 

 of the intestine a velvety appearance. 



