176 



THE DIGESTIVE SYSTEM 



mucosa and part of the submiicosa. These folds, parallehng each 

 other and extending part way around the knuen at irregular inter- 

 vals, are called valvule conniventes, or plictv circulares. The 

 mucosa is modified by extension into the lumen of i)apilla-like pro- 

 jections which may be leaf-like, finger-like, or broadly club-shaped. 

 These structures are known as villi and are diagnostic features of 







Fig. 113. — Diagram of tissues in the wall of the small intestine of a mammal. 

 A, mucosa showing villi with the glands of Lieberki'ihn extending basally between 

 them. At the base of these glands is the museularis muc-osa. B, submucosa with 

 an artery and vein. (', muscularis coat with an inner circular and an external 

 longitudinal muscle coat. D, adventitia with its external limiting membrane of 

 mesothelium. 



the small intestine of mammals. They .serve to increase the surface 

 and function in the ])rocess of absorption. 



The villi in a relaxt^d condition of the tract nearly fill the lumen. 

 Each villus has a core of the loose fibroelastic tissue from the tunica 

 propria and scattered smooth muscle cells from the muscularis 

 mucosa. Simple cohunnar epithelium with basally located nuclei 

 cover the villi. The cells have a striated cuticular border, the 

 striated appearance being claimed by some observers to be due to 

 minute canaliculi through which absorption of digested material 

 is effected. (Fig. 114.) Scattered among the simi)le columnar cells 



