224 



TEXT-BOOK OF PHYSIOLOGY. 



arteries, capillaries, veins, and lymphoid corpuscles. In the center 

 of the villus there is usually a single though at times a double club- 

 shaped lymph- capillary, the walls of which are composed of epithe- 

 lioid cells with sinuous margins. This capillary probably begins by a 

 blind extremity and opens at the base of the villus into the subjacent 

 lymph-vessels. The communicating orifice is guarded by a valve. 

 It is also surrounded by a layer of non-striated muscle-fibers, arranged 

 longitudinally, derived from the muscularis mucosae and attached to 

 the apex of the body of the villus. 



The arteries which penetrate the villi are derived from those of the 



submucous coat of the intes- 

 tine, which are the ultimate 

 branches of the intestinal artery, 

 and serve the purpose of deliv- 

 ering nutritive material to the 

 capillary plexus (Fig. 93). 

 /While passing through the lat- 

 ter a portion of the blood- 

 plasma transudes through the 

 capillary walls into the spaces 

 of the reticulated tissue, consti- 

 tuting lymph. At the same 

 time products of tissue metab- 

 olism pass through the capil- 

 lary walls into the blood.,/The 

 blood then passes into the ven- 

 ules, which, leaving the villus 

 at its base, unite with the veins 

 of the submucous coat to form 

 the intestinal veins^ These 



finally unite with 



FIG. 94. DIAGRAM OF THE PORTAL 



VEIN (pv) ARISING IN THE ALIMEN- 

 TARY TRACT AND SPLEEN (s), AND 

 CARRYING THE BLOOD FROM THESE 

 ORGANS TO THE LIVER. ( Yeo's 

 " Text-book of Physiology."') 



(Fig. . .. 

 within the 

 the club- 



The excess of lymp 

 villus passes into 

 shaped lymph-capillary, to be 

 finally carried by the lymph- 



atics^of the mesentery into the thoracic duct./ During the intervals 

 of digestion and in the absence of food from the intestine there is, 

 of course, no absorption of food nor the removal from the villus of 

 anything but the excess of lymph and metabolic products^/ 



Function of the Villi. The villi, and especially th epithelial 

 cells covering them, are the essential agents in the absorption of the 

 products of digestion. It is by the activity of these cells that the 



