ABSORPTION 



219 



composed of granular bioplasm containing a distinct nucleus. At its free 

 extremity a narrow border of the cell presents a striated appearance, as if it 

 were composed of small rods embedded in some cement substance. Goblet 

 or mucin-holding cells are also to be found among the columnar cells. The 

 body of the villus, that portion within the basement membrane, consists of a 

 reticulated connective tissue supporting 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 endothelial 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 sub- 

 mucous coat of the intestine, which are 

 the ultimate branches of the intestinal 

 artery, and serve the purpose of de- 

 livering nutritive material to the capil- 

 lary plexus. While passing through 

 the latter a portion of the blood- 

 plasma transudes through the capil- 

 lary walls into the spaces of the retic- 

 ulated tissue, constituting lymph. 

 At the same time products of tissue 

 metabolism pass through the capillary 

 walls into the blood. The blood then 

 passes into the venules, which, leav- 

 ing the villus at its base, unite with 

 the veins of the submucous coat to 

 form the intestinal veins. These fi- 

 nally unite with the gastric and splenic 

 veins to form the portal vein, which enters the liver at the transverse fissure 

 (Fig. 89). The excess of lymph within the villus passes into the club- 

 shaped lymph-capillary, to be finally carried by the lymph-vessels 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 the 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 new materials are 

 taken out of the alimentary canal and transferred into the lymph-spaces in 

 the interior of the villi, from which they are subsequently removed by the 

 blood-vessels and lymph-vessels. As to the mechanism by which the epi- 

 thelial cells accomplish this result, nothing definite can be asserted. In- 



FIG. 89. DIAGRAM OF THE PORTAL 

 VEIN (pv) ARISING IN THE ALIMENTARY 

 TRACT AND SPLEEN (5), AND CARRYING THE 

 BLOOD FROM THESE ORGANS TO THE LIVER. 

 (Yeo's "Text-book of Physiology.") 



