ABSORPTION BY LYMPHATICS. 291 



will be presently described. But the lacteals appear to absorb 

 only certain constituents of the food, including particularly 

 the fatty portions. The absorption by both sets of vessels is 

 carried on most actively, but not exclusively, in the villi of 

 the small intestine ; for in these minute processes, both the 

 capillary bloodvessels and the lacteals are brought almost into 

 contact with the intestinal contents. 



It has been already stated that the villi of the small intestine 

 (Figs. 81 and 82), are minute vascular processes of mucous 

 membrane, each containing a delicate network of bloodvessels 

 and one or more lacteals, and are invested by a sheath of cylin- 

 drical epithelium. In the interspaces of the mucous mem- 

 brane between the villi, as well as over all the rest of the 

 intestinal canal, the lacteals and bloodvessels are also densely 

 distributed in a close network, the lacteals, however, being 

 more sparingly supplied to the large than to the small in- 

 testine. 



There seems to be no doubt that absorption of fatty matters 

 during digestion, from the contents of the intestines, is effected 

 chiefly by the epithelial cells which line the intestinal tract, 

 and especially by those which clothe the surface of the villi 

 (Fig. 81). From these epithelial cells, again, the fatty parti- 

 cles are passed on into the interior of the lacteal vessels (Figs. 

 81 and 82), but how they pass, and what laws govern their so 

 doing, are not at present exactly known. 



It is probable that the process of absorption by the epithe- 

 lial cells, is assisted by the pressure exercised on the contents 

 of the intestines by their contractile walls ; and that the ab- 

 sorption of fatty particles is also facilitated by the presence of 

 the bile, the pancreatic and intestinal secretions, which moisten 

 the absorbing surface. For it has been found by experiment, 

 that the passage of oil through an animal membrane is made 

 much easier when the latter is impregnated with an alkaline 

 fluid. 



Absorption by the Lymphatic Vessels. 



The real source of the lymph, and the mode in which its 

 absorption is effected by the lymphatic vessels, were long mat- 

 ters of discussion. But the problem has been much simplified 

 by more accurate knowledge of the anatomical relations of the 

 lymphatic capillaries. It is most probable that the lymph is 

 derived, in great part, from the liquor snnguinis, which, as be- 

 fore remarked, is always exuding from the blood-capillaries 

 into the interstices of the tissues in which they lie; and 

 changes in the character of the lymph correspond very closely 

 with changes in the character of either the whole mass of 



