310 HANDBOOK OF PHYSIOLOGY: 



vessels and lacteals distributed in the mucous membrane. The lacteals 

 appear to absorb only certain constituents of the digested food, includ- 

 ing 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 blood- 

 vessels and the lacteals are brought almost into contact with the intes- 

 tinal contents. There seems to be no doubt that absorption of fatty 

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

 chiefly between the epithelial cells which line the intestinal tract (Wat- 

 ney), and especially those which clothe the surface of the villi. Thence, 

 the fatty particles are passed on into the interior of the lacteal vessels, 

 but how they pass, and what laws govern their passage, are not at pres- 

 ent exactly known. 



The process of absorption is assisted by the pressure exercised on the 

 contents of the intestines by their contractile walls ; and the absorption 

 of fatty particles is also facilitated by the presence of the bile, and the 

 pancreatic and intestinal secretions, which moisten the absorbing sur- 

 face. For it has been found by experiment, that the passage of oil 

 through an animal membrane is made much easier when the latter is im- 

 pregnated with an alkaline fluid. 



(b.) By the Lymphatics. The real source of the lymph, and the 

 mode in which its absorption is effected by the lymphatic vessels, were 

 long matters of discussion. But the problem has been much simplified 

 by more accurate knowledge of the anatomical relations of the lymph- 

 atic capillaries. The lymph is, as has been pointed out, diluted liquor 

 sanguinis, which is always exuding from the blood-capillaries into the 

 interstices of the tissues in which they lie ; and as these interstices form 

 in most parts of the body tho beginnings of the lymphatics, the source 

 of the lymph is sufficiently obvious. In connection with this may be 

 mentioned the fact that changes in the character of the lymph corre- 

 spond very closely with changes in the character of either the whole mass 

 of blood, or of that in the vessels of the part from which the lymph is 

 exuded. Thus it appears that the coagulability of the lymph, although 

 always less than, is directly proportionate to that of the blood ; and 

 that when fluids are injected into the blood-vessels in sufficient quantity 

 to distend them, the injected substance may be almost directly after- 

 wards found in the lymphatics. 



Some other matters than those originally contained in the e-xuded 

 liquor sanguinis may, however, find their way with it into the lymphatic 

 vessels. Parts which having entered into the composition of a tissue, 

 and, having fulfilled their purpose, require to be removed, may not be 

 altogether excrementitious, but may admit of being reorganized and 

 adapted again for nutrition ; and these may be absorbed by the lymph- 



