DIGESTION. 373 



cellulose from plants, and mucin from the secretions of the 

 alimentary tract, will all remain unchanged. 



Absorption from the Small Intestine. The chyme leav- 

 ing the stomach is a semi-liquid mass which, mixed in the 

 duodenum with considerable quantities of pancreatic secre- 

 tion and bile, is further diluted. Thenceforth it gets the 

 intestinal secretion added to it, but the absorption more 

 than counterbalancing the addition of liquid, the food- 

 mass becomes more and more solid as it approaches the ileo- 

 colic valve. At the same time it becomes poorer in nutritive 

 constituents, these being gradually removed from it in "its 

 progress; most dialyze through the epithelium into the sub- 

 jacent blood and lymphatic vessels, and are carried off. 

 Those passing into the blood capillaries are taken by the por- 

 tal vein to the liver; while those entering the lacteals are 

 carried into the left jugular vein by the thoracic duct. As 

 to which foodstuffs go one road and which the other, there is 

 still much doubt; sugars probably go by the portal system, 

 while the fats, mainly, if not entirely, go through the lacteals. 

 How the fats are absorbed is not clear, since oils will not dia- 

 lyze through membranes, such as that lining the intestine, 

 moistened with watery liquids. Most of them, nevertheless, 

 get into the lacteals as oils and not as soluble soaps; for one 

 finds these vessels, in a digesting animal, filled with white 

 milky chyle; while at other periods their contents are watery 

 and colorless like the lymph elsewhere in the Body. The 

 little fat-drops of the emulsion formed in the intestine, go 

 through the epithelial cells and not between them, for during 

 digestion these cells are loaded with oil-droplets; as their 

 free ends are striated and probably devoid of any definite 

 cell- wall, it is possible that the intestinal movements squeeze 



Lrops into them, but the cells may play a more active part. 

 yThe \triation of the border is due to closely-set rods which 

 fiown to be able to change their form, and it is possible 



'they actively seize oil-droplets and other minute solid 

 particles. The cell passes the fat to its deeper end 

 thence, out into the subjacent lymphoid tissue. It is 

 probable that here certain amoeboid cells of the adenoid 

 tissue pick it up, and carry it into the central lacteal of a villus, 

 where they break up and set it free. In the villus there are 

 all the anatomical arrangements for a mechanism which shall 

 actively suck substances into it. Each is more or less 



