Chap. IX.] NUTRITION AND METABOLISM. 187 



duodenum. At the same time, however, the process of absorp- 

 tion begins. Probably some little soluble matter is absorbed in 

 the mouth. But in the stomach a little of the peptones there pre- 

 pared, together with the glucose prepared by the saliva, and other 

 diffusible fluids, may pass by osmosis through the membranous 

 walls of the stomach, and so entering the blood of the capillaries, 

 are carried to the portal vein, and thus to the liver. 



When it passes into the duodenum the acid chyme is subject 

 to the action of the alkaline fluids, bile, pancreatic juice, and 

 succus entericus. The fats are emulsified and partly saponified. 

 Under the influence of the pancreatic fluid the conversion of 

 starch into su^ar is renewed. The bile causes the precipitation 

 of the peptones, while the gastric pepsin is also precipitated and 

 rendered inert, and thus the field is left open for the digestive 

 influence of the trypsin of the pancreatic fluid. Thus, as it passes 

 onwards through the small intestine by the wave-like peristaltic 

 contractions of its muscular walls, the three chief constituents of 

 the food, the proteid, the starch, and the fat, are so acted upon as 

 to be converted, the first two into diffusible fluids, the fat partly 

 into a soluble soap, but chiefly into an emulsion. 



Absorption of these materials, which are still, it must be re- 

 membered, strictly speaking outside the body, is in the small 

 intestine effected by the villi. Each of these minute processes 

 has in its centre the termination of a lymphatic canal, which is 

 here called a lacteal ; and surrounding this is a close plexus of 

 blood-capillaries. The diffusible fluids pass through the epi- 

 thelial layer, and are caught up by the blood-stream in the 

 capillaries and hurried away to the liver. The emulsified fats, 

 and such soluble matters as have escaped absorption into" the 

 blood-stream, pass into the lacteal rootlet, and so into the lym- 

 phatic system. This passage is aided by the alternate shortening 

 and elongation of the villus. By the former process the con- 

 tained fluid is forced through the lacteals, whence its return is 

 prevented by valves : by the latter the lacteal rootlet in the 

 villus tends to suck in the surrounding material. But still the 

 question how the fats pass into the lacteals, through the cells or 

 between the cells, is not well understood. It certainly is largely 



