112 DIGESTION 



the body before the residue passes into the large in- 

 testine. This mechanism consists of minute finger-like 

 processes, the villi, which project from the inner wall 

 of the intestine into its cavity, where they are bathed 

 by the chyle. The villi are covered with a continuous 

 layer of cells. Within this wall of cells lies a network 

 of connective tissue cells, muscle fibres and blood-vessels. 

 Scattered among these are irregular spaces, into which 

 open the mouths of tube-like channels, called lacteals. 



Absorption of food. The solutions of food materials 

 absorbed by the villi from the chyle pass into the body 

 in two ways. The peptones and the sugars diffuse di- 

 rectly into the fine blood-vessels in the villi and are 

 thence carried in the blood stream to the liver. The 

 fats, which remain in the villi after the peptones and 

 sugars have been absorbed, are pumped by them into 

 the lacteals, which in turn take them into the blood but 

 not by way of the liver. 



In this process of absorption, it is difficult to see how 

 the solutions get through the cells which form the mem- 

 brane covering the villi. Experiments show conclusively 

 that the cells must have the power of passing the solu- 

 tions of peptones and sugars and the emulsion of fat 

 through themselves. It is true that peptone and sugar 

 can diffuse through animal membrane, but, unless they 

 were assisted by some action of the cells themselves, it 

 would be at a much slower rate. Fats do not seem 

 to be able to pass through animal membranes even slowly 

 and therefore their absorption certainly requires the 

 active assistance of the cells. Indeed, it is probable 

 that the cells even change the fats into soaps in order 

 to facilitate their passage. 



Food after absorption. As the digested food passes 

 through the membrane covering the villi, its character 



