236 UNCONSCIOUS NERVOUS OPERATIONS 



and small intestine, so that what is left to pass into the 

 large intestine is indigestible matter and the remains of 

 the juices. The tubular glands in the mucous membrane 

 here absorb what is left of nutrient material and a con- 

 siderable amount of water, which is carried by the small 

 veins to the portal vein. The residuum, becoming more 

 solid as the water is absorbed, passes into the rectum and 

 is discharged as faces. The chief alteration in the con- 

 tents of the large intestine is due to the action of micro- 

 organisms which cause chemical changes, giving rise to 

 organic acids. 



341. Absorption. When the food has been digested it 

 is ready to be absorbed. It cannot be used for the sup- 

 port of the body until it reaches the blood, and there are 

 two ways by which it may reach the blood. First, it may 

 be absorbed directly into the blood. The mucous lining 

 of the alimentary canal is richly supplied with absorbing 

 cells. Little of the food, however, is taken up in the 

 mouth and esophagus, because of the thickness of the 

 epithelium in those cavities, and because the food passes 

 quickly through them. In the stomach, and especially in 

 the small intestine, the absorbents are very numerous, and 

 a large amount of digested food passes directly into the 

 blood stream by the capillaries through the columnar cells 

 of their epithelium ; while the walls of the large intestine 

 also absorb food, but to a less extent. This direct pas- 

 sage into the blood is now regarded as the more important 

 of the two channels of absorption. The living cells, while 

 allowing water and soluble salts to pass through them 

 unchanged, effect certain changes in the organic food 

 materials while in contact with them. The carbohydrates 

 and proteids are mainly absorbed into the blood in this 

 immediate way. 



