THE LARGE INTESTINE 113 



is once again changed. In the blood, peptone is not 

 found, but in its place appear certain forms of albumin 

 and other proteids peculiar to the blood. The emulsion 

 of fat becomes again ordinary fat. The maltose, the spe- 

 cial form of sugar derived from the starches and sugars 

 eaten, alone remains as maltose. When this reaches 

 the liver, however, most of it also is changed back into 

 a form of starch, the so-called liver or animal starch 

 known as glycogen. Thus we see that the main purposes 

 of digestion are to change the food so that it can be 

 readily absorbed and then manufactured, in the case 

 of proteids and fats, into the forms peculiar to the 

 blood. 



Undigested residue. After the chyle has been ab- 

 sorbed from the small intestine, there still remains a cer- 

 tain amount of undigested material. This ordinarily 

 consists of the cellulose from the vegetables and grains 

 eaten, of the connective tissue from meat, and of any 

 digestible matter which has escaped the action of the 

 various digestive juices. This residue is passed from the 

 small intestine into the large intestine through the con- 

 necting ileocecal valve. Occasionally, when masses of 

 indigestible material are unable to pass readily through 

 this valve, a pronounced distention of the intestine re- 

 sults, producing the pain known as colic. 



Large intestine. The large intestine is about five 

 feet long in an adult and varies from one and a half to 

 two and a half inches in diameter. Its walls are made 

 up of a series of pockets, the partitions of which serve 

 to prevent its contents from passing forward too rapidly. 

 In it, a certain amount of digestion and absorption, espe- 

 cially of water, still goes on. As a result, the mass of 

 undigested food and waste becomes firmer and dryer. 

 It passes forward around the partitions in the walls and 



