88 ELEMENTARY BIOLOGY 



of the digested matter is withdrawn into the villi. From the 

 surface cells of the villi the absorbed material is passed on, by 

 osmosis, to the blood vessels and to the lymph vessels. Chem- 

 ical changes take place in the course of the transfer, so that 

 the material taken into the blood is not in exactly the same 

 state as the material absorbed from the intestine, although, of 

 course, it is made up of the same elements. 



By the time the dinner you have eaten has reached the end 

 of the small intestine, most of the proteins, fats, and carbo- 

 hydrates that it contained have been absorbed by the villi and 

 passed on into the blood and lymph. There is left in the in- 

 testines at this point chiefly the undigested (for the most part 

 indigestible) fibrous and cell-wall material of the plant or animal 

 tissues eaten, and the modified secretions of the various glands 

 that have poured into the food tube all along the way. This 

 mass of refuse now passes into the large intestine (Fig. 28, k). 



124. The large intestine. In the large intestine the fer- 

 ments of the digestive juices may still continue to act for some 

 time. But gradually, as the mass proceeds along the canal, it 

 becomes drier, through the continued absorption of material 

 by the lining of the intestine (there are no villi in the large 

 intestine), so that toward the end the only chemical changes 

 going on are those produced by the millions of bacteria that 

 are present in the intestines of all animals. 



The mass of material that has accumulated toward the end 

 of the large intestine is of no further use to the body, and 

 should be removed from time to time. Birds, having no large 

 intestine, throw off the refuse about as fast as it passes from 

 the small intestine to the rectum (Fig. 32). Other animals 

 and infants throw off the refuse automatically. But older 

 children, absorbed in their games or other activities, are apt 

 to postpone emptying the bowels, and thus become irregular. 

 This neglect of the bowels often brings about serious conse- 

 quences, so that it is important for us to acquire regular bowel 

 habits while we are still young (see p. 1 18). 



