198 



Honjo a Co?nplex A?iimal Uses Food unit iv 



Absorption is more than simple diffu- 

 sion through a Hfeless membrane. The 

 cells that absorb take an active part in 

 the absorption as is shown by the fact 

 that they use more oxygen and produce 

 more carbon dioxide while they are ab- 

 sorbing. 



Absorption by the small intestine. If 

 you were to slit open the small intestine 

 along its length and examine the inside 

 with a powerful magnifying glass, you 

 would find it moist and pink like the 

 lining of your cheeks. But in other re- 

 spects it would be different. The inside 

 of your mouth is smooth; the lining of 

 the small intestine is wrinkled into deep 

 folds, sometimes one third of an inch 

 deep. If you rubbed your hand over the 

 folds and if your sense of touch were 

 delicate enough you would discover that 

 the folds feel like a soft brush or like 

 plush, for they are covered with micro- 

 scopic, hairlike projections. These are 

 called villi (vilPeye), plural of villus. 

 They are soft because they are made of 

 delicate cells. They sway back and forth, 

 now lengthening, now shortening. They 

 and the folds increase the lining surface 

 enormously. It has been estimated that 

 the surface of this narrow tube is more 

 than five times as great as the skin sur- 

 face of your whole body. Study the 

 drawing of a villus (Fig. 206) to see how 

 the digested foods can diffuse through 

 the thin layer of mucous membrane cov- 

 ering the villus and go into the tiny 

 blood vessels just underneath. Once the 

 food is in the blood vessels it can be car- 

 ried to larger and larger vessels and sent 

 to every part of the body. In the center 

 of the villus is a lacteal (lack'tee-al) into 

 which the fatty acids and glycerin go. 



Capillaries 



Lacteal {lymphatic) 



Mucous membrane 



Muscle cells 



Vein Artery 



Fig. 206 One of the villi of the sjnall intestine 

 cut through lengthwise. How many kinds of 

 tnbes does the villus contain? Can you see the 

 opening of one intestinal gland alongside the 

 villus? 



These products of fat digestion reach the 

 blood stream later. 



The large intestine. Parts of our food 

 are never digested because we have no 

 enzymes to act on them. This is true of 

 the thick cellulose walls of plant cells, 

 known as roughage, and other portions 

 of our food. These substances that have 

 not been digested are pushed on into 

 the large intestine. Here much water is 

 absorbed and the residue is ejected, or 

 eliminated, throuoh the anus. But the 

 nondigestible foods have actualh" been 

 useful. While in the small and large in- 

 testines they stimulated the walls to con- 

 tract, helping peristalsis. 



How does the liver function? You have 

 read how the bile helps in emulsifying 

 oils. It is of even greater importance in 



