BIGGEST AND BUSIEST FACTORY 205 



extent I have mentioned. In reality it probably amounts 

 to about 16 square feet. 



The interior of the bowel is paved with microscopic 

 living blocks of the usual kind — mucus epithelium. The 

 absorbing surface, which is formed by these units, must 

 be kept fresh, moist, and lubricated. These conditions 

 are secured by a constant formation of mucus in the 

 paving epithelium. Its units or blocks are continually 

 forming a fresh covering of mucus. But the paving 



Peritoneal coat 

 ! Nerve network 



Artery 



hatii 



Lump 

 ,' .Muscle 



c vessel 



v;ih 



A'rteru Lining epithelium 



Test-tube gland 



\ \ 'Vascular \auer 



\ Muscle Circular layer 



Muscle Lonait.- layer 



Fig. 42. — Section across a part of the wall of the small bowel showing the 

 structure of two villi ; the test-tube glands and lining epithelium are also 

 represented. 



epithelial units have much more important services to 

 perform than merely keeping the surface fresh and lubri- 

 cated ; they are the agents which select the products of 

 digestion, absorb them, and pass them on. They make 

 the substances which are absorbed part of the living body. 

 Those which come from starch foods or from nitrogenous 

 foods are passed into the blood of the capillary field and 

 are thus carried to the liver, but the soaps have a special 

 destination ; they are handed or directed to the lacteals, 

 and have several filters to pass through before they are 

 discharged into the great veins. Much, then, depends on 



