158 



FOUNDATIONS OF BIOLOGY 



FIG. 89. Chemical activities of the human digestive tract. 



about the solution of the food materials. Every cell of the 

 body receives .^rom the circulatory system the materials nec- 

 essary for its own life, but some cells take in addition sub- 

 stances which they do not need and, after transforming them 

 chemically, contribute the product as a SECRETION for the 

 good of the whole organism. Such cells may act more or less 

 independently as UNICELLULAR GLANDS, but generally, for 

 economy of space and adequate blood supply, many cells are 

 grouped together to form MULTICELLULAR GLANDS. This is 

 usually brought about by sinking the glandular area below 

 the level of the membrane to which it really belongs. Such 

 is the origin of complex glands as the liver and pancreas, 

 which are outpocketings of the wall of the digestive tract; 

 the sole remaining connection in each case being a narrow 

 tube, or DUCT, which delivers the products of the glands to 

 the intestine. (Fig. 90.) 



