100 DIGESTION 



structures like the bones and teeth. When it is liquid or 

 semiliquid the matter is caUed a secretion. There are 

 many kinds of secretion given out by the various cells of 

 the body, and some of the secretions have an important 

 part to play in the action of the body. So important are 

 some of these secretions, in fact, that certain cells are set 

 aside whose sole work is their preparation. These special- 

 ized cells often become grouped together in certain regions, 

 and ai-e then spoken of collectively as glands. A gland, 

 then, is merely a collection of cells whose special duty is 

 the giving off of a certain kind of secretion. To this group 

 of secretions belong all the digestive fluids, and the cells 

 or group of cells which secrete them are called the diges- 

 tive glands. In this group are included the salivary 

 glands, the pancreas, the liver, and the mucous membrane. 



The form of tissue which furnishes most of these speci- 

 alized cells is the epithelial, and the simplest form of a 

 gland would be a layer of these cells so placed as to receive 

 food from the blood and give off their secretion. Such a 

 glandular structure is called a secreting membrane, and an 

 example is the mucous membrane which lines aU parts of 

 the alimentary tract. (See Fig. 29.) 



In so complicated a structure as the body, however, it 

 is necessary to economize space, and it is easy to see that 

 one of the simplest methods of doing this is to arrange a 

 layer in a simple fold or sac. In this form (see Fig. 29) 

 the secretion is poured out into the central cavity or 

 lumen, which, when filled, overflows through the mouth 

 or opening of the gland. Of such a character are the 

 gastric glands of the stomach, and the simple glands 

 (crypts) of the intestine. Evidently, the next step in 

 saving space would be to collect a series of these simple 



