THE TISSUES 23 



to the attached margin of the cells, which are somewhat 

 irregular in shape, and are packed close together. Their 

 appearance varies according to whether the gland has been 

 at rest or has been actively secreting. 



Bestiny State. In the former case, in the fresh condi- 

 tion, the cells are large, and pressed closely together. Their 

 protoplasm is filled with large shining granules. After treat- 

 ment with reagents, each cell becomes distended with clear, 

 transparent mucin formed by the swelling and coalescence 

 of the granules. 



After Activity. After the gland has been actively secret- 

 ing, the cells are smaller and the granules are much less 



KX\<*^.'.?y r*r 



vl-<y/v.-M \&*&<' - 



FIG. 6. A Zymin-secreting Gland, to show an acinus lined by secreting 

 cells containing zymogen granules and the duct. 



numerous, being chiefly situated at the free extremity of the 

 cell, and leaving the nucleus much more apparent. 



This form of epithelium, during the resting condition of 

 the gland, takes up nourishing matter and forms this niucin- 

 yielding substance. During the active state of the gland, 

 the mucin-yielder is changed to mucin, and is extruded from 

 the cells into the lumen of the gland. 



Muein is a substance of great importance in the animal 

 economy. When precipitated and freed from water it is 

 white and amorphous. On the addition of water it swells 

 up and forms a glairy mass. In the presence of alkalies it 

 forms a more or less viscous solution, and from this solution 

 it is precipitated by acetic acid. In composition it is a pro- 

 tein linked to a molecule allied to the sugars, glucosamine 



