EPITHELIUM 35 



acini containing sucli cells are usually large. The cells 

 themselves are large, and are placed on a delicate basement 

 membrane, a condensation of the subjacent fibrous tissue, 

 which bounds the acinus. The nuclei are situated near to 

 the attached margin of the cells. These 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. 



Resting State. — In the former case, in the fresh condition, 

 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, and the cells tend to burst, so that the lumen 

 of the gland becomes filled with the glairy mass. 



After Activity. — After the gland has been actively 

 secreting, the cells are smaller and the granules are much 

 less 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 mucin- 

 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. 



Mucin is a substance which occurs in many tissues. 

 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 emulsoid, and from this it is pre- 

 cipitated by acetic acid. It is a conjugated protein 

 (Appendix) — a protein linked to glucosamine — a glucose 

 molecule in which one of the hydroxyls is replaced by 

 amidogen — NH2 — 



NH../H\ H 



H— C-C— C — C— H 



H \OHJ3 OH 

 It is therefore called a glyco-protein. When boiled 



