76 



HUMAN TISSUES 



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tion to mucous cells, there are other epithelial cells in the alimen- 

 tary canal adapted for secreting digestive juices. 



The epithelial cells lining the closed cavities of the body com- 

 pose structures called serous membranes. The serous membranes 

 line blood vessels, cover the lungs, line the chest, cover the 

 heart, cover the abdominal organs and line the abdomen. The 



serous membranes that 



cover the lungs are the 



ii M%\ ... ])leurae {pleura — rib). 



In breathing, when the 

 lungs become inflated or 

 larger, and compressed or 

 smaller, there would be 

 friction between the lung 

 and the wall of the chest, 

 were it not for this moist 

 epithelial tissue. If the 

 pleurae become inflamed, 

 the person is said to have 

 pleurisy. If the secretion 

 of fluid is interfered with, there is friction between the lungs and 

 the walls of the chest and breathing becomes painful. 



The covering of the heart is the pericardium {peri — around; 

 cardium — heart). This membrane prevents friction between the 

 heart and lungs and other organs. The membrane lining the 

 abdomen is the peritoneum. Infection or inflanunatioii of the 

 peritoneum is known as peritonitis. This condition is sometimes 

 brought about when an infection extends from a ruptured appen- 

 dix and attacks a part of the peritoneum. Friction among the 

 abdominal organs such as stomach, liver, and intestines is avoided 

 by the moist epithelial peritoneum. The abdominal organs do 

 not float in the abdominal cavity. If they did, jumping or bend- 

 ing might throw them out of place. They completely fill the 



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Epithelial cells may manufacture secretions. Certain 

 cells, globlet cells, are found in the gullet. They give 

 out mucin. Groups of epithelial cells often form simple 

 or compound glands which give off certain secretions. 



