EPITHELIAL TISSUES . 7 



In the digestive organs the epithelial layer plays an important part in the 

 formation of digestive fluids, and also in the absorption of digested food. 



In the lining of closed cavities it -assists in the formation of the fluids which 

 they contain (example, the pleura). 



Included under this heading are (with brief notes of functions) : 

 Gland tissue, where a layer of cells has the power to form 

 a special substance from the blood. (Adenoid tissue 

 resembles gland tissue.) (See p. 8.) 



Mucous membranes, which line all interior surfaces to which 

 air has access. Their special cells produce a clear thick 

 fluid called mucus which keeps the surfaces moist. 



Serous membranes, which line the closed cavities of the body. 

 They are themselves closed sacs containing a clear thin 

 fluid called serum which prevents the surfaces from 

 rubbing together. 



Synovial membranes, which line the interior of movable joints; 

 they contain a thick fluid called synovia which like serum 

 prevents friction. 



The epithelial lining of the heart and blood-vessels, serous membranes, 

 and lymph vessels, is called endothelium. 



Clinical notes. Mucous membranes are well supplied with blood-vessels 

 and bleed freely when wounded, as seen in operations upon the nose and 

 throat. 



An accumulation of serum- in the large serous membrane of the abdomen 

 causes the condition called asciles (a variety of dropsy). 



The processes of secretion and excretion are carried on through 

 epithelial cells. (In specialized epithelial tissues.) 



Secretion is the process of separating substances from the blood 

 (generally in fluid form). Such substances if useful to the body 

 are called secretions; if they are waste matters to be thrown off or 

 eliminated, they are called excretions. 



Secreting organs mucous and serous membranes, all glands. 



Excreting organs lungs, kidneys, liver, cutaneous glands. 



To summarize the functions of epithelial tissues they are 

 protective, secretory, excretory, absorptive. 



An organic substance is a substance formed by living cells, 

 whether they are single or arranged together in organs. Organic 

 substances disappear in burning. Inorganic substances are mineral. 



