V- 



CHAPTER II 

 EPITHELIAL TISSUES 



EPITHELIAL tissues, the epithelia, include all those cellular 

 membranes which cover the free surfaces of the body, either exter- 

 nal or internal, together with the cellular portions of the secreting 

 glands directly connected with, or developed from, these free sur- 

 faces. They thus include the epidermis of the skin, the mucous 

 membranes of the digestive, respiratory, and geni to-urinary tracts, 

 the cellular structures of the salivary glands, pancreas, liver, ovary, 

 testicle, kidney, and all other glands connected with these systems. 

 Portions of the organs of special sense the nose, eye, and ear 

 are also included within the scope of the term. 



The epithelial tissues are composed of cells which vary greatly 

 in their shape and histological characters, and which may be 

 arranged either en masse as in the Graafian follicles of the ovary, 

 or in tubules or acini as in most secreting glands, or as membranes 

 consisting of a variable number of cell layers. 



These membranes are either mucous or serous. Mucous mem- 

 branes include all those which are connected, directly or indi- 

 rectly, with external surfaces of the body, such as those of the 

 esophagus, stomach, bronchial tubes, bladder, etc. Their epithe- 

 lium is of epiblastic origin. Serous membranes, construing the 

 term in its broadest sense, include those cellular layers which line 

 all of the closed cavities of the body, viz., the arachnoid mem- 

 branes of the brain and spinal column, the pleurae, pericardium, 

 peritoneum, tunica vaginalis of the testicle, and the synovial mem- 

 branes, bursae, and sheaths of the tendons. The epithelial cells 

 which line these latter structures, together with those forming the 

 lining membranes of the circulatory system heart, arteries, capil- 

 laries, veins, and lymphatic vessels are of mesoblastic origin and 

 are classified as endotlielium, the term referring to their distribu- 

 tion within the closed cavities of the body. When epithelial cells 

 are arranged to form a membrane, they may occur either as a 

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