CHAPTER IV. 



THE TISSUES. 



Classes of tissues : Cells and intercellular elements make 

 up tissues ; tissues make up organs. A full knowledge of the 

 different varieties of tissues is essential to an adequate and 

 easy understanding of the structure of organs. The simple 

 tissues of the human body may be divided into five classes : 



Epithelium and endothelium ; 



Connective tissue ; 



Muscle ; 



Nerve-tissue ; 



Free cells and body-fluids. 



Of these the first four are made up of fixed elements ; the 

 last comprises the free and fluid parts of the organism. These 

 classes are sharply distinguished from one another by morpho- 

 logical and functional characters and by the fact that each 

 class, in general, is developed from and in turn produces its 

 own kind of cell and tissue exclusively. 



Endothelium and Epithelium. 



Location : The cells lining the exposed surfaces of the body 

 form the class of epithelium and endothelium. 



Those lining the closed internal cavities, as the vessels and 

 serous sacs, and derived from the mesoblast, are called endo- 

 thelial cells. 



Those lining the " external " surface of the body and the 

 channels and tracts continuous with it (that is, those lining the 

 skin, mucous membranes, and tributary glands), also the 

 closed thyroid and other ductless glands, central canal of the 

 spinal cord, and ventricles of the brain, are called epithelium. 



Epithelium consists almost entirely of cells, there being only 

 a slight amount of intercellular substance to cement them to- 

 gether. It is derived from the epiblast and hypoblast, except 



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