CHAPTER XX 



TISSUES 



As a result of differentiation a variety of cells is produced. These 

 tend to be associated in groups of similar cells to which is applied, in 

 general, the term tissues. 



120. Definition. — A tissue is a group of somatic cells which are 

 similarly differentiated — that is, which are similar in structure and which 

 perform one or more functions in common — together with the structures 

 produced by them. In some tissues is found intercellular material which 

 is developed from the cells and which is very important in the performance 

 of the particular function belonging to the tissue. 



Among the various tissues in the body are recognized four distinct 

 types, classified on the basis of both structure and function: (1) epithelia, 

 or epithelial tissues; (2) supporting and connective tissues; (3) muscular 

 tissues; (4) nervous tissues. 



121. Epithelia. — An epithelium, or an epithelial tissue, is the type of 

 tissue which covers any free surface, either the outside of the body or the 

 walls of cavities within it. In the simplest Metazoa this is the only kind 

 of tissue present and there may be little differentiation in it in the differ- 

 ent parts of the body. In the more complex animals, however, the 

 epithelia found in various parts of the body become quite diversified and 

 are named according to the shape of the cells or to the functions which 

 they perform. For example, an epithelium which on its outer surface 

 is made up of very flat cells is termed a pavement epithelium; one in which 

 the surface cells are in the shape of long prisms, set at right angles to the 

 surface, is called a columnar epithelium; and one in which the cells on the 

 surface bear cilia is known as a ciliated epithelium. If an epithelium 

 possesses only one layer of cells it is termed simple; if it has several layers, 

 stratified (Fig. 38). Examples of epithelia named from their function are 

 sensory, glandular, protective, and reproductive. 



The functions which epithelia perform are several. Some serve to 

 protect the structures below them. Others contain sensory cells and 

 serve to receive and transmit stimuli from the outside. Through 

 epithelia all food has to enter the body, and also all waste matter has to 

 leave. They also produce many of the secretions which, when poured 

 out upon a surface, serve to moisten it, to lubricate it, or to digest food. 

 Reproductive cells arise from what are called germinal epithelia. 



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