CHAPTER IX 



TISSUES OF THE BODY 



The tissues and organs of the body illustrate perfectly the prin- 

 ciple of unity of structure in the individual organism. We speak 

 of the body as being made of tissues, each specialized for cer- 

 tain particular functions. Various tissues make up the organs 

 and systems of organs, and the systems make the individual. It 

 is therefore well to understand the fundamental tissues before 

 considering organs. 



Ordinarily five types of tissues are recognized: (A) epithelial 

 or covering tissues which cover the body and line all cavities; 

 (B) connective tissues which include all the supporting elements 

 of the body; (C) fluid tissues, the blood and lymph, which are 

 often included with the connective tissues; (D) muscular tis- 

 sues, those which are specialized for contractility; and (E) the 

 nerve tissues, the most highly specialized cells of the body, func- 

 tioning as carriers of stimuli. These five types are treated sep- 

 arately below. 



A. Epithelial tissues are perhaps the most diverse in struc- 

 ture and function of any found in the body. Most epithelial 

 cells retain the ability to reproduce themselves and replace 

 those cells which are lost. Epithelia include the outer layers of 

 the skin; the lining cells of the blood vessels and digestive tract; 

 and the secretory cells in the glands of the body. In fact, all cells 

 are included which act as a cover to other tissues. These cells 

 differ widely in origin and structure. The epithelium covering 

 the body is ectodermal; that which lines the digestive tract and 

 the glands derived from it are endodermal; and those lining 

 the blood vessels, the coelomic cavity, and other spaces, are 

 mesodermal in origin. Structurally several types are recognized. 



1. Squamous tissues have flat polygonal cells which are very 



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