CHAPTER II. 



THE ELEMENTARY TISSUES. 



The various parts of the body are composed of a small number of 

 " elementary tissues." Each of these elementary tissues has a definite 

 structure, but the details of that structure may vary within certain lim- 

 its in different parts of the same mass or in different situations within 

 the body. Such variations can usually be referred to differences in the 

 functional activity assigned to the tissue, which is not always exactly 

 the same throughout the body. For example, epithelium is an ele- 

 mentary tissue consisting of cells which are nearly always rich in 

 cytoplasm and are separated from each other by a very small amount 

 of homogeneous intercellular substance. Wherever epithelium is 

 found it has these general peculiarities of structure. But the func- 

 tions demanded of epithelium are of widely diverse character in 

 different situations, and its structure shows a corresponding diversity 

 in its details. The fact that it is made up almost exclusively of 

 cells leads to the natural inference that the usefulness of epithelium 

 depends upon cellular activities. Inasmuch as these may be of 

 very different character, we should expect the tissue to vary chiefly 

 in the structure and arrangement of its component cells according 

 to the particular activity which was needed and the manner in 

 which it was utilized. Such, as a matter of fact, is the case. These 

 considerations will be made clearer if we follow a little more closely 

 the example offered by epithelium. 



In some situations epithelium serves to protect the underlying 

 tissues from injury. But the usual injurious influences which 

 threaten the tissues differ in different parts of the body, and 

 must, therefore, be averted by different means. Upon the sur- 

 face of the skin they are chiefly of a mechanical or chemical 

 nature, and to resist them the cells of the epithelium forming the 

 epidermis undergo a modification in structure, resulting in the 

 formation of a superficial horny layer which is highly resistant to 

 abrasion and chemical change. Upon the inner surfaces of the 



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