CHAPTER II. 



THE EPTTHELIAL TISSUES. 



The various types of epithelial tissues are composed of one or 

 more layers of cells lyino; so close to one another that there is 

 practicall}' no intercellular material. Membranes of this tissue 

 cover the free surfaces of the body and line not only the ducts con- 

 necting with the surface, but also cavities within the body not 

 connected with the outer surface. One of the chief general func- 

 tions of these tissues is ])rotection, such as preventing the loss of 

 body fluids or the invasion of foreign material. Most types of 

 epithelia have some secretory activity, but those lining numerous 

 glands are ])rimarily secretory. Epithelia are intimately concerned 

 in processes involved in respiration, in the assimilation of nutritive 

 materials, and in the elimination of wastes. 



Classification. — It must be admitted that any classification of the 

 various types of epithelia will be artificial, and the justification of 

 any scheme will lie in the ease and completeness it oft'ers for dealing 

 with the types in question. A classification of e])ithelia is not 

 possible on the basis of function alone, for the exact function of many 

 types is not yet clear, and others may have more than a single 

 function. Embryological origin likewise fails, since often the same 

 type is derix'ed from ectoderm, endoderm, and mesoderm. The 

 most satisfactory scheme uses the form and arrangement of the cells. 

 Even on this basis, allowances must })e made for intergrading forms. 

 Two large groups are separated depending upon the presence of 

 one or more than one layer of cells in the tissue. Tissues belonging 

 to the first group have a single layer of cells and are called simple 

 epitheha. In the second group are the stratified epithelia, where 

 more than a single layer of cells compose the tissue. Various types 

 within these two groups are then separated with reference to the 

 form and arrangement of the cells composing each. 



SIMPLE EPITHELIA. 



This group is usually subdivided into three types, namely, 

 squamous, cuboidal, and columnar. (Fig. 5.) These terms are 

 convenient for indicating types commonly appearing in histological 



