194 STRUCTURE OF EPITHELIAL TISSUE IN GENERAL. 



THE EPITHELIAL TISSUES. 



An epithelium is a tissue which is composed entirely of cells with a compara- 

 tively small amount of intercellular substance. It usually takes the form of a 

 membrane covering the free surfaces of the body. Thus it is well known, that 

 when the skin is blistered, a thin, and nearly transparent membrane, named the 

 cuticle or epidermis, is raised from its surface. In like manner a transparent film, 

 similar in nature to the epidermis, may be raised from the lining membrane of the 

 lips and mouth, in which situation it first received the name of " epithelium ; " and 

 under the latter appellation, a coating of the same kind exists on nearly all free 

 surfaces and membranes of the body. 1 



The following are the most important situations in which a covering or lining of 

 epithelial tissue is found : viz., 1. On the surface of the skin. 2. On mucous mem- 

 branes ; a class of membranes to be afterwards described, which line those internal 

 cavities and passages of the body that open exteriorly, viz., the alimentary canal, 

 the lachrymal, nasal, tympanic, respiratory, urinary, and genital passages ; as well 

 as the various glandular recesses and ducts of glands, which open into these passages 

 or upon the surface of the skin. 3 3. Lining the ventricles of the brain and the 

 central canal of the spinal cord. 4. In the organs of special sense where the cells 

 are often greatly modified and receive the endings of the nerves of special sense. 

 5. On ,the inner or free surface of serous membranes, which line the walls of closed 

 cavities in the chest, abdomen, and other parts, and on the inner surface of the 

 heart, blood-vessels and lymphatics. In these situations the epithelial lining has 

 received the name of endothelium (His). 



In many parts of the connective tissue the cells of that tissue are flattened out and 

 arranged close together, edge to edge, in such a manner as to form a membrane of cells, which 

 so far would come under the definition of the term epithelium. But the cells in question 

 exhibit every transition to the other cells of the connective tissue, so that their enumeration 

 under epithelium would create an artificial separation between cells of the same elementary 

 tissue. They may, however, be conveniently described as epithelium-like (epithelioid). Many 

 histologists are of opinion that a similar distinction should be made for the epithelium of 

 the serous membranes and of the vessels, because these are developed within the mesoblast, 

 and it is the following up of this idea which has led to the adoption of the word 

 " endothelium " to express an epithelium so derived. But, if every epithelium which originates 

 in the mesoblast is to be so designated, we shall be compelled to separate from the other epithelial 

 tissues, with which they are in every way closely allied, the epithelia of the renal, and of the 

 generative organs, since these appear to be derived from the same layer of cells. And 

 indeed, since it has been shown in several instances amongst both invertebrates and vertebrates, 

 that the epithelium of the serous cavities, and even that of the heart, is directly derived 

 from an undoubtedly epithelial layer the entoderm it is probable that this is the original 

 and typical mode of origin of all the so-called endothelia. although it has become obscured 

 in the development of higher vertebrates. 



Structure of epithelial tissue in general. Every epithelial tissue is 

 formed entirely of cells united together by cohesive, matter, often in too small 

 quantity to be apparent without the employment of nitrate of silver staining. -The 

 layer or layers thus formed take the shape of the surface-to which they are applied, 

 following accurately all its eminences and depressions. As a rule no blood-vessels 



1 The term "epithelia," which has passed into "epithelium," was introduced by Ruysch to desig- 

 nate the cuticular covering on the red part of the lips. The word y epidermis ".he considered inappro- 

 priate, as the subjacent surface is not skin (derma) ; butj as 'it is beset with papillae, he named the 

 covering layer " epi-thelia," from eirt and 6ri\t], a nipple or papilla. The use of the term has, by a not 

 unusual license been extended so as to signify the same kind of coating when it spreads over non- 

 papillary surfaces. 



2 The hairs and nails and the enamel of the teeth, as the study of their development shows, 

 are also undoubtedly of epithelial origin, but in the case of the enamel and in a lesser degree of the hairs, 

 they have become so specialized that their epithelial structure is scarcely longer recognizable. 



