VAEIETIES OP EPITHELIUM. 43 



received the name of " epithelium ;" * and under the latter appellation, 

 a coating of the same kind exists on nearly all free surfaces of the 

 body. It is true that in many situations the epithelium cannot be 

 actually raised from the adjacent surface as a coherent membrane, still 

 its existence as a continuous coating can be denionstrated ; and, al- 

 though in different parts it presents important differences, its several 

 varieties are connected by certain common characters. 



The existence of a cuticular covering composed of cells has in one fonn 

 or other been demonstrated in the following situations : viz., 1. On the 

 surface of the skin. 2. On mucous membranes; a class of membranes to 

 be afterwards described, which line those internal cavities and passages 

 of the body tliat 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. On the inner or free 

 surface of serous membranes, which line the walls of closed cavities in 

 the head, chest, abdomen, and other parts. 4. On the inner surface of 

 the heart, blood-vessels and lymphatics. f 



Structtire in general. — This tissue has no vessels, although nerves 

 have been demonstrated in it in various situations ; apart from tliese, 

 however, it possesses a decidedly organised structure. Wherever it 

 may exist, it is formed essentially of nucleated cells united together by 

 cohesive matter, often in too small quantity to be apparent. The cells, 

 where consisting of more than one layer, in whatever way they may 

 be produced, make their appearance first in the deepest part of the 

 structure, where they receive material for growth from the blood-vessels 

 of the subjacent tissue ; then, usually undergoing considerable changes 

 in size, figure, and consistency, they gradually rise to the surface, where, 

 as shown at least in various important examples, they are thrown off 

 and succeeded by others from beneath. In many situations the cells 

 form several layers, in which they may be seen in different stages of 

 progress, from their first appearance to their final desquamation. The 

 layer or layers thus formed take the shape of the surface to which they 

 are applied, following accurately all its eminences, depressions and 

 inequalities. Epithelium when destroyed or cast off", is, for the most 

 part, very readily regenerated. 



Varieties. — In accordance with the varied purposes whioh the epi- 

 thelium is destined to fulfil, the cells of which it is composed come to 

 differ in different situations, in figure and size, in their position in 

 respect of each other, their degree of mutual cohesion, and in the 

 nature of the matter they contain, as well as in the vital endowments 

 which they manifest ; and, founded on these modifications of its con- 



* The term "epithelia, " which has passed into "epithelium," was introduced by 

 Ruysch to designate the cuticular covering on the red part of the lips. The word 

 * ' epidei-mis " he considered inappropriate, as the subjacent surface is not skin (derma) : 

 but, as it is beset with papilke, he named the covei-ing layer " eiti-thelia," from (tti and 

 CtjAtj, a nipple or papilla. The use of the term has, by a not unusual license, been ex- 

 tended so as to signify the same kind of coating when it spreads over non-papillary 

 surfaces. The word "endothelium," recently applied by some German WTiters to dis- 

 tinguish what has heretofore been spoken of as the epithelium lining the serous mem- 

 branes, and the inner surface of blood-vessels and lymjdiatics, appears to me a needless 

 innovation, and, considering the literal meaning of the word, not a happy one. — W. S. 



t The flattened cells which are enumerated under 3 and 4, and which have a close 

 affinity with the ceils of the connective tissue to be afterwards described, may be con- 

 veniently distinguished by the term " epithelioid." 



