STRATIFIED EPITHELIUM. 195 



penetrate into epithelial tissue, although in some cases minute channels may exist 

 between the cells into which the plasma of the blood, derived from the blood-vessels 

 of the subjacent connective tissue, may pass for the nutrition of the epithelium-cells. 



Nerves are abundant in many epithelia, the nervous fibrils passing in the form 

 of fine varicose filaments among the epithelium-cells. 



In certain situations branched " migratory cells " which may contain pigment lie 

 in the intercellular substance of an epithelium. 



Epithelium-cells vary in structure as well as in shape, and some of these differ- 

 ences will be mentioned in speaking of the varieties of epithelium. The nucleus 

 varies, however, far less than the rest of the cell : in most cases it has an intra- 

 nuclear network and one or more nucleoli. In the division of epithelium-cells, it 

 undergoes the changes which have already been described. 



Classification of epithelia. The varieties of epithelium may be classified in 

 various ways, but none perhaps are altogether satisfactory. Thus we may dis- 

 tinguish an epithelium according to its origin, as epiblastic, mesoblastic, or 

 hypoblastic, and this distinction is partially indicated when a separate term 

 (endothelium) is used to denote mesoblastic epithelium. Or again, the epithelia 

 may be classed according to their function, and in this way we distinguish between 

 the protective, the secreting, the ciliated, and the sense-epithelia. But without 

 failing to recognize that these modes of classification have a certain amount of 

 importance, it will be most convenient here to follow the prevalent custom, 

 and to classify the varieties of epithelium-cells according to their shape and 

 arrangement. 



In the first place we may distinguish an epithelium which is composed of only a 

 single layer of cells as a simple epithelium in centra-distinction to a stratified epi- 

 thelium, in which the layers of cells are numerous. Where, on the other hand, the 

 cells are in more than a single layer, but the two or three layers dove-tail the one 

 into the other, so that the structure is not distinctly stratified, the term transitional 

 may be employed. 



Stratified epithelium. In a stratified epithelium the cells are disposed 

 in a number of layers, and it is commonly found that the constituent cells of the 

 various strata exhibit every variety of shape. As a rule the "cells of the deepest or 



ESS 



Fig. 222. SECTION OP THE STRATIFIED EPITHELIUM COVERING THE FRONT OF THE CORNEA OF THE EYE. 



HIGHLY MAGNIFIED. (E. A. S.) 



c, lowermost columnar cells ; p, polygonal cells above these ; fl, flattened cells near the surface. The 

 intercellular channels bridged by minute processes of the cell, are well seen. The lower part of the 

 section on the right is somewhat broken. ,. > 



attached layer are columnar (fig. 222, e), and the superficial cells are flattened scales 

 (fig. 222, fi) which may be of considerable size, but which do not, like the cells of pave- 

 ment or simple scaly epithelium, fit together by their edges, but, on the contrary, over- 

 lap one another (fig. 223). The cells of the layers immediately external to the columnar 

 layer are rounded in shape or at least only so far modified as to enable them to fit 

 to the columnar cells and to one another (fig. 222, p) ; but as we trace the strata 



