38 



HANDBOOK OF PHYSIOLOGY. 



superficial cells are as a rule of the squamous, or scaly variety, and the 

 deepest of the columnar form. 



The cells of the intermediate layers are of different shapes, but those 

 of the middle layers are more or less rounded. The superficial cells are 

 broad and overlap by their edges (figs. 33 and 34). Their chemical corn- 



Fig. 33. Squamous epithelium scales from the inside of the mouth, x 260, (Henle.) 



position is different from that of the underlying cells, as they contain 

 keratin, and are therefore horny in character. 



The nucleus is often not apparent. The really cellular nature of 

 even the dry and shrivelled scales cast off from the surface of the epi- 

 dermis can be proved by the application of caustic potash, which causes 

 them rapidly to swell and assume their original form. 



The equamous cells exist in the greatest number of layers in the epi- 

 dermis or superficial part of the skin; the most superficial of these are 

 being continually removed by friction, and new cells from below supply 

 the place of those cast off. 



The intermediate cells approach more to the flat variety the nearer 

 they are to the surface, and to the columnar as they approach the lowest 



Fig. 34. Vertical section of the stratified epithelium covering the front of the cornea. 

 Highly magnified. (Schafer.) c, Lowermost columnar cells; p, polygonal cells above these ; 

 fl, flattened ceils near the surface. The intercellular channels, bridged by minute cell processes, 

 are well seen. 



layer. There may be considerable intercellular intervals; and in many 

 of the deeper layers of epithelium in the mouth and skin, the outline of 

 the cells is very irregular, in consequence of processes passing from cell 

 to cell across these intervals. 



Such cells (fig. 35) are termed " ridge and furrow/' " cogged " or 

 " prickle " cells. These " prickles " are prolongations of the intracellular 

 network which run across from cell to cell, thus joining them together, 



