THIRD LECTURE. 



THE EPIDERMIS, OR THE EPITHELIUM. 



Under epithelium we understand closely-arranged cell lay- 

 ers, held together by a minimal quantity of cement (p. 16); it 

 covers the surface of the body, the external as well as the 

 internal. 



All three plates of the germinal layer (p. 26), participate in 

 the production of the tissue under examination. The horn layer 

 supplies the covering of the corium, the so-called epidermis. 

 The lower germinal plate forms the epithelium of the diges- 

 tive apparatus and the organs arising from the latter. Not 

 less important is the role of the middle cell layer. Manifold 

 cavities originate in it; the passages of the vascular system, 

 the so-called serous sacs, the articular cavities, down to innu- 

 merable small and diminutive tissue spaces. All these again 

 have their epithelial cell covering. The latter is now called 

 endothelium. The principle is correct ; but the boundaries 

 cannot yet be sharply drawn throughout. 



Epithelium consists either of a simple cell layer, or the 

 cells are stratified more or less manifoldly over each other. 

 We distinguish, therefore, unstratified and stratified epithe- 

 lium. The latter originates in the horn layer. The former 

 is due to the intestinal gland, as well as the middle germinal, 

 plate. 



The form of the cell varies. Many kinds of epithelium 

 present only thin, flat, scale-like cells (Figs. 7 and 20). We 

 speak now of flattened or pavement epithelium. In other 

 varieties the cell is tall and slender. This is called cylindrical 

 epithelium (Figs. 6 and i<p. When the surface of the cylin- 

 drical cells has vibratory ciliae (Fig. 33) we have the vibratile 

 or ciliated epithelium. 



