u 



EPITHELIUM. 



stitnent cells, or, at any rate, those forming the superficial layer, four 

 principal varieties of epithelium have been recognised, as follows : — 



1, The cells may become flattened into plates or scales, and the 

 variety of epithelial tissue thus constituted is termed scal/j, or tesseUafed 

 {imvemmt cintheltum of German histologists). It might be well to 

 employ the former term when the flattened cells overlap at their edges 

 (as in fig. 17), the latter where the adjoining edges meet ; in which 

 case the lines of junction may be even (as in fig. 18), or more or less 

 sinuous, as in various parts of the lymphatic system (fig. 19). 



Fig. 17. 



Fig. 18. 



Pig. 17. — Epithelium-Scales from the inside op the Mouth; magnified 260 



DIAMETERS (Henle). 



Fig. IS. — Epithelioid Cells from a Serous Membrane (Peritoneum) ; magnified 



410 diameters. 



a, cell ; h, nucleus ; c, nucleoli (Henle). 



2. In a second variety named columnar (cylinder-epithelium of the 

 Germans) the cells assume a prismatic figure, and are set upright on 

 the surface which they cover (fig. 20). 



Fig. 19. 



Fig. 20. 



\}'M-' 



Fig. 19. — Epithelioid Cells of Commencing Lymphatic ; magnified 240 diameters 



(Auerbacli). 

 Fig. 20. — Columnar Epithelium from intestinal villus of a Rabbit ; magnified 300 



diameters. 

 a, TLick border (from KiJllikcr). 



3. The cells may retain their primitive roundness, or, being flattened 

 where they touch acquire a polyhedral or cubical figure, in which no one 

 dimension remarkably predominates : in some places, however, the cells 

 show a tendency to lengthen into columns and in others to flatten into 

 tal)les, presenting thus transitional forms between the other varieties. 

 This variety of ei)ithclium has been named spheroidal and transitiojial. 



4. Lastly the cells, which in this case are mostly prismatic in form, 



