8 THE ELEMENTARY TISSUES 



Varieties. The varieties of epithelium may be classified 

 as follows: (i) Squamous, (a) simple, consisting of a single 

 layer, (b) stratified, consisting of several layers; (2) Col- 

 umnar, (a) simple, (b) stratified; (3) Modified, (a) ciliated, 

 (b) goblet, (c) pigmented, (d) glandular, (e) neuro-epi- 

 t helium. 



(i) Squamous Epithelium. {a) Simple squamous epithe- 

 lium consists of a single layer of cells which, when viewed 



FIG. 3. From a section of the lung of a cat, stained with silver 



nitrate. 



N, Alveoli or air-cells, lined with large, flat, nucleated cells, with some 

 smaller polyhedral nucleated cells. (Halliburton after Klein and Noble Smith,) 



from above, appear as flattened polyhedral nucleated plates 

 like a regular mosaic. It occurs in but a few places, lining 

 the air sacs of the lungs, the mastoid cells, the membranous 

 labyrinth and crystalline lens (Fig. 3). 



(b) Stratified squamous epithelium is composed of several 

 layers of epithelial cells placed upon one another. The deep- 

 est layer, which rests upon the basement membrane, is com- 

 posed of irregularly columnar cells which have their nuclei 



