8 



THE ELEMENTARY TISSUES 



Varieties. The varieties of epithelium may be classed as 

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

 (b) stratified, consisting of several layers; (2) Columnar, (a) 

 simple, (b) stratified; (3) Modified, (a) ciliated, (b) gob et, (c) 

 pigmented, (d) glandular, (e) neuro-epithelium. 



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

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



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.) 



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 deepest 

 layer, which rests upon the basement membrane, is composed 

 of irregularly columnar cells which have their nuclei near the 



