42 HISTOLOGY. 



Dimple epithelium are usually all at one level, are here placed 

 at various depths from the surface. Such cells usually bear 

 cilia on their free surface, as, for example, in the larynx, etc. 



Stratified epithelium may have cylindrical cells at the base 

 and transition forms above this, until at the surface the cells are 

 flat. This is known as stratified flat epithelium or pavement 

 epithelium (Fig. 24), and is characteristic of the epidermis, the 

 mouth cavity, the oesophagus, the vagina, etc. In the epidermis 

 the cells of the superficial layers lose their nuclei and at the 

 same time undergo a chemical change, the so-called cornification 

 (see Skin). 



Stratified epithelium may consist also of a layer of columnar 

 cells on the surface, with or without cilia, and below this transi- 

 tional forms, until a row of cubical or polyhedral cells at the 

 base is reached (Fig. 24, c}. Such an epithelium may be called 

 a stratified cylindrical epithelium. We find it in the main ducts 

 of many glands. 



Epithelial cells are joined together, as we have already said, 

 by means of a cement substance, which occurs usually only in 

 very small quantity between the cells. It is recognized always 

 in tissues treated by silver nitrate. If the epithelium be sub- 

 merged in a weak (0.1-1.5 per cent.) solution, the cement sub- 

 stance enters into some sort of a combination with the reagent, 

 which under the action of sunlight becomes dark brown or black. 

 The surfaces of the cells which the cement substance connects 

 are often quite smooth, but show sometimes inequalities and 

 depressions due to the pressure exerted by the cells on one 

 another. This is seen in the epithelium of the mouth cavity 

 (Fig. 21) and the urinary bladder. 



In the line of the cement substance there is often seen a 

 number of rod-shaped bodies connecting the two adjacent cells. 

 These form the so-called intercellular bridges, and can readily be 

 seen, for example, in the deeper layers of the epidermis. Where 

 the cells are isolated, the rods stand out from the surface and 

 give rise to the term prickle cells. The prickles or rods are 

 essentially connecting bridges passing through the cement sub- 

 stance from one cell to the other. They are plainly processes 



