HISTOLOGY 



89 



Its thickness varies with the size and habits of the animal, and, in a 

 particular animal, it varies locally depending upon the degree of exposure 

 to mechanical wear. 



In a thick stratified epithelium the cells of the bottom layer are 

 usually columnar and those of the outer layers are more or less flattened. 

 The intermediate cells have a form such as would result from crowding 

 tightly together a mass of compressible spheres, that is, polyhedral. 

 Yet the cells are not actually packed tightly together. They are separated 

 by excessively thin intercellular lymph spaces through which seeps lymph 



Stratum 

 comeum. 







Stratum 

 germinativum. 



Corium 

 (Tunica 

 pronria.) 





Fig. 81. — Epidermis from the sole of the foot of an adult man. Section perpendicu- 

 lar to surface of skin. External to the stratum germinativum, the strata show successive 

 stages in the production of the stratum corneum. X360. (From Bremer, "Text-book 

 of Histology.") 



derived from underlying blood-vessels and serving to provide for the 

 metabolic needs of the individual cells. Cells on opposite sides of the 

 intercellular space are connected by delicate strands of solid, or at least 

 dense, substance. Presumably protoplasmic, the strands are called 

 protoplasmic bridges or plasmodesms. 



Many epithelia, although "simple" in the sense of being only one cell 

 thick, are not the ideally simple tissue of the definition (page 85), con- 

 stituted of cells all "alike in their internal differentiation." Among the 

 special functions of an epithelium are the following: (i) production of a 



