76 



THE TISSUES 



from the underlying connective tissue are usually small regularly 

 placed cuboi^l_cells of uniform size which in tangential sections 

 appear as a distinct row of cells. Passing toward the surface the 

 cells, still small, become irregular in shape and arrangement, then 

 larger and as the surface is approached flatter, until the typical 

 squamous surface cells are reached. The deeper cells are the softer 

 younger cells and in the layers just above the basal layer mitotic 

 figures arc frequent. Here also are often found \ery distinct proto- 

 plasmic intercellular connections ("intercellular bridges," (Fig. 20), 

 see also p. 72). Extensions of cell protoplasm similar to the inter- 

 cellular bridges have been described as passing from the deepest 

 cells toward or into the underlying connective tissue. The inter- 



Flat surface cells 



Polyhedral cells 



Large polyhedral cells \r." '- ■ 



Small polyhedral cells i;i 

 Cuboidal cells ' 



Fig. 19. — Stratified Squamous Epithelium from Cat's Oesophagus. 



cellular spaces appear first as vacuoles in the exoplasm, the proto- 

 plasmic walls of the vacuoles being the "bridges.'' The spaces are 

 best developed in the thicker epithelia thus lending support to the 

 view that they have to do with the nutrition of the cells. As the 

 cells become flatter, cells without nuclei are seen and become more 

 numeroiLS as the surface is approached, until on the surface, especially 

 of parts exposed to air and unmoistened, hardcornified, non-nucleated 

 cells containing keratin predominate. Such cells are in process of 

 dying and being cast off. In the e])ithelium of mucous membranes 

 where the surface is kept moist, there are few^r non-nucleated cells, 

 the cells are softer and contain less keratin. In hair and nail, the 

 surface cells though hornitied are nucleated. 



