22 THE ESSENTIALS OF HISTOLOGY 



rounded or polyhedral, and those of the deepest layer generally some- 

 what columnar in shape (fig. 18, c}. Moreover, the deeper cells are 

 soft and protoplasmic, and are separated from one another by a system 

 of intercellular channels, which are bridged across by numerous pro- 

 cesses passing from cell to cell. 



The deeper cells multiply by division, the newly formed cells tending 

 as they enlarge to push those external to them nearer to the surface, 

 from which they are eventually thrown off. As they approach the 

 surface they become hard and horny, and in the case of the epidermis 

 lose entirely their cellular appearance, which can, however, be in a 

 measure restored by the action of potash ( 2). The cast-off super- 

 ficial cells of the stratified epithelium of the mouth, which are seen in 

 abundance in the saliva (1), are less altered, and the remains of a 

 nucleus is still visible in them. 



FIG. 19. EPITHELIUM-SCALES FROM THE 

 INSIDE OF THE MOUTH. (Magnified 260 

 diameters.) 



FIG. 20. PAVEMENT-EPITHELIUM FROM 

 A SEROUS MEMBRANE. (Magnified 410 

 diameters.) 



a, cell ; b, nucleus ; c, nucleoli. 



Simple scaly or pavement epithelium is found in the saccules of 

 the lungs, in those of the mammary gland when inactive, in the kidney 

 (in the tubes of Henle), and also lining the cavities of serous mem- 

 branes (fig. 20), and the heart, blood-vessels, and lymphatics. When 

 occurring on internal surfaces, stfch as those of the serous membranes, 

 blood-vessels, and lymphatics, it is often spoken of as endothelium. 



Polyhedral or spheroidal epithelium is characteristic of many 

 secreting glands ; columnar and ciliated epithelium are for the most 

 part found covering the inner surface of mucous membranes, which 

 are membranes lining passages in communication with the exterior, 

 such as the alimentary canal and the respiratory and generative 



The detailed study of most of these may be reserved until the 

 organs in which they occur are respectively dealt with. 



The hairs and nails and the enamel of the teeth are modified 

 epithelial tissues. 



