STRATIFIED EPITHELIA 



85 



alterations as they move eloser to the surfaee of the membrane, 

 until finally they die and become part of the surface layers which 

 continually wear away. ^Microscopic examination of saliva reveals 

 a number of such desquamated cells from the lining of the mouth. 

 In mammals this type is found in such structures as the lips, oral 

 cavity, esoi)hagus, and epidermis of the skin; there being consider- 

 able variation in the thickness of the membrane and its composition 

 in these different regions. Although the best examples occur in 

 mammals, stratified squamous e})ithelium forms the epidermis of 

 many lower forms. In fishes and amphibians some of the cells in 



Fig. 14. — Photograph of stratified squamous epithelium from esophagus of monkey. 



the epidermis become spherically distended by accumulation of 

 secretions which are later liberated on the skin surface. 



Stratified Cuboidal Epithelium.— The basal cells are cuboidal or 

 low colunniar, the intermediate region of several cell layers has 

 irregularly polygonal cells, and the surface cells are roughly cuboidal 

 with a rounded outer margin. This type is commonly found form- 

 ing the epidermis of urodeles, where scattered cells of the inter- 

 mediate layers are secretory and become distended with their 

 secretion ])r()ducts until the,\' appear almost spherical. (Fig. 90.) 



Stratified Columnar Epithelium.— The basal layers and intermediate 

 cell layers are similar to those of the preceding stratified types, 

 but the superficial cells are elongated prisms that appear col- 

 umnar in sections. It is found in mammals lining, the vas 

 deferens and some of the larger excretory ducts of certain glands. 



