54 



NORMAL HISTOLOGY AND ORGANOGRAPHY. 



(c) Stratified columnar, ciliated or non-ciliated. 

 This is found in the olfactory mucous membrane, the 

 first part of many gland ducts, palpebral conjunc- 

 tiva, portions of the male urethra, vas deferens, and 

 portions of the larynx. 



General Considerations. The epithelial cells are 

 simpler and more embryonic than the cells of the 

 other tissues. They are continually multiplying 



throughout life, to re- 

 place the superficial 

 layers that are con- 

 stantly exfoliating 

 from the surfaces. If 

 any of these surfaces 

 are injured, the cells 

 marginal to the injury 

 repair the loss by a 

 gradual growth cover- 

 ing the denuded sur- 

 face. As a consequence 

 of this mitotic activity, 



Fig. 22. Section of stratified epithe- 

 lium from esophagus. 



we find these cells fre- 

 quently in pathological growths as epithelial growths 

 or epithelioma. If the tumor is malignant it is a 

 carcinoma or cancer. It is a remarkable fact that, in 

 the adult, epithelium is able to produce cells only 

 of its own kind, i. e., squamous cells produce 

 squamous epithelioma, and columnar cells columnar 

 epithelioma. Epithelial tissue, however, is easily 

 modified, as is evidenced by calloused hands, pro- 

 duced by heavy labor, and the cornification of nails, 

 hair, horns, and teeth. 



