CHAPTER IV. 



EPITHELIUM. 



THE skin, mucous surfaces of the body and various pas- 

 sages in connection with them, are evenly coated with bodies 

 of peculiar shape, which are united together to form a cover- 

 ing of one or more layers. 



In some places, as upon the external portions of the epider- 

 mis, the corpuscles are more or less flattened. Elsewhere, as in 

 the ducts of secreting glands and in the trachea and fallopian 

 tubes, they are cylindrical, and the free extremities are often 

 surmounted by cilia fine, hair-like processes, which have a 

 vibratile movement that propels solid matters, such as sputa 

 and ova, in some special direction. In other parts, again, as in 

 the collecting tubes of the kidney, near the apices of the pyra- 

 mids, a cuboidal variety is found. Intermediate or transitional 

 forms are also frequently met with in all parts of the body. 



A characteristic of epithelium which is especially note- 

 worthy is that the same species is not found uniformly in the 

 same position. Sometimes this mutation of type is governed 

 by the physical laws that regulate the growth and development 

 of the subject, or it may be a consequence of disease. An ex- 

 ample of the former peculiarity is to be noted in the larynx, 

 where the ciliated corpuscles of infancy part with their cilia 

 from advancing age, or indeed may become flattened. 



As an example of pathological change it is not uncommon 

 to find villosities covered with the most beautifully marked 

 cylindrical epithelium, springing from the ordinary mucous 

 membrane, just where the superficial corpuscles happen to be 

 somewhat flattened in their normal state. 



The use to which the part is put has also an important influ- 

 ence in governing the shape and other attributes of the corpus- 

 cles. Where they are exposed to the drying action of the air, 

 to harsh usage, and continued friction, as upon the hands and 



