OF CELLS DEVELOPED ON FREE SURFACES. 29 



The nails and hairs are also modifications of epidermic cells — 

 for description of ivhich sec Anatomy. 



The ejntlLeliimi is the layer of cells which covers the internal free 

 surfaces of the body. In some instances it serves, as the epidermis, 

 to protect the subjacent membranes ; in others, it takes an important 

 part in some of the organic functions. 



In the arrangement of the epithelial cells, two principal forms are 

 seen, viz. : the tessellated or 2Javement-like epithelium ; and the 

 cylindrical^ or cylinder epithelium. 



The tessellated, covers the serous and synovial membranes, the 

 lining membrane of blood-vessels, and the ultimate follicles or tubuli 

 of the glands connected with the skin or mucous membranes. It is 

 called tessellated., from the fact, that the cells which compose it are 

 polygonal, and arrange themselves like the pieces of marble in a 

 tessellated pavement. Sometimes, however, the cells retain their 

 rounded form, and are separated by considerable intervals. 



The cylinder epithelium is so called because the component cells 

 are cylinders, and arrange themselves side by side, one extremity 

 resting on the basement membrane, while the other forms part of the 

 U'ee surface. It is found in the mucous membrane of the alimentary 

 canal, from the cardiac orifice downwards; in the larger ducts of 

 the glands which open into it ; or, upon the external surface. 



Both these forms of epithelium are frequently seen to be fringed 

 at their free extremities with delicate filaments, termed cili<x^ from 

 their resemblance to an eyelash. These, although exceedingly 

 minute, are of great importance in the economy, through the ex- 

 traordinary motor power with 



which they are endowed (Fig. 6). -^^S- 6.* 



In form the cilise are usually a , 

 little flattened, and tapering gra- '■ 



dually from base to point. Their 

 size is variable, ranging from 

 l-SOOOth to l-13000th of an inch 

 in length. 



When in motion they present the appearance of a field of wheat 

 over which the wind is blowing, first depressed, and then returning 

 to its original state. The direction of this motion is towards the 

 outlets. 



The cause of ciliary motion is unknown, it is not dependent on 

 muscular action, since it continues after separation, and contains not 

 even a fibrilla of muscle. It seems to be entirely molecular and in- 

 dependent of muscular influence, and bf both the vascular and 

 nervous systems. It continues many hours after separation from the 

 rest of the system has taken place, and when all supply of blood to 

 the parts endowed with it has been cut off*. It resists the action of 



* a. Nucleated cells resting on their smaller extremities ; 6. Ciliae. 



3* 



