SEVERAL FORMS OF EPITHELIUM. 219 



the cells may throw out processes or buds at one part of the 

 circumference that subsequently separate. 



Epidermic, Epithelial, or Cuticular Tissue. The type of 

 an epidermic or cuticular tissue is that thin membrane raised 

 by the application of a blister to the skin of an animal. It is 

 certain that a cuticular covering of this character, variously 

 modified, exists 1, on the surface of the skin ; 2, on mucous 

 membranes ; 3, on the inner or free surfaces of serous mem- 

 branes which line the walls of the closed cavities in the head, 

 chest, abdomen, and other parts ; 4, on the membranes termed 

 synovial, within the joints; 5, on the inner surface of the blood- 

 vessels and lymphatics. 



This texture has neither vessels nor nerves, and is wholly 

 destitute of sensibility. It has, however, an undoubted organic 

 structure. It is made up essentially of nucleated cells joined 

 together by a more or less cohesive intercellular matter. The 

 cells are formed from a blastema or germ-fluid derived from 

 the blood-vessels of the subjacent tissue. Appearing first in 

 the deepest part of the structure, they gradually rise, while 

 they undergo various changes, to the surface, where they are 

 given off to be succeeded by others. During all the changes 

 the original nucleus remains little altered. Its diameter is from 

 the 6000th to the 4000th part of an inch. 



Epithelium takes on several forms in different situations, 

 which have been distinguished by separate names namely, 

 the scaly, columnar, spheroidal, and ciliated. 



The scaly is what is called by some pavement or tesselated 

 epithelium. As a simple layer the scaly occurs on serous 

 membranes ; in stratified layers it covers the skin, forming the 

 scarf-skin or epidermis. In the columnar variety of epithelium, 

 the cells are elongated in a direction perpendicular to the sur- 

 face of the membrane, so as to form short upright columns. The 

 columnar variety of epithelium is found only in the mucous 



