DIFFERENT KINDS OF CELLS. 



19 





Fig. 4. 

 Various forms of Epithelium.* 



of these membranes, and, according to their functional 

 activity,' they present differing forms ; if they are situated 

 in a region where their functions are not very active they 

 are few in number and in order to occupy completely the 

 surface given up to them, they are flattened out, forming a 

 sort of pavement, and hence are called pavement (or tessel- 

 lated) epithelium (Fig. 4, A). If, 

 on the contrary, as, for example, 

 on the more important mucous 

 membranes, their vital functions 

 are very active, they become 

 multiplied, accumulate in large 

 numbers upon the same place, 

 and make room for each other, 

 by being compressed sideways ; 

 and so instead of being round 

 they become cylindrical, hence 

 they are called cylinder (or col- 

 umnar) epithelium (Fig. 4, B). 

 Finally, if a simple layer is insufficient, the globules are 

 superposed, and hence are called stratified epithelium (Fig. 

 4,0). 



Moreover, for the purpose of offering a large surface with- 

 out occupying too much space, these epithelium cells over- 

 lap each other, an instance of which may have been re- 

 marked in the blastoderm, and according to whether the 

 overlapping is on the superficial surface or on the side near 

 the deeper tissues, these make up papillae or glands ; more 

 particular mention will be made of this subject when we have 

 occasion to speak of the formation of the epithelium of the 

 mucous membrane of the mouth. 



The functions are of far greater importance than the form 

 of the epitheliums ; these may be divided into three classes. 

 Certain of the globules present an obstructing surface to the 

 passage of fluids, &c., and are impermeable, as, for instance, 

 in the epithelium of the bladder and of the serous mem- 

 branes in general. These might be called neutral globules. 



Another class, on the contrary, absorb very actively the 

 substances (gas or liquid) brought in contact with them, and 

 transport these to the more distant and deeper portions of 

 tissues, as is done for instance by the blood globule. These 

 may be called absorption globules. 



* A, Pavement epithelium. B, Columnar epithelium. C, Stratified epi- 

 thelium. 



