THE EPITHELIAL TISSUES. 



FIG. 19. 



Prickle - cells from 

 middle strata of the 

 epidermis. 



stitute the prickle-cells. During the journey to the free surface 



the character of the protoplasm also alters, the cells losing in 



vitality and becoming keratose or horny to a 



greater or less degree. TFeTextent to which these 



changes occur depends upon the external conditions 



affecting the tissue : on mucous surfaces kept con- 



tinually moist by secretions the cells retain their 



plasticity and nuclei ; where, on the contrary, they 



are exposed to the desiccating; influences of the 



atmosphere, they lose their nuclei and become dry 



and horny, as conspicuously seen in the superficial 



cells of the epidermis. Fatty granules and small 



oil-drops, sometimes, also, adherent masses of bacteria, are common 



in the superficial cells. As the young growing cells of the deeper 



layers increase in size and numbers, they push those of the super- 



imposed strata towards the free surface, where the older superficial 



cells become loosened and gradually set free, constituting the physio- 



logical desquamation continually taking place. 



In certain localities, as in the urinary bladder, the columnar cells 

 of the deep layer rapidly assume the scaly character of the superficial 

 strata ; such epithelium possesses relatively few layers, and, from the 

 facility with which the type of the cells changes, is often described 

 as ''trajisitionaL" It is to be remembered that such epithelium 

 constitutes not a distinct variety, but only a modification of the 

 stratified scaly group. 



Columnar Epithelium. The columnar epithelium, when occur- 

 ring as a single layer of cells, constitutes the simple columnar 

 variety, which, however, enjoys a much wider distribution than the 

 corresponding squamous group. The taller or shorted columnar 

 cells rest upon the membrana 

 propria with their bases, and 

 join their neighbors with 

 more or less accuracy. The 

 free or outer ends of the cells 

 in some localities, as, conspicu- 

 ously, in the intestine, are char- 

 acterized by the presence of a 

 narrow marginal zone, or 

 basal border : this exhibits a 

 vertical striation, which, on the 

 addition of a reagent, as water, 

 often breaks up into a series of 

 rods, resembling very robust cilia. When the single layer of these 

 epithelial cells is replaced by several, as in the stratified columnar 



FIG. 21. 



FIG. 20. 



Simple columnar epi- 

 thelium from intestine : 

 the free ends of the 

 cells present a peculiar 

 striated border - zone. 

 Highly magnified. 



Stratified columnar epi- 

 thelium from vas defer- 

 ens : the deepest layer 

 consists of small cells, 

 between which the co- 

 lumnar cells extend. 



