EPITHELIUM. 



69 



since within those next the basement membrane the relatively large nucleus — the 

 nutritive organ of the cell — occupies the end nearest the subjacent connective tissue ; 

 in the middle and superficial strata, the nucleus, comparatively small in size, is 

 placed about the centre of the cell. 



The irregularly polyhedral cells of the deep or middle strata frequently are 

 connected by delicate processes which bridge the intervening intercellular clefts ; 

 when such elements are isolated, the delicate connecting threads are broken and the 

 disassociated elements appear beset with minute spines, then constituting xhe. prickle- 

 cells. 



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

 layer rapidly assume the scaly character of the superficial strata ; such epithelium 



Fin. S4. 





Transitional epithelium from bladder of child. 

 X 300. 



Fig. 85. 



Fig. 86. 



-- ^--r 



-■••■) 





Simple columnar epi- 

 thelium from intestinal 

 mucosa. X 750. 



Stratified columnar epithelium 

 from vas deferens. X 500. 



possesses relatively few layers, and from the readiness with which the type of the 

 cells changes, is often described as transitional cpitheliiun ; the latter cannot be 

 regarded as a distinct variety, but only as a modification of the stratified scaly group. 

 Columnar epithelium, when occurring as a single layer of cells, constitutes 

 the simple columnar variety, which enjoys a much wider distribution than the cor- 

 responding squamous group, the lining of the stomach and of the intestinal tube 

 being important examples. When the single layer of such epithelial tissues is re- 

 placed by several, as in the stratified colwnnar variety, the superficial cells alone 



Fig. 87. 



Stratified ciliated columnar epithelium from trachea of 

 child. X 550- 



Fig. 88. 



// 



Ciliated epithelial cells. A, from intes- 

 tine of a mollusk (cycles) ; B, from nasal 

 cavity of frog. X 750. {Engelmann.) 



are typically columnar. The free ends of the columnar elements not infrequently 

 present specializations in the form of a cidicidar border or of cilia, while their ends 

 which rest upon the basement membrane are pointed, forked, or club-shaped. The 

 intervals thus formed by irregularities of contour are occupied by the cells of the 

 deeper stratum next the basement membrane. Each cell is provided with a nucleus, 

 which is situated about midway between the ends of the superficial elements and 

 nearer the base within the deeper ones. The surface cells often contain collections 

 of mucous secretion which distend their bodies into conspicuous chalice forms known 

 as goblet-cells, which occur in great profusion in the lining of the large intestine and 

 the respiratory mucous membrane. 



