EPITHELIAL TISSUES. 



smaller and smallest bronchioles of the lungs, in certain portions of 

 the uriniferous tubules and their collecting ducts, in the smaller 

 ducts of salivary and mucous glands, liver, pancreas, etc. 



(c) Simple Columnar Epithelium. In this type the cells take 

 the form of prisms or pyramids of varying length. Cuticular 

 structures are especially well developed. Columnar epithelium 

 occurs in the entire intestinal tract from the cardiac end of the 

 stomach to the anus, in certain portions of the kidney, etc. 



Goblet cell. 



Cuticular border. 



Fig. 40. Simple columnar epithelium from the small intestine of man : a, Isolated cells ; 

 b, surface view ; c, longitudinal section. 



Simple ciliated columnar epithelium is found in the oviduct and 

 uterus, central canal of the spinal cord, and smaller bronchi. 



(^) Pseudostratified Columnar Epithelium. This type is 

 one in which all the cells rest on a basement membrane, but they 

 are so placed that the nuclei come to lie in different planes. Thus, 

 in a longitudinal section the nuclei are seen to be placed in several 

 rows. 



The development of this type from the 

 simpler forms occurs when the cells are too 

 crowded to retain their normal breadth. As 

 a result, they become pyramidal, alternate 

 cells resting their bases or apices on the base- 

 ment membrane. As the nucleus is usually 

 situated at the broader portion of the cell, 

 the result is that there are two rows of nu- 

 clei simulating a stratified epithelium. Occa- 

 sionally there are spindle-shaped cells wedged in between the pyra- 

 midal cells, and as the broad portion of these cells is midway 

 between the basement membrane and external surface, a third row 

 of nuclei is seen midway between the other two. Such epithelia 

 usually possess cilia (portions of the respiratory passages). 



2. STRATIFIED EPITHELIUM. 



Should the increase of the cells forming the last type of simple 

 epithelium proceed to such an extent that all the cells no longer 

 rest on the basement membrane, an epithelium is formed having dis- 



Fig. 41. Diagram 

 of pseudostratified col- 

 umnar epithelium. 



