SIMPLE E PI Til ELI A 



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lining the digestive tract from stomach to anus in mammals, 

 and lines the many small glands in the wall of the stomach and 

 intestine of vertebrates generally, as well as some of the duct sys- 

 tems of the larger digestive glands, such as the liver, pancreas, and 

 salivary glands. 



The e])ithelial sheet in the alimentary tract of many lower verte- 

 brates is composed of \'ery long cells whose tapering basal portions 

 extend between shorter, irregularly fusiform cells. Although all 

 the cells rest upon the same base, there is the appearance of 

 a number of layers present. Even in the more typical columnar 

 epithelia there are a number of smaller polyhedral, fusiform, or 

 spherical cells, often intercalated basally. This brings us to the 

 term pseudostratiiied epithelium, which is commonly used for an 

 epithelium falsely resembling the stratified columnar type. (Fig. 13.) 

 It has the appearance of several 

 strata of cells with columnar cells 

 forming the layer nearest the sur- 

 face. It has been shown, however, 

 that if a section passes parallel to 

 the long axis of this epithelial mem- 

 brane all the cells will be seen to rest 

 upon a common basement mem- 

 brane adjacent to the underlying 

 connective tissue. The cells of the 

 superficial stratum have long, taper- 

 ing basal extensions, reaching down to the basement membrane, 

 and polyhedral cells of various sizes fill in about these slender por- 

 tions and also reach to the basement membrane. ^Nlost preparations 

 do not differentiate the lateral boundaries of the cells clearly enough 

 to show the absence of stratification, but the rows of nuclei at dif- 

 ferent levels indicate cells of different lengths. This type, with 

 cilia present on the free borders of the columnar cells, forms the 

 lining of the trachea and bronchi of mammals. The epithelial 

 membranes in the case of the lining of the alimentary tract of many 

 lower vertebrates appears to be of the same structure but the 

 columnar cells are longer and it is more difficult to separate the 

 tissue into truly stratified and pseudostratified epithelia. 



A simple columnar epithelium is associated with secretion and 



single cells of the membrane accumulate the secretion product until 



the free end is much distended and resembles a goblet. (Fig. 12.) 



Such cells are called goblet cells and occur abundantly in the lining 



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Fig. 13. — Diagram of ciliated 

 pseudostratified epithelium. 



