COLUMNAR EPITHELIUM. 539 



it is more easily seen, because the cells are not so closely 

 aggregated together as in the skin ; therefore a piece of 

 epithelium from the mouth is recommended for display 

 under the microscope, and by the addition of a drop of the 

 solution of iodine the cells are still better seen. The cells 

 from serous and mucous membranes are acted upon by 

 acetic acid, and dissolved if the acid be of considerable 

 strength ; but if the acid be weaker, the cells swell up. 

 Cells are not affected by alcohol, aether, ammonia, or its 

 salts ; but they are dissolved by caustic potash, which dis- 

 solves the intercellular substance also. 



Columnar or cylindrical epithelium, Fig. 274, a. The 

 nucleus is generally better seen than in the former kind of 

 cells, although formed from them. If we examine a portion 

 sideways, it resembles those at d, the upper being broader, 

 and the nucleus being midway between the two extremi- 

 ties. When the cells of the cylindrical epithelium are 

 closely aggregated together, they become compressed into 

 the prismatic form ; when they are less so, the rounded 

 shape prevails. Consequently, when we take a bird's-eye 

 view of them, from above or below, they appear like the 

 pavement epithelium, as at c, and thus error might creep 

 in : but we must satisfy ourselves by examining them side- 

 ways, and with various reagents. Their chemical composi- 

 tion is the same, and the cells dissolve in strong acetic 

 acid. As examples of the situations in which this form of 

 epithelium may be found, we may instance the intestinal 

 tract, along the ducts of the glands, as the liver, &c. 



In no situations do we find these two kinds of epithe- 

 lium terminating abruptly the one in the other ; but there 

 is a gradual change of the one kind into that of the 

 adjoining : as, for example, where the tesselated epithelium 

 is gradually supplanted by the cylindrical, as it passes from 

 the sesophagus to line the interior of the stomach. This is 

 transition epithelium. 



its submucous tissue. The upper surface of one villus is se3n covered with 

 cylindrical epithelium ; the other is denuded, and with the dark line of base- 

 ment membrane only running round it; b, pavement epithelium scales, separated 

 and magnified 200 diameters ; in the centre of each is a nucleus, with a smaller 

 spot in its interior, called the nucleoli. c, pavement epithelium scales, from the 

 mucous membrane of the bronchial or air tubes of the lung, showing nuclei, 

 with a double nucleoli in some, d represents another form of epithelium, termed 

 the ribralile or ciliated; the nuclei are visible, with cilia at their upper or free 

 surfaces, magnified 250 diameters. 



