670 THE MICROSCOPE. 



experienced, presents a distinctly stratified, appearance. 

 These layers of cells are held together by intercellular 

 substance, which exists in quantities ; if the epithelium 

 be taken from these membranes it is more easily seen, be- 

 cause 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 

 much better seen. The cells from serous and mucous mem- 

 branes 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, ether, 

 ammonia, or its salts; but they are dissolved by caustic 

 potash, which also dissolves the intercellular substance. 



Columnar or Cylindrical Epitheliuin, Fig. 303, 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 (/, the upper part being 

 broader, and the nucleus being midway between the two 

 extremities. 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, at c, and thus error might creep in ; 

 but we must become fully satisfied by examining them 

 sideways, and with various reagents. Their chemical com- 

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

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

 epithelium is 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; for example, where the tesselated epithelium 

 is gradually supplanted by the cylindrical, as it passes from 

 the oesophagus to line the interior of the stomach ; it is 

 then termed transition epithelium. 



Ciliated Upithelium,Fig. 303, d. — The cells do not differ 

 materially from those of the cylindrical ; the great distinc- 

 tion between he tv.^o is, that in the former there are no 



