354 THE MICROSCOPE. 



pericardium, or covering membrane of the heart, are much smaller than 

 those of the covering membrane of the lungs, or the serous surface 

 of the cornea, &c. On some surfaces we have many layers ; in the 

 skin it will be found of comparatively great consistence. If a vertical 

 section of such be made, and viewed under the microscope, it will be 

 seen to be composed of numberless layers, as shown in fig. 167. The 

 skin taken from the sole of the foot, in con- 

 sequence of the continued pressure there expe- 

 rienced, presents this distinctly stratified appear- 

 ance. These layers of cells are held together by 

 intercellular substance, which exists in quantities 

 in the epithelium of the mucous membranes; if the 

 epithelium is taken from these membranes, 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 recom~ 

 mended for display under the microscope, and by 

 the addition of a drop of the solution of iodine the 

 fig. 167. ce ijg are s till 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, sether, ammonia or its salts, 

 but they are dissolved by caustic potash, which dissolves the intercel- 

 lular substance also. 



Columnar or cylindrical epithelium, No. 2, a, Plate XII. The nu- 

 cleus is generally better seen than in the former kind of cells, although 

 formed from them. If we examine a portion sideways it appears as 

 at a, the upper being broader, and the nucleus being midway 

 between the two extremities. When the cells of the cylindrical epi- 

 thelium 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 would creep in ; but we must satisfy ourselves by examining them 

 sideways, and in various modes. Their chemical composition 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 in- 

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

 In no situations do we find these two kinds of epithelium terminat- 

 ing 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 tesse- 



