42 



PRACTICAL MICROSCOPY. 



on the slide, and with a needle lead it to the edge of the saliva. 

 The dye will pass under the cover slowly; and, gradually, whatever 

 anatomical elements there may be present will be stained. Observe 

 that the nuclei of the flat scales first take the dye, and appear of a 

 deep pink; while the other portions are either colorless or very lightly 

 stained. 



Find a typical field and sketch it with pencil, afterward tinting 

 with dilute eosin. 



PAVEMENT EPITHELIUM. 



When thin flat cells are disposed in a single layer, like tiles, the 

 epithelium is termed pavement or tessellated. These cells are often 

 quite regularly polygonal (although this obtains more frequently with 

 tissue from the lower animals), and they are always connected by their 

 dges by means of an albuminous cement. 



FIG. 26. PAVEMENT EPITHELIUM. DIAGRAMMATIC. 



This structure is very extensively distributed. Most serous sur- 

 faces, e. g., the pleurae, omenta. mesenteries, and peritoneal surfaces 

 generally, are so covered. The lining of the heart , of arteries and 

 veins, and of lymph channels is constructed with these cemented 

 cells. Blood capillaries are formed almost entirely of such elements. 



The best demonstration is made by coloring the cement which 

 unites the cells. If a tissue, covered with this epithelium, be placed 

 for a few minutes in a solution of nitrate of silver a chemical union 

 ensues; an albuminate of silver is formed which blackens in the light, 

 thereby mapping out the cells with great precision and clearness. 



It is nearly impossible to procure human tissue for this purpose, as 

 the cement substance decomposes soon after death. The mesentery 

 of the frog affords a good example of pavement cell structure; and 

 -differs but little from the arrangement on human serous surfaces. 



