506 HISTOLOGY 



cover, and another clean dry cover is immediately dropped upon it. The 

 blood should spread between the two cover glasses, forming a film which 

 cannot be too thin. The covers are then drawn rapidly apart (they should 

 slide along one another and not be lifted apart). The blood film dries 

 from exposure to the air, and remains stainable for weeks. 



To stain the blood film, the cover glass is to be held in cover-glass 

 forceps with the film side uppermost. The stain is applied as follows: 



1. Cover the preparation with a noted quantity of the stain by means 

 of a drop-bottle or medicine dropper. 



2. After i minute add to the staining fluid on the preparation the 

 same quantity of distilled water, by means of the dropper, and allow the 

 mixture to remain i\ minutes, not longer. Longer staining may produce 

 a precipitate. The total quantity of diluted fluid on the preparation 

 should not be so much that some runs off. A metallic scum forms when 

 the stain is properly diluted, but the stain should not become transparent. 



3. Wash the preparation in tap water for 30 seconds, or until the 

 thinner portions of the film become yellow or pink. Disregard the thick 

 parts, which are blue. The process of decolorizing may be watched 

 through the microscope by placing the cover glass with film side upper- 

 most on a slide. 



4. Dry and mount directly in damar. 



Silver Nitrate. Intercellular cement spaces and the boundaries of 

 endothelial cells may be blackened by a i to i per cent, aqueous solution 

 of silver nitrate, which acts chiefly upon free surfaces. The fresh tissue 

 should be kept flat, the mesentery, for example, being tied over a detached 

 bottle neck, while it is immersed in the solution for i to 10 minutes. 

 Transfer to distilled water, and expose to direct sunlight. As soon as it 

 becomes brown, usually in 5 to 10 minutes, it is washed in 0.6 per cent, 

 salt solution. If desired, the nuclei may be lightly stained in alum haema- 

 toxylin. Examine in glycerin, or dehydrate clear in xyol and mount in 

 damar. 



Blood vessels may be injected through glass tubes with the silver 

 solution. Sections are made and exposed to the light, and the outlines 

 of the endothelial cells become dark. 



5. Clearing and Mounting. 

 CLEARING. 



Before satisfactory permanent preparations can be obtained, the 

 sections or object must be cleared,. This is accomplished by infiltrating 

 the tissues with substances which, by reason of their high index of refrac- 

 tion, render the tissues more or less transparent. Structures to be studied 

 are previously stained and thus easily rendered prominent. 



