METHODS OF INJECTION. 53 



table with the specimen upward. A drop of Canada balsam is now 

 placed on the section (usually on its left side), and a clean cover-slip 

 grasped with a small forceps. It is then gently lowered in such a way 

 that the Canada balsam spreads out evenly and no air-bubbles are im- 

 prisoned under the glass. When this is done the preparation is finished. 



If one is dealing with loose sections, a spatula or section-lifter is 

 very useful in transferring them from absolute alcohol into the clearing 

 fluid carbol-xylol or bergamot oil (xylol evaporates very rapidly) and 

 from this onto the slide. In doing this it is necessary that the sec- 

 tion should lie well spread out on the section -lifter, wrinkles being re- 

 moved with a needle or small camel's-hair brush. In sliding the section 

 off the spatula (with a needle or brush) a small quantity of the clearing fluid 

 is also brought onto the slide. This must be removed as far as possible 

 by tilting or with blotting-paper. The section can now be mounted 

 in Canada balsam as before. For esthetic and practical reasons the 

 student should see that during the spreading of the drop of Canada balsam 

 the section remains under the middle of the cover-slip. Should it float 

 to the edge, it is best to raise the cover-slip and lower it into place again. 

 The cover-slip should never be slid over the specimen. 



Glycerin. To mount in glycerin the sections are transferred from 

 water to the slide, covered with a drop of glycerin and the cover-slip ap- 

 plied. This method is employed in mounting sections colored with a 

 stain that would be injured by contact with alcohol, and where clearing is 

 not especially necessary. 



Farrant's Gum Glycerin. 



In place of pure glycerin the following mixture may be used : 



Glycerin 5 c - c - 



Water 50 " 



Gum-arabic (powder) 5 g m - 



Arsenious acid I " 



Dissolve the arsenious acid in water. Place the gum-arabic in a glass mortar and 

 mix it with the water; then add the glycerin. Filter through a wet filter-paper or 

 through fine muslin. 



To preserve such preparations for any length of time the cover- 

 glasses must be so fixed as to shut off the glycerin or acetate of potash 

 from the air. For this purpose cements or varnishes are employed which 

 are painted over the edges of the cover-slip. These masses adhere to the 

 glass, harden, and fasten the cover-slip firmly to the slide, hermetically 

 sealing the object. The best of these is probably Kronig's varnish, pre- 

 pared as follows : 2 parts of wax are melted and 7 to 9 parts of 

 colophonium stirred in, and the mass filtered hot. Before employing 

 an oil-immersion lens it is advisable to paint the edge with an alcoholic 

 solution of shellac. 



METHODS OF INJECTION. 



The process of injection consists in filling the blood- and lymph-ves- 

 sels with colored masses in order to bring out clearly their relation to the 

 neighboring tissue elements. The instruments required are a syringe of 

 suitable size or a constant pressure apparatus and cannulas of various sizes. 

 Serviceable and instructive injections of blood-vessels are readily made ; 

 good injections require skill, experience, and patience. Injection masses 

 may be classed under two heads cold injection masses and warm injection 



