414 HISTOLOGY. 



cover glasses, forming a film which cannot be too thin. The covers are 

 then drawn rapidly apart, sliding over one another, and the blood dries 

 from exposure to the air. It remains stainable for weeks. 



To stain the blood film, the cover glass may be held in the forceps 

 devised for this purpose (cover-glass forceps), with the film uppermost. 

 Stain sufficient to cover it is poured upon it, and after one minute several 

 drops of distilled water are added to the stain, until a delicate metallic scum 

 forms upon the surface. The stain should not be so diluted as to become 

 transparent. After two or three minutes, the stain is washed off. The 

 preparation appears blue. Distilled water is placed upon it to extract the 

 excess of stain and the color changes to orange, or pink if the decolorization 

 proceeds further. The general color of the specimen is due to that of the 

 red corpuscles which at first are blue. When they have become orange 

 or pink as is desired, the water is removed by applying several layers of 

 filter paper, and the preparation is mounted in damar. The process of 

 decolorizing may be watched through the microscope by placing the cover 

 glass (with the film side up) on a slide. Thicker portions of the film which 

 remain blue when the thinner parts are orange, should be disregarded. 

 The leucocytes are figured on page 147. 



Intercellular cement spaces and the boundaries of endothelial cells 

 may be blackened byi a i to i per cent, 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 bottle neck, while it is immersed in 

 the solution for from i to 10 minutes. Then it is placed in distilled water 

 and exposed to direct sunlight. As soon as it becomes brown (usually in 

 5 or 10 minutes) it is washed in dilute salt solution and slowly hardened in 

 graded alcohols. Larger blood vessels may be injected through glass tubes 

 with the silver solution, and after sections have been made and exposed to 

 the light, the endothelial cell outlines become dark. 



The courses of blood and lymphatic vessels and of ducts are studied 

 by means of injections. Colored fluids, usually such as harden by cooling or 

 otherwise, are forced into them by pressure from a syringe. The syringe 

 is connected by a short rubber tube with a tapering glass tube or cannula; 

 the latter is inserted into the vessel which is then tied securely around it. 

 Pressure may also be obtained by having the injection mass in a receptacle 

 which connects with the cannula by a long flexible tube; pressure is in- 

 creased by elevating the receptacle. The organs to be injected must be 

 fresh; they may be left within the body or removed and injected separately. 

 To avoid undue distention of the vessels and to allow the injection to flow 

 more readily, the efferent vessels may be cut, so that the blood escapes. 

 Sometimes the vessels are washed out by a preliminary injection of salt 



