646 TECHNIQUE 



Very small pieces of tissue should be used. They should be left 

 in the fluid three to twelve hours, and the vessel tightly closed to 

 prevent evaporation of the volatile fluid. The tissues are then 

 transferred to several changes of absolute alcohol to remove the 

 fixing fluid and complete the dehydration. 



This fluid can not be too highly recommended for the preserva- 

 tion of finer cytological details. It is not applicable to large pieces 

 of tissue, and can be advantageously applied only to perfectly fresh, 

 viz., living tissues. 



Heat This is a useful agent for the fixation of blood, marrow 

 cells, and scrapings from glandular and other organs, which are not 

 to be afterward stained with methylen blue or its compounds. For 

 this purpose smears made upon glass slides or cover glasses are 

 quickly dried in the air, heated to 110 C. for twenty to thirty min- 

 utes, and are then ready for immediate staining. 



The smears are made in the following manner : Slides or cover 

 glasses should be thoroughly cleaned (see page 639) with a final 

 rinsing in equal parts of absolute alcohol and ether. A small drop 

 of blood or other fluid is collected by quickly touching the center 

 of a cover glass to a drop of ordinary size. This cover glass is then 

 immediately dropped upon the surface of a second one, and the two 

 are drawn apart by a rapid sliding motion, the two surfaces being 

 maintained parallel to one another during the motion. The success 

 of the maneuver depends upon its rapidity, and to obtain very thin 

 preparations some little dexterity is required. 



Fairly good smears are more easily made with slides. A drop 

 of blood is collected upon the end of one glass slide whose edge 

 must have been ground. The end of the slide with the drop of 

 blood is then touched to the middle of a second slide, the drop 

 spreads out between the two, and the first slide is rapidly drawn 

 over the surface of the second, while being held at an angle of 

 about 45. A broad smear is thus left upon the surface of the 

 second slide, some portions of which are sufficiently thin, other 

 portions too thick for use. Like the former maneuver the success 

 of this depends upon rapidity, cleanliness, and the use of a suffi- 

 ciently small drop of fluid. 



Fixation by Vapors. Smears of fluids or very thin pieces of 

 tissue may be fixed by a very brief exposure to the vapor of osmium 

 tetroxid, formalin, etc. This method is only useful in occasional 

 instances. 



In all methods of fixation where pieces of tissue are immersed 



