152 



SPECIAL PHYSIOLOGY 



The blood will then spread between the surfaces of the glasses by 

 capillary attraction. As soon as the blood has entirely stopped 

 spreading, take firm hold of the upper glass by the projecting 

 corner with the thumb and first finger of the right hand, on the flat 

 surface this time, as shown in Fig. 75. Now, with a quick, full swing 

 of the whole arm, pull the upper glass away from the lower one as 

 quickly as possible. Always pull the glass away in the same plane 

 it was in, and do not tilt one glass upon the other, otherwise the 

 spread will show ridges and will be of no use. Make a number of 

 specimens for further study. 



Fixing Blood Films. After the blood is spread the glasses are 

 to be placed under a bell jar, with blood side upward, and allowed 

 to air-dry until perfectly dry. The length of time for drying depends 

 upon the thickness of the film; usually five to fifteen minutes are 

 sufficient. The method of fixing blood spreads may be divided into 

 two classes: chemical and thermal. 



FIG. 74 



Dropping cover-glass upon the 

 drop of blood. 



Showing manner of holding the cover-glass 

 to jerk them apart. 



The principal chemicals are absolute alcohol and ether, abso- 

 lute alcohol, ether, 95 per cent, alcohol, and 1 per cent, formal- 

 dehyde. The heating or baking method may be done crudely 

 by passing the spread back and forth through the Bunsen flame 

 for five minutes, while being held with the fingers. There are 

 also copper ovens, copper plates, and copper receptacles, con- 

 taining water, used to bake the blood. For staining with the 

 ordinary histological or bacteriological stains the chemical method 

 of fixing is sufficient, but when some of the finer differential staining 

 is desired, as with the Ehrlich triacid stain, the baking method is 

 necessary, and a copper oven controlled by a thermostat is the best. 

 Fifteen to thirty minutes is the time required for baking, with the 

 temperature about 100 C. The time depends upon the thickness 

 of the film. Some observers employ higher temperatures with suc- 

 cess. With the film on the slide, the blood-fixing jar is convenient for 

 the chemical method. It is a quadrilateral jar, wide enough to allow 

 four or five slides to stand in the jar, with a ridge of glass between 



