EXAMPLE 6 



Smear Preparation of Human Blood Stained 

 by the Method of Wright 



An illustrated account of the preparation of a blood smear has been 

 given in Chapter 10 and it is unnecessary to repeat the details here. Clean 

 slides are an absolute necessity— Fig. 138 shows a smear made on a 

 greasy slide— and an adequate number should be placed at hand before 

 the blood is taken. There should also be available a drop bottle of 

 Wright's stain and another either of distilled water or of a phosphate 

 buffer at pH 6.4. Most supply houses that handle prepared Wright's 

 stain offer a special buffer for use with it. 



Human blood for smears is commonly drawn from a puncture wound 

 in the ball of the forefinger, which should be compressed until the tip if 

 suffused. The wound should be made either with a sterile hypodermic 

 needle or with a special device manufactured for the purpose. A good 

 drop of blood is then squeezed out and touched to the slide about 1 in. 

 from the end (Fig. 84). A second slide (Figs. 85 and 86) is then used to 

 push the blood into a thin smear, which is waved in the air to dry. A 

 glance at the dried film through a X 10 objective should show an erythro- 

 cyte distribution similar to that in Fig. 137. Smears thicker than this, 

 which are used only for malaria diagnosis, require special staining meth- 

 ods, and thinner smears are unlikely to have an adequate distribution 

 of leukocytes. 



One of the dried smears is now taken and flooded with Wright's stain. 

 For this purpose it may conveniently be rested across the open mouth of 

 a coplin jar, although special racks are available if many slides are to be 

 made at once. After 1 min water, or buffer, is added to the stain on the 

 slide in the proportion of two parts of added fluid to one of stain. After 

 2 min the diluted stain is washed off with a jet of distilled water and the 

 slide allowed to dry. 



The success of the stain is judged by an examination ( Figs. 139 to 141 ) 

 of the leukocytes under a X90 immersion lens. The nuclei of polymorphs 



196 



