EXAMINATION OF BLOOD 377 



ear or preferably by wounding the tip of the tail ; in the 

 frog, the web of the foot should be selected. 



2. Puncture the selected vein with a sharp needle. 

 A flat Hagedorn needle (size No. 8) with a cutting 

 edge is the most useful for this purpose. If the vein 

 cannot be distended by proximal compression, vigourous 

 friction with a piece of dry lint may have the desired 

 effect or a test-tube full of water at about 40 C. 

 may be placed close to the vein. Failing these methods, 

 a drop or two of xylol may be dropped on the skin just 

 over the vein, left on for a few seconds and then wiped 

 off with a piece of dry lint. 



3. One of the short ends of a 3 by i glass slip is 

 brought into contact with the exuding drop of blood, 

 so that it picks up a small drop. 



4. The slide is then lowered transversely on to the 

 surface of a second 3 by i slip, which rests on the 

 bench near to one end at an angle of about 45, and 

 retained in this position for a few seconds, while the 

 drop of blood spreads along the whole of the line of 

 contact (see also Fig. 69). 



5. Draw the first slide firmly and evenly along the 

 entire length of the lower slide, leaving a thin regular 

 film which will probably show the blood cells only one 

 layer thick. 



6. Allow the film to dry in the air. 



7. Stain with one of the polychrome blood stains 

 (seepage 97). 



8. Examine microscopically. 



b. The bacteriological examination of the blood is 

 directed solely to the demonstration of the presence in 

 the circulating blood of the organisms previously in- 

 jected into the animal. For this purpose several cubic 

 centimetres of blood should be taken in an all-glass 

 syringe from an accessible vein corresponding to one of 

 those suggested as the site of intravenous inoculation 

 and under similar aseptic precautions. 



