36 CLINICAL BACTERIOLOGY AND H^EMATOLOGY 



pipette, which can be inverted and the blood shaken into the tip, 

 but take care not to do this until the glass is cold, as the serum 

 loses some of its properties when heated. If you are not 

 going to examine the serum for some time seal the other end 

 to prevent evaporation. 



When the blood has coagulated it will begin to contract 

 away from the sides of the tube, the serum being forced out 

 of the clot. When this process is complete there will be a 

 central dark-coloured clot suspended in clear serum. The tube 

 can now be notched with a file and broken, and the serum re- 

 moved with a pipette and used for Widal's reaction, the esti- 

 mation of the opsonic index, etc. If much serum is required, 

 the pipette can be hung by its crook (straight end downward) 



FIG. 13. 



in the bucket of a centrifugal machine and centrifugalized. In 

 this way the clot is driven to the bottom and a large crop of 

 serum obtained. 



Since some practitioners seem to have difficulty in collecting 

 the quantity of blood desirable for the opsonic and other tests, 

 a few more notes on the process may be added. I personally 

 prefer to obtain the blood from the ear, and proceed as fol- 

 lows : the lobe is well rubbed by means of a piece of lint, until 

 it is markedly hyperaemic, and is then punctured on its lower 

 border with a Hagedorn's needle or platino-iridium hypoder- 

 mic needle (previously sterilized in the flame), or with the 

 glass needle described below. This is done by a short, sharp 

 "jab," and should not be felt as pain at all; I have frequently 



