26 VACCINE AND SERUM THERAPY. 



well separated. The tube may now be opened by filing and break- 

 ing at "c." 



FIG. 6. 



According to the original method of Wright and Douglas, 

 blood was obtained from the patient and from a number of 

 healthy individuals. Equal parts of the normal serum were taken 

 and mixed together or "pooled." This was done so as to get a 

 better normal serum. After it has once been established that 

 serum from a certain healthy individual has an opsonic index 

 of 1 . for a particular organism when compared with pooled 

 serum, this individual's serum replaces the pooled serum. After 

 this has been established, the opsonic index of any patient for 

 this organism is determined by dividing his phagocytic index 

 by the phagocytic index of the "normal individual," as he may 

 now be called. 



LEUCOCYTES. 



The leucocytes used in the determination of the opsonic 

 index are usually obtained from the blood of supposedly healthy 

 individuals, most frequently from the blood of the investigator 

 himself. No particular stress, however, is laid upon the individ- 

 ual from whom the blood is obtained. 



To obtain blood, preferably, the middle finger is cleaned, con- 

 gested and punctured in the same manner as was followed in the 

 collection of serum. Ten drops of blood are collected in about 

 lOc.c. of a normal salt solution containing one per cent of sodium 

 citrate. The sodium citrate is added to keep the blood from 

 clotting. The tube containing the mixture is then centrifuged 

 in a centrifuge of not too great speed. When the speed exceeds 

 1,200 to 1,500 revolutions per minute, the cells are too closely 



