IMMUNIZATION. 129 



A lower serum standard is sufficient for protective purposes. 

 Larger amounts of the antitoxic serum must be used to pro- 

 duce a cure than for immunization against infection. 



When the horse possesses the desired tolerance, the jugular 

 vein is laid bare under antiseptic precautions and a sterile 

 trocar thrust into the vein. The blood is collected in sterile 

 bottles, and after it has coagulated it is placed on ice for a 

 few days, when the clear supernatant serum is pipetted off 

 with sterile pipettes. This serum contains the antitoxic sub- 

 stance, and is known as the "antitoxin." The serum is then 

 standardized and placed in glass containers, which are steril- 

 ized and then hermetically sealed. It is customary to add some 

 preservative to the serum. A 0.5 per cent, solution of car- 

 bolic acid, camphor, or formalin, or a 0.4 per cent, solution 

 of trikresol, is used for this purpose. Trikresol is the most 

 preferable, as it is not irritating and possesses slight anaesthetic 

 properties. 



9 Bact. 



