138 



VETERINARY BACTERIOLOGY 



second injection of a larger amount is given. Increasing doses of 

 the toxin are injected at intervals until as much as 500 c.c. of the 

 toxin may be administered at one time. Not all horses produce 

 enough antitoxin in their blood to be commercially valuable; 

 hence the antitoxin content is usually determined some time be- 

 fore the process of immunization is complete. When the animal's 

 blood contains the maximum amount of antitoxin, it is drawn 

 by a sterile trocar from the right jugular vein into wide-mouthed 

 sterile jars (Fig. 68). Usually a little less than one liter of blood 

 for every hundred pounds weight of the horse is removed, as this 



Fig. 67. Injection of a horse with toxin (Levaditi). 



amount may be withdrawn without appreciable injury. After 

 a rest the horse may again be injected and bled. 



The jars of blood are allowed to stand until the clot has shrunk 

 and the clear, straw-colored serum has separated. This contains 

 the antitoxin and is the serum antidiphtheriticum of the Pharma- 

 copeia. It must now be standardized, that is, the amount of 

 antitoxin per cubic centimeter determined. Several methods 

 of determining the potency of the antitoxin have been pro- 

 posed. Inasmuch as the unit formulated by Ehrlich has been 

 generally adopted (except by the French) its development will 

 be briefly traced. Behring first proposed as an antitoxic unit, 



