460 PRINCIPLES OF VETERINARY SURGERY 



The antitoxic effect of the serum is preserved for a long 

 time, even more than a year, and its strength is measured 

 according to the recommendations of Behring: The quan- 

 tity of serum required to immunize a gram of white mouse 

 against a fatal dose of toxin is taken as a unit. A gram of 

 young cavy will answer for a gram of white mouse. When 

 antitoxic serum is said to possess an activity of a million 

 units, that means that a cubic centimeter of the serum is suf- 

 ficient to immunize i,o6o kilograms (1,000,000 grams) of 

 mouse ; or, again, that one motise of 20 grams is rendered im- 

 mune by two hundred thousandths of a cubic centimeter. 



In practice antitoxic activity is measured in the following 

 manner: Mice are injected with gradually increasing doses 

 of the serum, starting with a dose supposed to be inefifectual, 

 then from ten to twelve hours later, each mouse is given a 

 fatal dose of the toxin. Thus the extent to which the mouse 

 has been vaccinated is determined, that is to say, the amount 

 of resistance to the poison is made known. 



Serums of exceptional activity may be obtained. They 

 may be represented by figures o.oooooooooi or even 

 0.0000000002 according to Nocard. At each bleeding, the 

 activity of the serum should be tested as to its antitoxic po- 

 tency. The antitoxic potency of blood of the horse used 

 will be found to diminish regularly from the time the vac- 

 cinating injections cease. Although the immunity lasts for 

 a long time, often for two years, the decrease in the anti- 

 toxic strength of the serum is already manifest 15 to 20 days 

 after the last vaccinating injection. In order to maintain 

 an active serum it is necessary to regularly inject toxin into 

 the animals which supply it. This does not only prevent 

 the enfeeblement of the toxic and immunizing products, but 

 also increases them incessantly in proportion to the quantity 

 of toxin injected. 



The antitoxic substance exists only in the serum. It is 



