nS AIDS TO BACTERIOLOGY 



to the animal. When sufficiently immune, a sterile 

 cannula is inserted in the jugular vein, and the blood 

 drawn off into sterile bottles. The bottles are placed 

 in an ice chamber to allow the clot to separate. The 

 clear serum is separated for use. A small quantity of 

 trikresol is added. For use in hot climates the serum 

 may be evaporated to dry ness in vacuo, and then forms 

 amber-coloured scales or granules, which, for use, may be 

 dissolved in five to ten parts of sterile water (one part 

 of dry serum = about ten parts of the fluid serum). 



There is gradual diminution of antitoxic power in the 

 serum yielded by horses, even though they continue to 

 receive toxin, so after some months fresh animals have to 

 be employed. 



The preparation of a horse to give serum of very high 

 antitoxic value by the above method involves treat- 

 ment extending to six months or more. Cartwright 

 Wood grows the diphtheria bacillus in ordinary peptone 

 broth containing serum for three or four weeks at 37 C., 

 and, after filtration, heats for an hour at 65 C. It is 

 thereby claimed that powerful antitoxic serum can be 

 produced in a short time. 



During immunisation a small quantity of blood is with- 

 drawn from time to time, and its antitoxic power tested. 



Standardisation of the Serum. The serum is tested 

 against a certain amount of the toxin, the result being 

 recorded in units. The methods of Behring and of Roux 

 were formerly employed for this purpose. 



According to Behring's standard, a serum that contains 

 one normal antitoxin unit per c.c. is of such a strength 

 that ^o f a c - c - completely neutralises the action of ten 

 lethal doses of toxin. 



The lethal dose of toxin is the amount required to kill a 

 given weight of guinea-pig. Of this toxin, ten times the 

 amount required to kill a guinea-pig of about 250 grammes 

 weight is injected, together with the antitoxin to be tested. 

 By noting the absence or presence of local reaction and 

 the increase or loss of weight, it is stated that an opinion 

 may often be formed after twenty-four hours, but that 

 after forty-eight hours a decision can always be given. 

 When the toxin is completely neutralised, as it should be, 

 the animal should not only live, but there should be no 

 trace of local reaction (oedematous swelling). This 



