126 IMMUNITY, NATURAL AND ACQUIRED 



(c ) By inoculation with toxins or bacterial products. This 

 is a most important advance, and has done much to disprove 

 Metschnikoff's doctrine of phagocytosis as being the means by 

 which natural protection was acquired by the animal. 

 Virulent organisms are grown in a culture medium for a certain 

 time and then filtered off through a Pasteur-Cham berland filter ; 

 the filtrate then contains the toxins elaborated by the bacilli. 

 This can be standardised and used with the same exactitude in 

 dose as an a.lkaloid. The toxin is administered to the animal at 

 first in small doses, and gradually increased — for example, 

 Calmette and Fraser's experiments with snake poison to produce 

 anti-venene. Inmost casfs the toxins are in solution, but a 

 similar result may be obtained by sterilising the cultures, and 

 not filtering, but injecting the dead bacilli with the culture 

 medium, as in Haft'kine's plague prophylactic. — (The method 

 of preparation of Haft'kine's plague prophylactic was here 

 explained.) — Albumoses in the beef-tea perhaps cause fever ; an 

 improved method by two Italian scientists has led to the 

 production of a crystalline substance which causes no 

 fever. By this method a high degree of immunity can be pro- 

 duced, but as before, only up to a certain point. This is 

 practically a chemical process, and is comparable to the toleration 

 of laudanum in the devotees of the opium habit or of arsenic in 

 the Styrian peasants. Similar results, but on a more restricted 

 scale, have been obtained by feeding animals on toxins or dead 

 bacterial cultures. The important axiom to be drawn from this 

 method is, to quote Kanthack, " immunity therefore implies 

 resistance both to bacteria and their products." 



((/) Immunity may be procured by inoculating an animal 

 with serum derived from animals protected by a previous attack 

 or immunised by the above methods. Practically, this is the 

 method which has found most general use in the prevention and 

 treatment of certain infectious diseases. Take, as example, 

 protective inoculation agahist diphtheria by the injection of 

 serum from horses highly immunised by injections of cultures of 

 diphtheria bacilli. This must not be confused with the curative or 

 antitoxic effect of anti-diphtheria serum, as that belongs to a 

 dift'erent category, though it may be well now to refer to this part 

 of theisubject. The chief of these methods are treatment of diphthe- 

 ria by antitoxic serum, of tetanus by anti-tetanic serum, immun- 

 isation of cattle against tick fever by the injection of serum ob- 

 tained from cattle which have survived an attack. The draw- 



