20 IMxVlUNlTr AND HEREDITY. 



Calraette has made animals immune from the bites of certain P°i^°"°^^ 

 snakes by repeated injections of .the venom, in, at first, small (non-ietnai, 

 doses which were gradually increased, without mjuriously affecting ine 

 patient, to many times the quantity capable of kilhng under normal circum- 

 stances. He further found that the serum of the blood of an immunisea 

 animal was a powerful antidote to the poison of the snakes by whose means 

 immunity was obtained. , i, , ,. j 



Roux, in experimenting with snake poison, has proved that the supposed 

 ar.ti-toxin does not destroy it ; although it undoubtedly neutralises it 

 Taking advantage of the fact that a temperature of lo^-^ *• \°° y-l 

 causes the immune serum to lose its protective properties, but has no in- 

 fluence in diminishing the virulence of snake poison, he mixed a lethal dose 

 of snake poison with a fully protective quantity of serum, and after having 

 heated the mixture to 68° C, he injected it into rabbits, with the result 

 that these animals died of snake poisoning in the same manner as they 

 would have done had the serum been omitted. When, however, the 

 mixture had not been heated, its injection proved harmless. 



It appears to have been assumed by Calmette that his serum was capable 

 of affording immunity from the injurious consequences of the bites of all 

 snakes. Dr. C. J. Martin, who made many experiments with Calmette's 

 serum in Australia, proved, that although it had a strong curative effect on 

 the venom of cobras, it had very little influence on that of Australian tiger 

 snakes. In explanation of this fact he showed that snake poison contains 

 two or more toxins which vary in the proportions they bear to each other, 

 according to the variety of the snake ; and that the chief toxin of cobra 

 venom is found only in very small quantities in Australian tiger snake 

 venom. Hence, the non-success of Calmette's serum in Australia. 



Immunity can be also obtained against certain diseases, tetanus, for 

 instance, by commencing with injections of pure toxin much below the 

 lethal quantity, and gradually increasing the dose, or by using injections, 

 in the same manner, of attenuated toxin, the attenuation being effected by 

 heat or by the addition of chemical agents. Thus, in rabies, the attenuated 

 virus or toxin is successfully employed as a preveiltive vaccine ; in other 

 words, the inoculated animals acquire immunity. 



Recent researches conclusively prove the existence of antitoxins in the 

 blood, spleen, and other glands of immune ajiimals ; and that these anti- 

 toxins are manufactured by the tissues — probably, for the most part, by 

 the glands. Hence, the addition to the blood of serum containing an anti- 

 toxin, may act as a reinforcement to the antitoxin secreted by the tissues. 

 It appears that the presence of a toxin in the blood stimulates the tissues 

 to form an antitoxin. Consequently, the healthier are tlie tissues, the 

 better able are they to produce an antidote to the poison. We here arrive 

 at the apparently sound theory that, in many cases, acquired immunity 

 is dependent on developing the power of the tissues to secrete antitoxins. 



The frequently successful resistance which the healthy body makes to 

 attacks of infective diseases, gives us strong grounds for assuming that the 

 production of one or more antitoxins is a normal function of its tissues ; a 

 view which has been borne out by experiments. Roux and Calmette have 

 also shown that rabbits which have been made immune from rabies, have 

 greatly increased power of resisting the action of cobra venom, and that 

 the serum of animals which have been rendered respectively immune from 

 tetanus and anthrax, has a strong counteracting effect on the venom of 

 this snake. Hence, the neutralising power of an antitoxin is not always 

 restricted to its own particular toxin. 



Disease-producing bacteria elaborate not only one or more special toxins, 

 but also form products which give rise to fever, by their action on the 

 heat-regulating centres of the brain. 



