20 IMMUNITY AND HEREDITY. 



the quantity capable of killing under normal circumstances. He further 

 found that the serum of the blood of an immunised animal Avas a powerful 

 antidote to the poison of the snakes by whose means immunity was obtained. 



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

 anti-toxin does not destroy it ; although it undoubtedly neutralises it. Taking 

 advantage of the fact that a temperature of 154.4° F. (68° C.) causes the 

 immune serum to lose its protective properties, but has no influence 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. 

 ]3r. 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 Avhich 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 success- 

 fully employed as a preventive 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 animals ; and that these antitoxins 

 are manufactured by the tissues — probably, for the most part, by the glands. 

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

 toxin. Consequently, the healthier are the 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 ha.s been borne out by the experiments of Dr. Marriatti-Bianchi. 

 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, have a strong counteracting 

 effect on the venom of this snake. Hence, the neutralising power of an anti- 

 toxin 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. 



Heredity. 



Heredity is the appearance, in descendants, of " like " of parents or of 

 ancestors. This production of " like " is limited to parental or ancestral 

 function, and, consequently, cannot begin after conception. Heredity in 



