38 TOXINS THEIR FRAGILITY 



substances, and their exact nature is as yet unknown. This may 

 be due in part to the minute amounts which are formed, and in 

 part to the difficulties which prevent their being obtained in a 

 pure state ; but there are other reasons, to which we shall revert 

 later, for this complexity. Further, they are, with a few ex- 

 ceptions, very fragile substances, and are readily destroyed by 

 the action of many agents, and especially by heat. Nearly all 

 the bacterial toxins are rendered inert by boiling, and many of 

 them by a short exposure to a temperature of .60 or 70 C. 

 They are usually destroyed by gastric digestion, so that they are 

 without action when administered by the mouth. 



A considerable amount of attention has been paid to this 

 question, since it would be desirable, if possible, to replace hypo- 

 dermic injections of vaccines, etc., by oral or rectal administration. 

 In general terms the statement made above holds good : toxins 

 administered by the mouth are not absorbed as such, and do not 

 produce the characteristic symptoms of the disease. In some 

 cases, however, there is reason to believe that a small amount 

 of absorption, probably of the toxin in an altered form, does 

 occur, and a certain degree of immunity may be produced by 

 the oral administration of killed cultures of typhoid bacilli, and 

 possibly of tubercle bacilli. But this method has only one advan- 

 tage its painlessness over the hypodermic method, whereas its 

 uncertainty renders it extremely undesirable. There can be no 

 doubt that the advantage of giving an exactly measured dose, with 

 the certainty that every particle will be absorbed and act in the 

 way desired, will, under ordinary circumstances, render the hypo- 

 dermic method infinitely preferable. To administer infinitesimal 

 doses of killed tubercle bacilli or of TR to an infant who may be 

 swallowing large doses of living and dead bacilli in milk, sputum, 

 etc., does not appear rational, and the clinical evidence in its favour 

 is entirely unconclusive. In the case of ricin, about a hundredth 

 part of the toxin given by the mouth is absorbed as such i.e., the 

 minimal lethal dose on oral administration is about 100 times 

 as large as the lethal dose of the same preparation given sub- 

 cutaneously (Stillmarck). Ricin is, however, far more resistant to 

 the action of digestive enzymes than are the exotoxins. 



The most important feature of the bacterial toxins is their 

 relation to immunity. It is possible in all cases to render a 

 susceptible animal immune to their action by the injection of the 

 toxins in suitable doses at suitable intervals, though in some 



