NATURAL SUSCEPTIBILITY TO TOXINS 501 



hence no bactericidal action such as occurs when the blood is shed. 

 In the case of the haemolytic action of a normal serum, it has 

 been shown in many instances that in addition to complement a 

 natural immune-body is also concerned (p. 481), and this would 

 appear to be the rule ; the process being analogous to what is 

 seen in the case of an artificially developed hsemolytic serum. 

 In certain instances an analogous condition appears to obtain in 

 a normal bactericidal serum. For example the dog's serum heated 

 at 58 C. contains a natural immune-body to anthrax which can 

 be activated by the addition of normal guinea-pig's serum so as to 

 produce a bactericidal action, though the latter is by itself with- 

 out any such effect. At present, however, the possibility of 

 bactericidal action by complement alone cannot be excluded, as 

 it appears to combine with many bacteria without any inter- 

 mediary. Further work is necessary to determine whether all 

 the facts regarding natural immunity are explainable by the 

 opsonic and bactericidal properties of the serum. 



2. Variations in Natural Susceptibility to Toxins. We must 

 here start with the fundamental fact, incapable of explanation, 

 that toxicity is a relative thing, or in other words, that different 

 animals have different degrees of resistance or non-suscepti- 

 bility to toxic bodies. In every case a certain dose must be 

 reached before effects can be observed, and up to that point the 

 animal has resistance. This natural resistance is found to 

 present very remarkable degrees of variation in different animals. 

 The great resistance of the common fowl to the toxin of the tetanus 

 bacillus may be here mentioned (v. p. 381), and large amounts of 

 this poison can be injected into the scorpion without producing 

 any effects whatever; the high resistance of the pigeon to 

 morphia is a striking example in the case of vegetable poisons. 

 This variation in resistance to toxins applies also to those which 

 produce local effects, as well as to those which cause symptoms 

 of general poisoning. Instances of this are furnished, for 

 example, by the vegetable poisons ricin and abrin, by the snake 

 poisons, and by bacterial toxins such as that of diphtheria. We 

 must take this natural resistance for granted, though it is 

 possible that ere long it will be explained. 



According to Ehrlich's view of the constitution of toxins, it 

 might be due to the want of combining affinity between the 

 tissue cells and the haptophorous group of the toxin ; or, on the 

 other hand, supposing this affinity to exist, it might be due to 

 an innate non-susceptibility to the action of the toxophorous 

 group. Certain investigations have been made in order to 

 determine the combining affinity of the nervous system of the 



