12 EXPERIMENTAL RESEARCHES 



monotropic, for receptors for the tetanus spasmin 

 are present only in the central nervous system ; 

 on the other hand, in the rabbit, suitable 

 receptors are present not only in the central 

 nervous system, but also in other organs and 

 in the connective tissue, and the type of distri- 

 bution in this animal is more complex, and 

 depends upon the point of entrance of the 

 infection or injection. Thus the varying sensi- 

 tiveness of different species to the same toxin 

 may be explained. The localisation of the 

 receptors is, then, of great moment for the 

 distribution of toxin in the organism, and con- 

 sequently for its sensitiveness to poisonous 

 action. One may therefore, in general, dis- 

 tinguish four different types of distribution : 



(1) In which fitting receptors are not present, 

 the animal possessing natural immunity (the 

 formation of antibodies cannot take place). 



(2) In which receptors are present, but only 

 in those organs on which the poison does not 

 act, or in organs of lesser importance; here, 

 again, the animal possesses natural immunity 

 (antibodies may be formed, and immunisation 

 is easily carried out). 



(3) In which the receptors are distributed 

 over various parts of the organism, and are 



