496 IMMUNOLOGY 



capacity of the .smooth muscle to respond to histamine need not be 

 altered. In fact, one would expect the desensitized horn to react 

 to histamine if it is added to the bath. 



In regard to the second objection tluit liistaniine will not pro- 

 duce prolongation of the clotting time of the blood as is com- 

 monly ob.served in anaphylactic shock, this may be a real objec- 

 tion and again it may be found either that a toxic substance 

 possessing properties of both histamine and peptone is liljerated 

 or that both chemical and physical mechanisms are involved. 

 Rous and Gelding (1930) have offered experimental evidence 

 which casts doubt upon the contention of Lewis that local vaso- 

 dilatation after different tissue injuries is due to a single factor 

 such as histamine. From their work they draw the following 

 conclusions : ' ' Experiment shows that the vascular contraction 

 responsible for Bier's spotting ]>revaiLs over the local vasodilata- 

 tion caused by liistamine pricked into the skin. The results raise 

 doubts concerning tlie validity of the hypothesis referring all 

 local vasodilatations to the action of a single chemical substance 

 or set of substances (H-snbstance) lil)erated within the tissues." 



The only serious physiological objections to regarding his- 

 tamine as the toxic factor in anaphylactic shock have been raised 

 as the result of studies carried out in our laboratory. These may 

 be summarized as follows : 



1. In histamine shock in the dog and rab])it each i)hase of the 

 blood pressure response is decidedly sliorter than in anaphylaxis. 

 In the rabbit histamine frequently does not produce a drop in 

 blood pressure below normal such as occui's almost invariably in 

 anaphylactic shock. When histamine ]n'oduces a lowering of the 

 l)lood pressure in the rabbit, both the drop in pressure and the 

 return to normal occur in much shorter time than in anaphylaxis. 



In histamine shock in the cat the blood pressure tracing re- 

 sembles that observed in anaphylaxis except that the phase of low 

 pressure is longer (20 min.) in histamine shock than in anaphy- 

 lactic shock (12.5 min.). It will l)e observed that this is the 

 opposite of the findings in the dog and rabjjit. 



2. There is in histamine and anaphylactic shock a noticeable 

 difference in the heart rate following the drop in blood pressure. 

 In histamine shock in the rabbit 62.5 per cent showed an increase 



