88 Messrs. J. A. Gunn and II. St. A. Heathcote. 



Observations on Natural and Acquired Immunity to Cobra Venorfi. {Ghmn and 



Heathcote^ 



The Congenital Immunity of the Cat to Cobra Venom. 



It has long been known that animals show different degrees of suscepti- 

 bility to the toxic action of venoms, and especially that the cat shows a high 

 degree of resistance as compared with rodents. Thus Fraser (24) in 1896 

 showed that the minimum lethal dose of cobra venom for the cat is thirty 

 times that for the rabbit, and Fraser and Grunn (25) that the minimum lethal 

 dose of Sapedon hcemachates — another colubrine venom — is for the cat 

 fifteen times that for the rabbit. This comparative tolerance of the cat is 

 most marked in the ease of colubrine venoms, but it is also true to a certain 

 extent of viper venoms, for Fraser and Gunn (26) found that the ratio of 

 lethality for the cat and rabbit in the case of Echis venom was 9 : 1. 



Minimum Lethal Dose for Babbit and Cat. 



The minimum lethal dose of the venom we used was found to be for the 

 rabbit 0012 grm. per kilogramme and for the cat 0"025 gramme per kilo- 

 gramme. Calculated per kilogramme, therefore, the minimum dose required 

 to kill the cat is twenty times greater than that required to kill the rabbit. 

 As this relation is similar to that found by Fraser and others, it is not 

 necessary to give the experiments in detail. 



The following experiments were made to ascertain whether or not, corre- 

 sponding to this relative natural immunity of the intact animal, the tissues of 

 the cat are less susceptible to the action of the venom than are the tissues of 

 the rabbit. With this end in view, experiments were made on the heart, 

 intestine, and red blood-corpuscles. 



(a) Heart. 



For perfusing the heart and for recording the flow through the coronary 

 vessels, the heart-perfusion apparatus (27) and syphon-recorder (28), described 

 by one of us were employed. As the perfusing solution, Locke's solution 

 without glucose was used. The hearts were perfused for at least twenty 

 minutes before the venom solution was tried, so that all the serum was 

 removed from the heart first. 



Cobra venom, if in sufficient concentration, kills the heart by arresting it 

 in the systolic position. The preliminary effects are somewhat variable, there 

 being sometimes a primary enfeeblement of systolic contraction, occasionally 

 a small and short-lasting augmentation or acceleration. 



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