96 Messrs. J. A. Gunn and R. St. A. Heathcote. 



heart of unimmunised rabbits, it is clear that as the result of immunisation, 

 the heart becomes much less sensitive to the action of the venom, quite 

 apart from any antitoxic action of the serum, because in these experiments 

 the serum is previously washed out by perfusion. A concentration of 

 1 in 400,000 has less effect on the excised heart of an immunised rabbit than 

 has a solution of 1 in 800,000 on the excised heart of an unimmunised rabbit 

 Figs. 5 and 6 show this difference in action. In fig. 5 is shown the perfusion 

 of a normal heart with 1 in 800,000, and in fig. 6 the perfusion of the heart 

 of the immunised animal with 1 in 400,000. It is clear that, though in the 

 latter case the concentration perfused is doubled, the toxic effect produced is 

 far less. 



In short, the heart of a rabbit immunised to 10 M.L.D. was able to 

 withstand with only slight changes in its action, twice the concentration of 

 venom that sufficed to arrest the heart of an unimmunised animal. 



Further, when the immunised heart was subsequently perfused with a 

 concentration of 1 in 200,000, it was only arrested in systole in 20 minutes, 

 corresponding to the time of arrest produced by 1 in 600,000 in the normal 

 heart, in spite of the fact that the former had been previously perfused for 

 45 minutes with 1 in 400,000. 



Though these experiments are insufficient to enable an exact numerical 

 ratio to the quantitative effects of the venom upon the immunised and non- 

 immunised heart to be given, it is justifiable to state that in a rabbit 

 immunised to 10 M,L.D., the heart is at least three times less sensitive to the 

 action of the venom than is the heart of an unimmunised rabbit. 



Table III. — Comparison of the Action of Cobra Venom on the Normal and 



Immunised Heart. 



Strength. 



Normal rabbit. 



Immunised rabbit. 



1 in 200,000 

 „ 400,000 

 „ 600,000 

 „ 800,000 



Systolic arrest in 13 mimifces 

 » » 15 ,, 

 „ 20 „ 

 34 „ 



Systolic arrest in 20 minutes. 

 Yery slight eifects in 45 minutes. 



(b) Coronary Mow. 



As has been stated in a preceding section (p. 90), when the normal 

 rabbit's heart is perfused with cobra venom, the rate of flow through the 

 coronary vessels is diminished from the first. When the heart of the 

 immunised rabbit was perfused with 1 in 400,000, there occurred a 

 preUminary increase of flow lasting 7 minutes and amounting at its 



