PHYSIOLOGICAL ACTION OF VENOM OF BLACK SNAKE. 215 
injected, the pressure begins to slowly decline, and continues to 
do so until it is only equal to a few millimetres of mercury, when 
the enfeebled circulation is no Jonger adequate to support life, 
and the animal dies. 
This fatal fall of blood pressure also invariably occurs in those 
cases where artificial respiration is maintained. Without this 
assisted respiration the pressure declines more quickly as the 
cardiac depression is accentuated by a diminished supply of oxygen, 
due to simultaneous failure of respiration. 
Up to within a very short interval before death, when the cir- 
culation is almost ended, vagus stimulation produces the usual 
slowing or stoppage of the heart. (of. Exs. 10, 11, and 14.) 
The intravenous injection of the poison, however, neutralises 
that normal retarding influence which is exercised by the vagus, 
and is so marked in some dogs. In these animals the large slow 
cardiac contractions are almost immediately superseded by con- 
tractions increased in rate, and much diminished in force after the 
injection of venom. Section of the vagi under these circumstances 
is without effect. 
When the venom is subcutaneously injected, the decline of the 
blood pressure is more gradual, and the primary sudden fall so 
characteristic of intravenous injection is absent. The time which 
elapses before the pressure begins to fall depends upon the amount 
injected and the rapidity of absorption. A small dose (-005 
gramme per kilo.) when introduced under the skin, may not 
materially affect the blood pressure for three hours subsequent to 
the injection, whereas the same amount introduced into the 
Peritoneal cavity may cause a marked fall in pressure during the 
first fifteen minutes, Larger doses however act much more rapidly, 
and if a sufficient quantity is injected the blood pressure often 
falls to one half of its previous height in three or four minutes 
after the introduction of the poison. 
