268 



DR JAMES A. GUNN ON 



The Cause of the Fall of Blood Pressure. 



It has been already stated that the rise in blood pressure produced by smaller doses 

 of harmaline is to be attributed to contraction of the arterioles ; it remained to be 

 determined whether the fall of blood pressure, when it occurs, is due to cardiac causes 

 or to dilatation of the arterioles. 



Experiment 35 (fig. 16). — Rabbit, 1900 grammes. In this experiment a record 

 was taken of the blood pressure and of the changes in intestinal volume. About four 

 inches of the rabbit's small intestine was enclosed in an oncometer, and the changes 

 in its volume were recorded by an air-piston recorder. A dose of 0"008 grm. per kilo 

 was injected, and in thirty seconds this lowered the blood pressure from 100 mm. to 

 78 mm., and the pulse rate from 19 to 16, per ten seconds. No increase of intestinal 



Fig. 16. 



volume occurred during this fall of pressure, which must therefore be ascribed to 

 slowing or to slowing and weakening of the heart. 



Experiment 36. — Cat, 3300 grammes. A record was taken of the blood pressure 

 and volume of the left kidney. A dose of 0'005 grm. per kilo was given, and this 

 reduced the blood pressure in one minute from 109 mm. to 78 mm., and the pulse rate 

 from 21 to 15 per ten seconds. During this fall of blood pressure there occurred 

 a distinct diminution of kidney volume, showing that there was certainly no dilatation 

 of the kidney vessels. In the cat, therefore, as in the rabbit, it is probable that the fall 

 of blood pressure produced by large doses of harmaline is due to slowing or to slowing 

 and weakening of the heart's action. 



The effects produced by harmaline on the blood pressure differ in some respects from 

 those produced by quinine. With quinine "the heart is often accelerated at first, but 

 is afterwards slow and weak, while the blood pressure, after a slight increase, declines 

 progressively. The changes are caused by a preliminary contraction of the arterioles 



