292 EXPERIMENTAL INVESTIGATION OF THE ACTION OF MEDICINES. 



This comparison between the effects which atropia produces in 

 different animals, and in large and small doses in the same 

 animal, shows us that it sometimes raises and sometimes lowers- 

 the blood-pressure, but that it always quickens the pulse,, 

 except when a large quantity of the poison is introduced at 

 once into the heart of the rabbit. On consulting the table 

 already given (p. 278),* it will be seen that quickening of the 

 heart may be due to stimulation of the sympathetic, either 

 directly by the drug or indirectly by diminution of the blood- 

 pressure ; to stimulation of the cardiac ganglia ; or to weaken- 

 ing or paralysis of the vagus. Any one of these conditions may 

 cause quickened pulsation ; and, in order to determine which of 

 them really does it, we must test each one of them separately 

 by farther experiment. 



Mode of determining the exact cause of S?/mptoms. — The plan? 

 which we follow is this : we suppose for the time being that the- 

 cause which we are testing is the true one, and consider wlmt. 

 effects it will produce under certain conditions. We tlien 

 supply these conditions experimentally, and see whether or not. 

 the results we obtain correspond with those which we should 

 find if our supposition were correct. So in the present instance- 

 we first ask, Is the quickening of the pulse due to indirect 

 stimulation of the sympathetic roots by diminished blood- 

 pressure or not ? We suppose for the moment that it really is. 

 so, and we consider that if we raise the blood-pressure we shall 

 remove the cause of quickening and bring the pulse down again, 

 to its normal rate. We then proceed to raise the pressure, and 

 see whether or not the pulse is rendered slow, as we expect it to 

 be. In the case of atropia, a special experiment is not neces- 

 sary for this purpose, as we have seen that small doses do not 

 loiver but raise the blood-pressure, at the same time that they 

 quicken the pulse ; consequently the quickening cannot be due 

 to indirect stimulation of the sympathetic. Other drugs, how- 

 ever, such as nitrite of amyl, even in small doses, lower the 

 blood-pressure at the same time that they quicken the pulse,, 

 and in their case we must raise the blood-pressure artificially. 



* Brilish Medical Journal, June 3rd, page 538. 



