430 EXPERIMENTAL PHARMACOLOGY 



ingly poisonous and the doses must at first be small and 

 guarded or the animal may be killed immediately and the 

 details of the action of the drug cannot be made out. The 

 object now is to give small, repeated injections and bring 

 on the action of the drug very sloiclij. 



If everything is working satisfactorily you should ob- 

 serve after a time a slight slowing of the heart. This is 

 manifested by a gradual increase in the amplitude of the 

 manometer tracing. The pressure will slowly become low- 

 er. Do not crowd on the drug but wait patiently for the 

 vagus center to be roused to greater and greater activity. 

 Meanwhile the heart muscle is becoming slowly more and 

 more irritable. But if the inhibitory apparatus is normal 

 it will at first get the upper hand and the heart should fi- 

 nally be greatly slowed. Is there any way by which you could 

 prove that the heart slowing is due to stimulation of the 

 vagus center? Continue the injections very caution sly and 

 when the heart becomes noticeably slower then stop the in- 

 jections and wait to see if the action of the drug does not 

 become progressively more marked without further injec- 

 tions. In some experiments the heart may become so 

 slowed that the manometer tracing will show an amplitude 

 of one-half to three-fourths of an inch for each separate 

 heart beat. When this stage is approached then carefully 

 lift up both vagus nerves and quickly cut them with the 

 scissors. What effect has this on the blood-pressure and 

 heart beat? 



Caution: Do not mistake the carotid artery and cut it instead of one vagus 

 nerve as the author has seen a student do. What would you do if this accident 

 occurred ? 



Wait a while now and see if the action of the drug de- 

 velops further without giving any more of the poison. How 

 does the heart beat? Can you determine the true action of 

 the heart by watching the mercury manometer ? Does the 

 pressure become exceedingly irregular f It should do so 

 and then after a longer or shorter period (usually only a 



