DRUGS ACTING ON THE CIRCULATION 35 



of the apex, which is comparatively free from nerve supply. 

 The action of drugs on the inhibitory apparatus is of more 

 importance than that exerted on the accelerator nerve, 

 and we know more about it. Drugs usually affect the roots 

 of the vagus nerve in the centre, or its peripheral termina- 

 tions in the heart, rather than its trunk. The heart is 

 influenced by drugs as follows : ' 



1. Stimulation of tbe inhibitory apparatus leads to slowing or 

 weakening of the heart-beats, or to both. 



3. Depression of the inhibitory apparatus results in quickening or 

 strengthening the heart-beats, or both. 



3. Stimulation of the accelerator apparatus causes an increase in 

 the rate or force of the heart-beats, or both. 



4. Depression of the accelerator apparatus induces decrease in the 

 rate or force of the heart-beats, or both. 



5. Stimulation of the heart-muscle produces increase in the rate or 

 force of the heart-beats, or both. 



6. Depression of the heart-muscle lowers the rate or force of the 

 heart-beats, or both. 



The vagus centre is stimulated by agencies increasing 

 blood-pressure, or causing asphyxia. On the other hand, 

 agencies reducing blood-pressure depress the vagus, or 

 stimulate the accelerator nerve, or both. Thus, the nitrites, 

 as amyl nitrite, nitro-glycerin and spirit of nitrous ether, 

 quicken the heart by lowering vascular tension. External 

 stimuli to sensory nerves reflexly stimulate the heart, as 

 also do many locally irritating agents taken internally ; e.g., 

 strong alcoholic or ammoniacal preparations. 



Since drugs commonly influence more than one part of 

 the mechanism controlling the heart, and since it is difficult 

 to determine the exact physiological details in such complex 

 actions, we shall content ourselves with tabulating the 

 actions of drugs ordinarily employed for their influence on 

 the heart, remembering that while moderate doses produce 

 the effects enumerated below, poisonous doses often give 

 rise to diametrically opposite actions. 



(a) Drugs increasing the force of the heart-beat. 



Digitalis "I r> ■ li. ^ 



Adrenalin Barmm salts ) Do not alter rate 



Strophanthus \ Slow the pulse Camphor V particularly 



Sparteine Physostigmme ) ^ y 



Squill 



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