LVII.] DRUGS AND CURRENT ON HEART. 



LESSON LVII. 



DRUGS AND CONSTANT CURRENT ON HEART 

 DESTRUCTION OF CENTRAL NERVOUS 

 SYSTEM. 



1. Action of Drugs on the Heart Muscarine, Atropine, and 



Nicotine. Either the excised heart, placed in a watch-glass, or the 

 heart in situ may be used, or Gotch's method may be employed 

 (p. 269). The heart may be attached to a GaskelFs lever (fig. 187) 

 and the result recorded. The last is the best plan, for by this means 

 a tracing can readily be obtained. 



(a.) Muscarine. Pith a frog, expose its heart, and if desired 

 attach its apex to a GaskelPs lever recording its movements. Record 

 the result (fig. 199). To get the full effect of the drug on cardiac 

 action snip the heart to allow the blood to run out. With a fine 

 pipette apply a few drops of serum or normal saline (0.6 p.c.) con- 

 taining a trace of muscarine, which rapidly arrests the rhythmical 

 action of the heart, the ventricle being relaxed i.e., in diastole 

 and if uncut distended with blood. Before it stands still the 

 heart-beats become less and less vigorous. (This is a good method 

 of collecting a considerable quantity of frog's blood when it is 

 wanted for any purpose from the heart.) 



(6.) When the ventricle is completely relaxed in the diastolic 

 phase, it is very inexcitable, responding only to strong stimuli, and 

 perhaps the auricles not at all. 



Atropine. To the heart arrested with muscarine, 



(c.) After a few minutes, with another pipette apply a few drops 

 of a 0.5 per cent, solution of sulphate of atropia in normal saline. 

 The heart gradually again begins to beat rhythmically. Thus the 

 atropine undoes the effect of the muscarine. This is sometimes 

 spoken of as " Antagonistic action" of drugs (fig. 199). 



(d.) Faradise the crescent or inhibitory centre of the atropinised 

 heart; the heart is no longer arrested, because the atropine has 

 paralysed the intracardiac inhibitory mechanism. 



(e.) Pilocarpine. In another frog, arrest the action, of the heart 

 with pilocarpine, and then apply atropine to antagonise it, observing 

 that the heart beats again after the action of atropine. 



(/.) Nicotine. Apply nicotine (.2 milligram). Stimulation of 

 the vagus no longer arrests the heart's action, but stimulation of the 

 sinus venosus does ; so that nicotine paralyses the fibres of the 

 vagus, and leaves the intracardiac inhibitory mechanism intact. 



