. Materia medica : pharmacology, therapeutics and prescription writing for students and practitioners. a leakage backward. Asa matter of fact, the normal ventricular contraction begins inthe papillary muscles. 2. Irritability or excitability is the susceptibility to , it does not determine the rate of the heart, for thenormal pacemaker is the sinus node. But an increase of irri-tability beyond the normal tends to result in spontaneous mus-cular contractions that do not have their origin in the sinus effects of these are harmful. They may be produced bydigitalis. Overir
. Materia medica : pharmacology, therapeutics and prescription writing for students and practitioners. a leakage backward. Asa matter of fact, the normal ventricular contraction begins inthe papillary muscles. 2. Irritability or excitability is the susceptibility to , it does not determine the rate of the heart, for thenormal pacemaker is the sinus node. But an increase of irri-tability beyond the normal tends to result in spontaneous mus-cular contractions that do not have their origin in the sinus effects of these are harmful. They may be produced bydigitalis. Overirritability or over excitability may show in auricular orventricular premature beats, in paroxysms of tachycardia, inauricular fibrillation, or in ventricular fibrillation. In someexcitable hearts there are alternations of premature beats,paroxysmal tachycardia, and auricular fibrillation. DIGITALIS 159 (a) Premature Beats.—One of the earliest indications of exces-sive irritability is the premature or interpolated or abortive beat,the so-called extrasystole, a beat which has its origin elsewhere. Fig. 9.—Ventricular extrasystoles developing in a heart with normal rhythm andmoderate dilatation. This resulted from 10 minims ( ) of tincture of digi-talis and 20 minims ( ) of tincture of nux vomica three times a day. Itceased within two days of stopping the medicine. (Top line, apex; lower, radial pulse.) than at the sinus node. The site of origin may be the auricle,the result being a premature auricular beat, followed by a cor-responding premature ventricular beat in response to theauricular stimulus. But muchmore commonly the prematurebeat has its origin in the ventri-cle, the ventricle alone giving apremature beat, while the au-ricular rhythm is not premature beat may appear atregular intervals or irregularly,and frequently or may follow the normal beatsso that the ventricle beats incouples. It may show in theradial pulse or it may not
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