§ 545. Action of the Digitalins on the Frog’s Heart.—The general action of the digitalins is best studied on the heart of the frog. Drs. Fagge and Stevenson have shown[576] that, under the influence of digitalin, there is a peculiar form of irregularity in the beats of the heart of the frog; the ventricle ultimately stops in the white contracted state, the voluntary power being retained for fifteen to twenty minutes afterwards; in very large doses there is, however, at once paralysis. Lauder Brunton[577] considers the action on the heart to essentially consist in the prolongation of the systole.
[576] Guy’s Hospl. Reports, 3rd ver., vol. xii. p. 37.
[577] On Digitalis, with Some Observations on the Urine, Lond., 1868.
Atropine or curare have no influence on the heart thus poisoned. If the animal under the influence of digitalin be treated with muscarine, it stops in diastole instead of systole. On the other hand, the heart poisoned by muscarine is relieved by digitalin, and a similar influence appears to be exercised by atropine. The systolic stillness of the heart is also removed by substances which paralyse the heart, as delphinin, saponin, and apomorphin.
Large doses of digitalin, thrown suddenly on the circulation by intravenous injection, cause convulsions and sudden death, from quick palsy of the heart. With frogs under these circumstances there are no convulsions, but a reflex depression, which, according to Weil[578] and Meihuizen,[579] disappears on decapitation. The central cerebral symptoms are without doubt partly due to the disturbance of the circulation, and there is good ground for attributing them also to a toxic action on the nervous substance. The arteries are affected as well as the heart, and are reduced in calibre; the blood pressure is also increased.[580] This is essentially due to the firm, strong contraction of the heart, and also to the “stop-cock” action of the small arteries.[581]
[578] Archiv f. Anat. u. Physiol., 1871, p. 282.
[579] Archiv f. d. Ges. Physiol., vol. vii. p. 201.