Abstract
By means of apparatus previously described, 1 a point source of radiation could be directed on the desired spot on the developing embryonic heart and its effect on the rhythm noted. Studies were made on embryos varying in age from 2 to 20 days, and the following results obtained: Short exposures of the sinus region of the heart produced an increase in rate of beat within 30 seconds after the exposure was begun. A long-continued exposure slowed the rate. Exposure on the ventral aspect of either auricle (at a point of visible condensation of tissue) also produced a marked increase. Exposure of the ventricle did not change the rate of beat except when the point of the quartz rod was placed on the interventricular septum at the base of the auricles. A slight increase was then obtained in hearts whose rate had already been slowed.
Also, in young hearts (2 to 6 days old), it was possible to obtain complete reversal of the direction of beat by a preliminary slowing brought about by lowering the temperature, and then radiating the ventricle or bulbus region.
The normal sequence of the heart cycle may be changed by radiating the sinus or auricle. When a heart has already slowed its rate of beat following continued exposure, a 1:1 auricular-ventricular rhythm may be gradually changed to a 2:1, and then a 3:1, and so on. (A 5:1 ratio was maintained in one heart for several minutes before a further change was produced.) In some cases it was possible to obliterate the ventricular beat entirely, while in others the sinus alone remained beating. Partial or complete heart block can be produced.
Ectopic beats of auricular origin have been obtained by local stimulation of the auricle.
Get full access to this article
View all access options for this article.