ABSTRACT
A seasonal difference in the temperature-pulse rate curve of the isolated frog’s heart was described by Barcroft & Izquierdo (1931). In the summer they found an almost linear relation between temperature and frequency of pulsation from 5 to 20° C., while in the winter the relation was exponential over the same temperature range. Carter (1933) investigated the effects of various endocrine and related substances on the form of the temperature-pulse rate curve. He found that the addition of thyroxine to the medium perfusing the heart transformed the curve given by the winter heart into the summer form, but had no effect on that of the summer heart. He concluded that the seasonal change in the temperature effect on heart rate was controlled by the endocrine system, and that the thyroid by increase in its activity in the summer was the effective agent.
