Rate of Heart-Beat in Limulus as Affected by Exposure to Ultraviolet Point Radiation.

Abstract

This is the third of a series of studies on heart-beat. Preliminary reports have appeared in this journal, covering experiments with embryos and newly hatched Fundulus, and with adult frogs and turtles., It has been possible, in each instance, to modify the rate of beat by exposure to ultraviolet radiation, using the unscreened spectrum of a Cooper-Hewitt quartz mercury vapor arc running at 110 volts, D. C., focussed through a quartz rod. The present report deals with experiments on the Limulus heart, the beat of which is controlled by a nerve cord which runs along the median dorsal surface of the heart. Exposures were made by placing the tip of the quartz rod on various points along the cord. In this way, an increase or decrease in rate of beat was obtained, with the length of the period of exposures and the locus of application of the rod as determining factors. In general, an increase in rate of beat was obtained by short exposures of a given region, and a decrease in rate following longer exposures. The rate of beat could also be slowed by placing the rod on the nerve cord at points of entrance of inhibiting nerves. Rapid recovery of normal rate followed exposures in which the induced variations were not marked. It was early noted that any pressure on the nerve cord due to too heavy a contact of the rod had to be avoided since it slowed the rate of beat or even blocked conduction. In many experiments where the normal rate of beat was studied for a considerable period of time, there were distinct indications of a rhythm consisting of a series of slightly faster beats alternating with a series of slower ones.