The pace Maker of the Cardiac Ganglion of Limulus .polyphemus

The ganglion on the dorsal surface of the heart of Limulus polyphemus originates the impulses which cause the contractions of the cardiac muscle (1). In large specimens the portion of the dorsal nerve cord which contains the ganglion cells may be 10 or even 12 cm. in length and one naturally wonders where in the extent of this long structure the impulse actually arises. Does it arise in a specific locus? Can it arise at different places? Does the whole ganglion function with each contraction, and if so what is the coordinating mechanism? These questions have been dealt with experimentally, treating the ganglion as if it were comparable and analogous to the pace maker of the vertebrate heart. Section and excision: a. If a transverse section be made across the entire tubular .heart at the level of the sixth segment, counted from the anterior end, the two pieces thus separated each contain about one half of the length of the ganglionated portion of the nerve {Jord. After the initial mechanical stimulation due to the operation the pieces settle down each to a perfect rhythm which measurement of tracings shows to be identical with the original rhythm, and each of the two pieces thus procured may be again cut into two sections and again each of the pieces will finally settle down to the original rhythm. The removal of the ganglion, i.e., the ganglionic cord and all outlying extensions which frequently are present, from any piece at once and forever stops all contractions of the muscle of that piece, proving the neurogenic origin of the beat as has been noted by Carlson. Two facts are noticeable: a. Although the rate of each piece remains the same as that of the original whole heart, the pieces no longer beat synchronously, but are out of phase; for example, while one piece is contracting another will be relaxing. The result is identical with that which Dr. Ida Hyde described for the movements of the gill plates of Limulus, which occur at a definite rate and in definite sequence, but if the ventral nerve cord is cut between the ganglia controlling each of the five gill plates the contractions' of each gill plate continue at the same rate as before the operation but different ganglia are out of phase. These experiments indicate that the