A study of the crustacean heart muscle

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Abstract
The following new experimental evidence was secured with relation to the heart of Maia squinado. When the perfusion pressure is gradually raised, the frequency of the natural rhythm also rises in accordance with either one of 2 types of curve. The limiting acidity with which the heart can be in equilibrium without undergoing important changes is of about pH 65. Thus far, the statement that it can resist greater acidity was not confirmed. Break-shocks of gradually increasing strength evoke responses growing in a step-like arrangement towards a definite maximum. After an effective stimulation the heart passes through an absolute refractory period of about 0.2 sec, which occupies approximately J of the time during which tension is maintained. This, together with the values found for the general characteristics of the contraction and the possibility of evoking summated contractions which follow a definite law, is much against the tetanic nature of the automatic contraction. Compensatory pauses are better observed in hearts driven at such quick rhythms that the refractory period is made to cover a considerable part of each cycle. A second break-shock falling at progressively increasing intervals after a previous one, begins to evoke summated contractions as soon as the refractory period is over. The recovery curve of the contractile power gave gradually rising curves, comparable to the similar curve for the frog''s heart, after neutral or alkaline perfusion. By repetition of stimuli of the same strength[long dash](a) at intervals for which relaxation is not yet complete, the heights of the contractions rise in a true staircase arrangement; (b) at shorter intervals, new steps of summation occur, followed by undulatory or complete tetanus. Whether the high frequency of stimulation is reached suddenly or slowly, a drop of a half-rhythm is never observed[long dash]as in the vertebrate heart[long dash]because fusion of contraction occurs earlier.