1. All inorganic perfusing solution for the heart of the lobster Homarus americanus, to allow prolonged normal beating (20 hours or more) must agree closely with the inorganic composition of the serum, which varies differentially with that of the environmental sea water.

2. All of the chief inorganic ions of the serum are necessary—Na, K, Ca, Mg, Cl, and SO4; the critical numbers of the ions being 100, 3, 5, 2–3, 116, and 1–2 respectively. Absence of Mg and SO4 will be tolerated for several hours.

3. The pH of the solution must agree with that of the serum within 0.2.

4. The osmotic pressure of the solution must agree with that of the serum within 15 per cent.

5. Beating of the heart will continue for several hours on improperly balanced solutions but changes in frequency, tone, or amplitude will occur. Hearts adapted to such solutions will show different responses to physical and chemical stimuli of the solution than those perfused on properly balanced solutions.

6. Arrest in systole is caused by isotonic NaCl, KCl, LiCl, and urea, and arrest in diastole by isotonic CaCl2, MgCl2, NaBr, NaI, MgSO4, and glucose.

7. Lithium cannot replace sodium; neither can bromide or iodide replace chloride ions.

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