1. Interneurones located in the circumoesophageal commissures were found to control the activity of the cardioinhibitor and cardioaccelerator regularity neurones. 2. These interneurones (cardiac command fibres) fell into three classes: (1) strong inhibitors, which caused cardiac arrest, (2) weak inhibitors, which caused bradycardia, and (3) accelerators, which caused tachycardia. 3. When the po...