IT has been known for many years that pressure on the neck over the carotid artery produces slowing of the heart. Parry1 in 1799 described a retardation of the heart in man by pressure over one carotid artery. Waller2 in 1862 made similar observations. Czermack3 in 1866 attributed the slowing of the heart to mechanical stimulation of the vagus nerve. Many observations were reported on the effects on the cardiac mechanism of pressure over the carotid sheaths, and the test was called the vagus pressure test. It was not until 1927, with the publication of Hering's excellent monograph,4 that it was definitely demonstrated that the vagus nerve was not affected directly but that pressure over the carotid sheath stimulated sensory endings in the carotid sinus, which resulted in reflex cardiac inhibition and other reflex effects.
The brilliant investigations of Hering,4 Heymans5 and Koch6 established the function of the