In 1866 Czermack1 demonstrated that digital pressure of the neck in the region of the vagus nerve caused a slowing of the heart rate in man. He attributed this effect to a mechanical excitation of the vagus nerve. Hering,2 while making observations on this phenomenon, noted that very slight pressure resulted in marked cardiac inhibition in some instances. These observations led him to doubt the theory of direct vagal stimulation. Hering and his co-workers demonstrated conclusively that the slowing of the heart by digital pressure in the neck was the result of a reflex which originated in a specialized portion of the internal carotid artery, the carotid sinus. It has been observed that the usual response to pressure on the carotid sinus is a moderate slowing of the heart rate, but that in some instances there results a complete arrest of the heart for many seconds. The present