“Though the left ventricle--dispensing its blood under a high pressure into the aorta--is the master hand in maintaining the arterial blood pressure, the height to which that pressure rises depends primarily on the resistance encountered in the peripheral arterial system, and in the capillaries, and, secondly, on the response of the ventricle to overcome that resistance. Given a normal ventricle the arterial tonus largely determines the ventricular response and the level of the arterial blood pressure; and this is the keynote of the physiological and pathological variations of that pressure.” Arterial tension may be described thus: During the resting-time of the heart, between each beat, the circulation of the blood is maintained by the contractile power of the arteries, which steadily urges the blood onwards into the capillary vessels, and so into the veins. This power is known as arterial tension or pressure, and it is estimated fairly accurately by the sphygmomanometer. A large number of observations have taught us what is the average pressure at different ages of life, and thus we are able to talk about a normal or abnormal pressure. If we hope to understand and successfully treat abnormal pressure and sclerosis, we must visualize the whole of this picture clearly, wisely, and in its due proportions. This we have not always done. The heart man, as Osler wittily said, “tinkers at the pump,” while the pressure man, with his manometer, concentrates on the pipes. To give a scientific description and explanation of the physics of the circulation is far beyond me, and would be inappropriate in a small practical work of this sort, but a study of some elaborate standard work, such as Sir Clifford Allbutt’s _Diseases of the Arteries_, or Halliburton’s _Physiology_, would do much to give a fair understanding of this intricate subject: but, as they so often acknowledge, the vital problem of the circulation, with its ever-changing demands, both general and local, can hardly be solved by the known laws of physics. Heat and cold, joy and grief, anger and fear, acting through the nerves of the vaso-motor system, all help to complicate the problem and often to confuse cause and effect.