Blood pressure is the expression used for a series of phenomena resulting from the action of the heart. As every heart beat is actual work done by the heart in overcoming resistance to the outflow of blood, this force is approximately measurable in a large artery such as the brachial. It has been determined that the pressure in the brachial artery is almost equal to the intraventricular pressure in the left ventricle. In animals it is easy to attach manometers to the carotid artery and to measure the blood pressure accurately. Formerly the method consisted in attaching a tube and allowing the blood to rise in the tube. The height to which the blood rose measured the maximum pressure. This is a crude method and has been replaced by the U-tube of mercury with connection made to the artery by saline or Ringer's solution. This apparatus is familiar to all physiologists. In man the measurement is most conveniently made from the brachial artery. There is some difference in the pressure in the femoral and the brachial and some use both arteries. However, the difficulty of adjusting instruments to the upper leg, the great force which must be used to compress the femoral artery and the relative inaccessibility of the leg as compared to the arm, make the leg an inconvenient part for use in blood pressure determinations. It is not to be recommended. Blood pressure is a valuable aid in diagnosis and of material help in many cases in prognosis, but it is not infallible neither can it be used alone to diagnose a case. Blood pressure is only one of many links in a chain of evidence leading to diagnosis. It has been badly used and much abused. It has been condemned unjustly when it did not furnish _all_ the evidence. It has been made a fetish and worshipped by both doctors and patients. A sane conception of blood pressure must be widely disseminated lest we find it being discarded altogether. Blood pressure consists of more than the estimation of the systolic pressure. The blood pressure picture consists of (1) the systolic pressure, (2) the diastolic pressure, (3) the pulse pressure which is the difference between the systolic and diastolic pressure, (4) the pulse rate. Expressed in the literature it should read thus: 120-80-40; 72. That tells the whole story in a brief, accurate form. This is recommended in history reporting. It must be ever kept in mind that a blood pressure reading represents the work of the heart at the _moment when it was taken_. Within a few minutes the pressure may vary up or down. There is no normal pressure as such, but an average pressure for any group of people of the same age living under similar conditions. The habit of speaking of any systolic figure as normal should be broken. A pressure picture may be normal but a systolic reading, whatever it may be, is not accurately designated as normal. This distinction is worth insisting upon. =Blood Pressure Instruments=