Up to within a very brief time all the statistical evidence of blood pressure was based on systolic readings alone. This data is most valuable and much has been learned as to diagnosis and prognosis, but it is a mass of data based on a one-sided picture and can not be as valuable as the statistics which will undoubtedly be published later when all the pressure picture figures can be analyzed. =Pulse Pressure= The pulse pressure is the actual head of pressure which is forcing the blood to the periphery. At every systole a certain amount of blood 75-90 c.c. (Howell) is thrown violently into an already comfortably filled aorta. The sudden ejection of this blood instigates a wave which rapidly passes down the arteries as the pulse wave. The elastic recoil of the aorta and large arteries near the heart contract upon the blood and keep it moving during diastole. Normally the blood-vessels are highly elastic tubes with an almost perfect coefficient of elasticity. The pulse pressure varies under normal conditions from 30 to 50 mm. Hg. There is a very definite relationship between the velocity of blood and the pulse pressure which is expressed thus; velocity = pulse rate x pulse pressure.[5] Further it has been demonstrated that under normal conditions and during various procedures--the pulse pressure is a reliable index of the systolic output.[6] [5] Erlanger and Hooker: An Experimental Study of Blood Pressure and of Pulse Pressure in Man, Johns Hopkins Hosp. Rep., 1904, xii, 145. [6] Dawson and Gorham: The Pulse Pressure as an Index of Systolic Output, Jour. Exper. Med., 1908, x, 484.