THE CIRCULATION". 303 



the small waves corresponds to heart-action, the slight ascent to sys- 

 tole, the slight descent to diastole. Jn studying a tracing it must be 

 remembered that the real blood-pressure is really twice what is re- 

 corded, since the needle moves through a space that represents the 

 difference of level between the mercury of the two tubes. 



Dawson has called attention to the fact that when we speak of 

 blood-pressure we should state whether systolic or diastolic, for the 

 systolic may exceed the diastolic pressure by as much as 100 per cent, 

 of the latter. These two may vary independently of each other. By 

 taking the average height of systolic pressure and obtaining the 

 average mean pressure, Dawson has found that the mean pressure 

 is nearer the diastolic level than the systolic level. He states the 

 pulse-pressure is the difference between the diastolic and systolic 

 pressure. For instance, in the right brachial artery of the dog the 

 systolic pressure was 156, its mean 118, the diastolic pressure was 101, 

 then the pulse-pressure would be 55. The pulse-pressure is an index 

 of the variation in pressure in any artery caused by the heart-beat. 



Each contraction of the heart stretches the artery by a sudden 

 increment of pressure equal in the right brachial to 55 millimeters of 

 mercury. The pulse-pressure becomes less as we approach the 

 periphery of the vascular system, as the oscillations between systolic 

 and diastolic pressure become less. Dawson found the mean pressure 

 to be practically constant throughout the large arteries, but when the 

 small arteries are reached there is a conspicuous decrease. The 

 diastolic pressure is also constant in the larger arteries. The diastolic 

 end pressure in the femoral is slightly and invariably lower than that 

 of the carotid. 



Blood-pressure in Man. 



Since it is impossible to ascertain blood-pressure in man as it is 

 practiced in. animals, numerous instruments have been invented that 

 can be used and applied to superficial arteries without dissection of 

 the tissues. These pieces of apparatus have been variously termed 

 sphygmometers, sphy go manometers, etc. 



Sphygmomanometer. The Riva-Rocci sphygmomanometer con- 

 sists of a canvas band bound close about the arm. Within the canvas 

 band is a rubber bag which communicates with a mercury mano- 

 meter and with the rubber bulb which produces the pressure. When 

 the canvas band is in place, the rubber bulb is rhythmically com- 

 pressed and air forced into the rubber bag around the arm. This 

 inflated bag exerts a pressure upon the arm and upon the brachial 



