THE CIRCULATION. 183 



in the case of the femoral artery showed a pressure of about 

 120 millimetres, or about 5 inches of mercury. Experi- 

 ments on tibial arteries made the pressure higher, results 

 indicating here 160 millimetres, or about 6| inches of 

 mercury, while the average pressure indicated by a sphyg- 

 mometer (an instrument which may be used on an artery 

 without injuring it), reaches about 180 millimetres, or 7 

 inches of mercury, in the case of the radial artery. As the 

 pressure in the aorta must be somewhat greater than in any 

 of these arteries, it is probably not far from 200 millimetres, 

 or 8 inches of mercury. 



To more fully understand what is meant by a pressure 

 of eight inches of mercury, comparison may be made with 

 the barometer. Here the pressure of the air is also meas- 

 ured by a column of mercury. As every one knows, the 

 air pressure supports about thirty inches of mercury. As 

 the atmospheric pressure to the square inch is about fifteen 

 pounds it means that every two inches of mercury indicate 

 one pound of pressure to the square inch. Thus, therefore, 

 an arterial pressure of eight inches of mercury means a 

 pressure against every square inch of arterial wall of four 

 pounds. As mercury is a little over thirteen times heavier 

 than water or blood, it means that the arterial pressure 

 would lift a column of blood or water thirteen times eight 

 inches, or not far from nine feet. To state the same thing 

 in still another way, it means that, disregarding friction, 

 blood would spurt from such an artery at such a pressure 

 vertically upward, nine feet. 



In the pulmonary artery the pressure is only about one- 

 third of that in the aorta. This relatively low pressure in 

 the pulmonary system is, of course, equally easily explained 

 when we recall that the blood traverses a much shorter 

 distance, and that the lung capillaries are more easily 

 traversed. 



Pressure in Capillaries. 



The arterial pressure sinks rapidly in the smaller arteries 

 and in the capillaries is very slight indeed. There are 



