234 HUMAN PHYSIOLOGY 



while the flow from the vein is copious but easily checked. 

 We say that arterial pressure is high and venous pressure 

 low. The contrast became apparent to the pioneer inves- 

 tigator Hales when he performed a certain memorable 

 experiment nearly two centuries ago. 



Hales established a connection between the femoral 

 artery of a prostrate horse and a vertical glass tube 9 

 feet high. The blood mounted in the tube to a height 

 of about 8 feet. The column then rose and fell by a 

 few inches in the rhythm of the heart-beat and showed 

 other fluctuations but in general remained at this high 

 level. When a vein was placed in communication with 

 a similar vertical tube the blood rose only a few inches 

 above the vessel. We have now to explain the high pres- 

 sure in the artery and the trifling pressure in the vein. 



The high pressure in the aorta and its branches is an 

 indication of the power which the contracting ventricle 

 has impressed upon the blood. If we find a liquid 

 passing along a tube we must conclude that, whatever 

 the pressure it exerts at a given point, a still higher 

 pressure prevailed at the source from which it sprang. 

 So if we find blood exerting a great pressure in an artery 

 we have to assume that the pressure was still greater in 

 the larger vessel of which this is a branch and highest of 

 all in the left ventricle which gave the initial thrust. 



The physiologist of the present day, if he wishes to 

 measure the pressure of the blood in an artery, does not 

 make use of the tall and inconvenient tube of Hales. 

 Instead he introduces a glass nozzle into the vessel and 

 opens communication through this with a U-shaped tube 

 containing mercury. The pressure then forces the mer- 

 cury down in the arm next to the artery and up in the 

 distant one. A difference of 5 or 6 inches between the 

 levels of the two ends will hold back the blood in the 

 arteries of a dog or cat and the pressure is read in terms 

 of the mercury column, usually expressed in millimeters. 

 A float on the surface of the mercury in the remote arm 

 of the U-tube is commonly arranged to write a record 

 upon a travelling smoked surface (Fig. 55). 



