THE CIRCULATION. 



299 



pressure upon the arterial walls, for, when they were punctured, an 

 intermittent jet of blood arose to a considerable height, the latter 

 depending upon the proximity of the wound to the heart. When a 

 vein was wounded the blood was noticed to exude with much less force, 

 and it was continuous, not intermittent. 



Hales was the first to make any improvement upon this rough 

 movement, which he did by inserting a brass pipe one-sixth of an 

 inch in diameter, in lieu of a cannula, into the femoral artery of a 



Fig. 98. Manometer of Mercury for Measuring and Registering 

 Blood-pressure. (EO.) 



a, Proximal glass tube. &, Union of the two glass tubes of the manometer. 

 d, Stop-cock through which the sodium carbonate can be introduced between 

 the blood and the mercury of the manometer, e, The rod floating on the 

 mercury carries the writing-point. 



horse about three inches from the abdomen. The brass pipe in the 

 artery was connected by means of another brass pipe to a glass tube 

 whose height was nine feet, its bore nearly the same diameter as the 

 brass pipe, and placed vertically. The first blood-pressure experiment 

 is, perhaps, best depicted in the words of Hales himself. He says: 

 "In December, 1733, 1 caused a mare to be tied down alive on her back. 

 She was fourteen hands high, and she had a fistula on her withers and 

 was neither very lean nor yet very lusty. Having laid open the left 



