CH. xxi.] STEPHEN HALES' EXPERIMENTS. 285 



fluid, the two ends of which are connected by an india-rubber 

 tube on which is a valved syringe to represent the heart. On 

 the course of the tube are a large number of open-mouthed 

 upright manometers which indicate the pressure when the syringe 

 is worked, and confer on the tube the elasticity necessary to cause 

 the disappearance of the pulse in the middle region which represents 

 the capillaries. The long leaden tube is twisted round a cylinder 

 so that the manometers are placed closely side by side. 



We can now pass on to the methods adopted in the investiga- 

 tions of blood-pressure in animals. 



The fact that the blood exerts considerable pressure on the arterial 

 walls may be readily shown by puncturing any artery ; the blood 

 is propelled with great force through the opening, and the jet rises 

 to a considerable height ; in the case of a small artery, where the 

 pressure is lower, the jet is not so high as in a large artery : the 

 jerky character of the outflow due to the intermittent action of the 

 heart is also seen. If a vein is similarly injured, the blood is expelled 

 with much less force and the flow is continuous, not intermittent. 



The first to make an advance on this very rough method of 

 demonstrating blood-pressure was the Rev. Stephen Hales, Vicar 

 of Teddington (1727). He inserted, using a goose-quill as a 

 cannula, a glass tube at right angles to the femoral artery of a 

 horse, and noted the height to which the blood rose in it. This 

 is a method like that which we used in the first schema described 

 (fig. 271). The blood rose to the height of about 8 feet, and having 

 reached its highest point, it oscillated with the heart-beats, and 

 also with the respiration ; each inspiration causing a rise, each 

 expiration a fall of pressure ; each cardiac systole causing a 

 smaller rise, each diastole a smaller fall. The method taught 

 Hales these primary truths in connection with arterial pressure, 

 but it possesses many disadvantages ; in the first place the blood 

 in the glass tube very soon clots, and in the second place, 

 a column of liquid eight feet high is an inconvenient one to 

 work with. 



The first of these disadvantages was overcome to a great extent 

 by Vierordt, who attached a tube filled with saturated solution of 

 sodium carbonate to the artery, and the blood-pressure was 

 measured by the height of the column of this saline solution 

 which the blood would support. 



The second disadvantage was overcome by Poiseuille, who 

 introduced the heavy liquid, mercury, as the substance on which 

 the blood exerted its pressure ; and the U-shaped mercurial 

 manometer was connected to the artery by a tube filled with 

 sodium carbonate solution to delay clotting 



