268 



THE CIRCULATION IN THE BLOOD-VESSELS [CH. XXI. 



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 investigations 

 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 in- 

 jured, 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 demon- 

 strating blood-pressure was the Eev. 

 Stephen Hales, "Vicar of Teddington 

 (1702). He inserted, using a small 

 brass tube 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. 267). 

 The blood rose to the height of about 

 8 feet, and having reached its highest 

 point, it oscillated with the heart- 

 beats, each cardiac systole causing a 

 rise, each diastole a fall. Hales also 

 noted a general rise during each inspiration. 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 intro- 



FIG. 269. Anderson Stuart's 

 Kymoscope. 



