en. XXIL] 



STEPHEN HALES EXPERIMENTS 



271 



that exerted by the atmospheric pressure on the other. The mercury, 

 however, is pushed up in the far limb of the manometer connected to 

 the artery, the pressure there being greater than that of the atmos- 

 phere ; this is therefore called positive pressure, and the total amount 

 of pressure is measured by the difference between the levels a and a. 

 The manometer B attached to the vein, however, indicates a negative 

 pressure (b V), that is a pressure less than that of the atmosphere, so 

 that the mercury in the limb nearest the vein is sucked up. 



Anderson Stuart's kymoscope (fig. 238) is a more complete schema. 

 It consists of a long leaden tube filled with 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 dis- 

 appearance 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 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 (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. 236). The 

 blood rose to the height of about 8 feet, and having reached its highest 



FIG. 238. Anderson Stuart's 

 Kymoscope. 



