266 



THE HUMAN BOD Y. 



to the veins, while their supply from the heart is cut off or 

 lessened. Hence a pressure-gauge attached to an artery 

 shows readily the influence of stimulation of the cardio-in- 

 hibitory fibres; and in order to avoid the serious operation of 

 opening the thorax to observe the heart directly, it is usual 

 to study indirectly the cardiac effect of stimulation of the 

 pnemogastric by observing its influence on arterial pressure. 



FIG. 101. Manometer for recording variations in arterial pressure, ddggg, glass? 

 U-tube partly filled with mercury, o; its limb, gg, is open to the air, and a float bear- 

 ing the light stem e on which is the pen / rests on the mercury: the limb dd is 

 filled above the mercury with magnesium sulphate solution and connected water- 

 tight by tubes and the cannula a with the heart end of a divided artery. The pen 

 writes on a horizontally travelling surface and rises and falls with the mercury on 

 the side gg, a rise indicating increase of arterial pressure, a fall the reverse: the 

 pressure 'in the artery at any moment is indicated by the vertical distance between 

 the top of the mercury in dd and that in gg. due allowance being made for the 

 weight of the magnesium sulphate and some other possible sources of error. 



For this purpose a small glass tube or cannula, a, filled with 

 solution of magnesium sulphate (to check blood-clotting) is in- 

 troduced into the cardiac end of a divided artery, say the fem- 

 oral, of a living animal, the artery being clamped at a place 

 nearer the heart than the p'oint where the cannula is tied on. 



