THE CIRCULATION OF THE BLOOD 



523 



pheric pressure. Close the aorta containing the 

 cannula by gentle pressure with a forceps the 

 blades of which are covered with rubber tubing. 

 Join the cannula-tube to the manometer, exclud- 

 ing air bubbles. Eemove the forceps. 



The mercury will fall in the proximal and rise 

 in the distal limb until the blood-pressure in the 

 aorta is balanced by the column of mercury. 

 With each ventricular beat, 

 the column rises a short dis- 

 tance above the mean level 

 and sinks again. 



Record the blood-pressure 

 curve on a very slowly 

 moving drum. To get the 

 actual pressure in milli- 

 metres of mercury multiply 

 by two the mean height of 

 the curve above the atmos- 

 pheric pressure line. 



The Effect on Blood-Pressure of Increasing the 

 Peripheral Resistance in the Prog. The peri- 

 pheral resistance may be increased by the nar- 

 rowing of the small arteries which follows the 

 stimulation of special vaso-constrictor nerve fibres. 

 The vaso-constrictor nerves may be stimulated 

 directly or reflexly. The latter method is chosen 

 here. 



Pig. 69. The small mercury 

 manometer. 



