THE CIRCULATION OF THE BLOOD 529 



Now turn the drum until the point of the 

 aortic lever lies beneath the notch seen in the 

 down stroke of the pulse curve (the dicrotic 

 notch, see page 546). Describe synchronous 

 ordinates. It is known that the dicrotic notch 

 in the aortic pulse curve corresponds closely to 

 the moment of closure of the aortic valves. It 

 marks, therefore, the end of the outflow period. 

 Note that this point is reached soon after the 

 ventricle begins to relax. Thus the period dur- 

 ing which the intraventricular pressure is higher 

 than the pressure in the aorta embraces part of 

 the relaxation as well as part of the contraction 

 of the ventricle. It includes approximately the 

 highest third of the intraventricular pressure 

 curve. 



Observe also the considerable interval between 

 the beginning of ventricular contraction and the 

 opening of the aortic valve, as shown by the 

 upstroke in the pulse curve consequent upon 

 the entrance of liquid into the aorta. 



The Visible Change in Form. Expose the heart 

 of a frog. Observe the great veins, the auricles, 

 the single ventricle, the two aortse, and the dila- 

 tation, or bulbus, by which the aortas are con- 

 nected with the ventricle. All these parts except 

 the two aortae are contracting. The veins con- 

 tract first ; the auricles next ; then the ventricle ; 



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