THE CIRCULATION 147 



'pulse pressure.' The degree of this increase depends 

 upon the volume of blood thrown into the aorta by the 

 left ventricle, and the amplitude of the pulse is usually 

 a guide to the amount of this volume. If, however, the 

 diastolic pressure within the artery be very low, as it 

 is, for example, when the arterioles are dilated, a com- 

 paratively small output on the part of the heart may 

 cause a considerable relative rise in the systolic pressure, 

 and so make the pulse appear one of large volume, and 

 the ' pulse pressure ' high. Further, if the radial artery 

 be of small size, or if its muscular coat be contracted, 

 the pulse volume may appear small, even although the 

 systolic pressure within the artery rises very considerably. 

 In such a case a sphygmometer is a useful corrective to 

 the indications furnished by the finger. 



The force with which the pulse wave is transmitted 

 indicates the energy of contraction of the left ventricle. 

 A forcible pulse is usually one of large volume, but an 

 ample pulse is not necessarily forcible if it be the result 

 of a low diastolic pressure within the arteries; nor is 

 a small pulse necessarily a weak one, for its want of 

 volume may be due to contraction of the arteries or to 

 the output of the ventricle being small owing to its 

 being imperfectly filled, as happens, for example, when 

 the total volume of the blood is reduced by haemorrhage. 



The tension of the pulse will be considered when we 

 come to deal with blood-pressure. 



The Sphyg-mogram. If one analyzes a pulse wave as 

 depicted by the sphygmograph, it will be found to consist 

 of an upstroke (called the ' primary ' or ' percussion ' 

 wave), an apex, and a downstroke. On the downstroke 



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