370 Effect of Lability of Arterial Wall on Blood Pressure. 



obtained with the sphygmomanometer. Much of the systolic force of the 

 heart is stored as potential energy in the distension of labile large arteries, 

 to be given up again on tbeir recoil during diastole ; a part is spent in over- 

 coming their resistance to distension ; the greater the lability the less will be 

 the amplitude of the systolic wave which reaches the peripherally placed 

 arteries ; the higher also will be the diastolic wave, the closer the approxima- 

 tion of the diastolic and systolic pressures, the less marked the dicrotic wave. 



It follows from these considerations that, supposing the force of the heart 

 remains constant, what has been termed a " high " or a " low " pressure form 

 of sphygmogram does not depend only on the resistance in the arterioles, 

 but may be obtained according to the greater or less " lability " of the 

 conducting arteries — e.g. aorta— subclavian — brachial — radial. So, too, will 

 the readings of systolic and diastolic pressure vary with the " lability " of 

 the conducting arteries. Observations on cases of aortic regurgitation have 

 shown us that the systolic readings may be 100, even 150, mm. Hg higher 

 in the leg than in the arm arteries. This great difference is entirely due 

 to conduction modified by the "lability" of the arteries. The arm arteries 

 in such cases are more " labile " than the leg arteries — the latter are con- 

 tracted, " harder," more rigid, and conduct the systolic wave from the aorta 

 with far less diminution of force. This is to ensure a circulation through 

 the capillaries of the leg, to compensate for the great fall of diastolic 

 pressure. By local modification of the contractile state of the arteries 

 either the full hammer-like stroke of the heart may be delivered to the 

 capillary vessels with a wide variation between the systolic and diastolic 

 pressures, or a more uniform pressure be conveyed with the systolic variations 

 of pressure approximated. 



In reading the systolic or diastolic pressure by means of the sphygmo- 

 manometer we read, not the actual pressure produced by the heart, but 

 this pressure as conducted by the arteries to that artery selected for 

 observation. In the normal young man, placed in the recumbent posture, 

 the arteries so conduct, that approximately the same readings are obtained 

 in the arm and in the leg arteries. The contraction of the muscular coat 

 of the arteries is controlled so as to effect this. In cases of aortic regurgi- 

 tation the conduction is widely different. A similar difference may also 

 pertain in conditions of functional activity, e.g. the leg arteries may be 

 more contracted and give higher readings of systolic pressure after running 

 up a flight of stairs. High readings of systolic pressure do not necessarily 

 indicate any greater systolic force exerted by the heart. They may indicate, 

 and probably often do indicate, less " lability " — arteries held in the con- 

 tracted state so as to conduct the systolic wave with almost undiminished 



