CIRCULATION 269 



If this is high, the walls of the artery are already somewhat 

 stretched, and therefore the pulse wave expands them only 

 slightly further. On the other hand, if the average pressure is 

 low, the arterial wall is lax, and is readily stretched to a greater 

 extent. 2nd. The force of the heart. To stretch the arterial 

 wall to a large extent requires an actively acting heart throw- 

 ing a sudden large wave of blood into the arterial system at 

 each systole. The full pulse is well seen after violent exer- 

 tion, when the heart is active and the peripheral vessels fully 

 dilated, thus allowing a free flow of blood from the arteries 

 and thus keeping the mean arterial pressure low 



4th. Tension of the pulse. Sometimes the pulse wave is 

 easily obliterated by pressing on the artery sometimes con- 

 siderable force is required to prevent it from passing. To 

 test this, two fingers must be placed on the artery. That placed 

 nearer the heart must be pressed more and more firmly on the 

 vessel until the pulse wave is no longer felt by the second finger. 

 In this way the tension or force of the pulse, the maximum 

 systolic pressure in the artery, may be roughly determined. 

 So important, however, is this point, that various instrumental 

 methods for determining it have been devised (see p. 273). 



The tension of the pulse varies directly with the force of 

 the heart and with the peripheral resistance. The first 

 statement is so obvious as to require no amplification. It is 

 also clear that if the peripheral resistance is low, so that blood 

 can easily be forced out of the arteries into the capillaries, 

 the arterial wall will not be so forcibly expanded as when 

 the resistance to outflow is great. Hence a high-tension 

 pulse is indicative of a strongly acting heart with constriction 

 of the peripheral vessels. It is well seen during the shivering 

 fit which so frequently precedes a febrile attack, since at that 

 time the peripheral vessels are constricted and the heart's 

 action excited. The tension of the pulse wave must not be 

 confused with the mean arterial pressure (see p. 273). 



5th. The form of the pulse wave may be investigated by 

 means of the finger alone or by means of the sphygmograph. 

 The points to be observed are 



(1) Does the wave come up suddenly under the finger ? 

 In the pulsus celer (or active pulse) it does so ; in the 

 pulsm tardus, on the other hand, it comes up slowly. The 



