THE HEART 301 



Ventricular systole causes a rise in the arterial blood- 

 pressure which is marked in the sphygmographic tracing by the 

 sudden upstroke (Fig. 107). At the end of ventricular systole, 

 the semilunar valves close and the pressure begins to fall. But, 

 as the valves close, the blood endeavours to pass back into the 

 ventricles and it rebounds from the valves, causing a secondary 

 increase in pressure, which is recorded on the tracing as the 

 dicrotic wave. Thereafter the arterial pressure continues to 

 fall during diastole and atrial systole. 



The orifices of the superior and inferior venae cavae are 

 devoid of competent valves, and therefore a wave of increased 

 pressure passes back along them with each atrial contraction. 

 No valves are found in the superior vena cava or in the in- 



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FIG. 107. Sphygmographic Tracing of a Normal Pulse. 



nominate veins (p. 314), which form it, and so the impulse is 

 transmitted to the subclavian and the internal jugular veins 

 (p. 315). About i inch above the sternal end of the clavicle, 

 the internal jugular possesses a valve of two crescentic cusps 

 (Fig. 1 08), which is almost invariably competent. The portion 

 of the vein below the valve is termed the jugular bulb, and, 

 since it lies behind the interval between the sternal and the 

 clavicular heads of the sterno-mastoid, it is readily accessible 

 to the receiver of the sphygmo-manometer. This venous 

 pulsation is perfectly normal, and tracings of it are of value in 

 determining the condition of the heart (p. 311). 



The subclavian vein, which lies behind the clavicle and is 

 consequently inaccessible, possesses no valves up to a point 

 just distal to its reception of the external jugular vein. There- 



