388 THE MECHANICS OF THE CIRCULATION, HEMODYNAMICS 



an occasional intermittency. If the former, the pulse is characterized as inter- 

 mittent, and if the latter, as deficient. Clearly, the absence of the pulse in a 

 peripheral blood-vessel does not signify that it is also absent near the heart or that 

 the cardiac contractions have ceased altogether. 



(/) Intercurrens, Alternans, and Bigeminus. — These types of pulses also indi- 

 cate a disturbance in the cardiac rhythm. If an occasional wave is forced in 

 between two regular ones, the pulse is said to be intercurrent. Its cause must 

 be sought in extra systoles. A true alternating pulse consists of rhythmic waves 

 of large and small amplitude, this abnormality being usually dependent upon a 

 degeneration of the myocardium. The prognosis, therefore, is grave. An en- 

 tirely different significance, however, must be attached to the pseudo-alternating 

 pulse and the pulsus bigeminus. As these types of pulses are dependent upon 

 extrasystoles, two waves must necessarily appear at the periphery whenever an 

 additional contraction results, but the wave produced by the extra contraction 

 is always smaller than the normal one. In the bigeminus variety the pulse-waves 

 appear in couplets, i.e., the normal and succeeding extra waves are separated from 

 the neighboring ones by a definite interval. In the pseudo-alternating pulse, on 

 the other hand, this separation is not clearly in evidence, because it is caused by 

 extra systoles of the premature type. 



B. THE CARDIAC VARIATIONS IN VENOUS BLOOD PRESSURE 



The Physiological Venous Pulse. — The venous entrances to the 

 heart are not guarded by valves; moreover, while the size of these 

 orifices is greatly lessened during systole in consequence of the con- 

 traction of the circular layer of muscle fibers, their complete closure 

 is not effected. For this reason it cannot surprise us to find that the 

 auricular pressure is propagated outward into the central veins, where 

 it influences the venous pressure as well as the flow. Thus, if a water- 

 manometer is connected with a central vein, the level of the water 

 immediately exhibits rhythmic fluctuations which occur synchronously 

 with the contractions of the heart. In addition to these oscillations 

 it also shows much larger wave-like variations which are dependent 

 upon the respiratory movements. The finer details of these waves 

 may be brought out more clearly by registering them with the help 

 «f a membrane manometer. 



The cardiac variations in venous pressure are most manifest near- 

 the heart and gradually decrease in amplitude in the direction of the 

 peripheral veins. They are usually absent from the abdominal 

 portion of the inferior vena cava as well as from the distal end of the 

 external jugular vein, but their presence in' these channels depends 

 very largely upon the force of the heart beat and the tension prevailing 

 throughout the venous system. These changes in pressure give rise to 

 pulsations which are generally obtained from the external jugular vein 

 in close proximity to the aperture of the chest. Distally to this point 

 they are usually so slight that they cannot be properly registered. 

 Tracings of the venous pulse may also be obtained from the central 

 veins of animals after the chest has been opened. A receiving and a 

 recording tambour are commonly employed for this purpose. This 

 record is known as a phlebogram. 



