92 A MANUAL OF PHYSIOLOGY 



student must, however, be warned that the proper interpreta- 

 tion of such tracings in the study of cardiac disease requires 

 special knowledge and training.* 



We have already said that a negative pressure may be detected in 

 the cardiac cavities by means of a special form of mercurial mano- 

 meter. This is confirmed by an examination of the tracings written 

 by good elastic manometers, for the curves of both ventricles may 

 often descend below the line of atmospheric pressure. The cause of 

 this negative pressure has been much discussed. In part it may be 

 ascribed to the aspiration of the thoracic cage when it expands during 

 inspiration (p. 210). But since the pressure in a vigorously-beating 

 heart may still become negative, when the thorax has been opened, 

 and the influence of the respiratory movements eliminated, we must 

 conclude that the recoil of the somewhat narrowed, or at least dis- 

 torted, auriculo-ventricular rings, and of elastic structures in the 

 walls of the ventricles, exerts of itself a certain suction upon the 

 blood. This, however, is not an important factor in the maintenance 

 of the circulation. 



The Arterial Pulse. At each contraction of the heart a quan- 

 tity of blood, probably varying within rather wide limits (p. 127), 

 is forced into the already full aorta. If the walls of the blood- 

 vessels were rigid, it is evident (p. 77) that exactly the same 

 quantity would pass at once from the veins into the right auricle. 

 The work of* the ventricle would all be spent within the time of 

 the systole, and only while blood was being pumped out of the 

 heart would any enter it. Since, however, the vessels are exten- 

 sible, some of the blood forced into the aorta during the systole 

 is heaped up in the arteries, beyond which, in the narrow arterioles 

 and in the capillary tract, with its relatively great surface, the 

 chief resistance lies. The arteries are accordingly distended to 

 a greater extent than before the systole, and, being elastic, they 

 keep contracting upon their contents until the next systole 

 over-distends them again. In this way, during the pause the 

 walls of the arteries are executing a kind of elastic systole, and 

 driving the blood on into the capillaries. The work done by 

 the ventricle is, in fact, partly stored up as potential energy in 

 the tense arterial wall, and this energy is being continually 

 transformed into work upon the blood during the pause, the 

 heart continuing, as it were, to contract by proxy during its 

 diastole. Thus, the blood progresses along the arteries in a 

 series of waves, to which the name of ' blood-waves ' or ' pulse- 

 waves ' may be given. Wherever the pulse-wave spreads it 

 manifests itself in various ways by an increase of blood-pres- 

 sure, an increase in the mean velocity of the blood-flow, an 

 increase in the volume of organs, and by the visible and palpable 

 signs to which the name of pulse is commonly given in a restricted 



* The necessary details must be sought in such works as Mackenzie's 

 ' Diseases of the Heart.' 



