MECHANICS OF THE CIRCULATION IN THE VESSELS 101 



the action of the auricle, information of the same kind as that 

 afforded by the arterial pulse-tracing and the cardiogram as to the 

 action of the ventricle. In the interpretation of the venous pulse- 

 tracings, a simultaneous record of the radial or, better, the carotid 

 pulse or of the apex-beat is always important, and often indispens- 

 able, for it enables the time of onset of the ventricular systole to be 

 marked upon the phlebogram (the venous trace), and this facilitates 

 the identification of the a wave, which must immediately precede, 

 and the c or s wave, which should coincide with the beginning of the 

 contraction of the ventricle. The student must, however, be warned 

 that the proper interpretation of such tracings in the study of cardiac 

 disease is often difficult and requires special knowledge and training.* 



Suction Action of the Ventricle. We have already said that a 

 negative pressure may be detected in the cardiac cavities by means of 

 a special form of mercurial manometer. 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. 225). 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 some- 

 what narrowed, or at least distorted, 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. 



SECTION III. PHYSICAL OR MECHANICAL PHENOMENA OF THE 

 CIRCULATION IN THE BLOODVESSELS. 



The Arterial Pulse. At each contract on of the heart a quantity of 

 blood, probably varying within rather wide limits (p. 139), is forced 

 into the already full aorta. If the walls of the bloodvessels were 

 rigid, it is evident (p. 85) 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 extensible, 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 ca'pillary 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, 



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

 ' Diseases of the Heart.' 



