88 A MANUAL OF PHYSIOLOGY 



and both sets of valves are closed. This is the period of rapid 

 relaxation. 



Of the four periods, the second and fourth are exceedingly 

 brief. The third is relatively long and constant, being but 

 slightly dependent on either the pulse-rate or the pressure 

 in the arteries. The duration of the first period varies in- 

 versely as the frequency of the heart ; with the ordinary 

 pulse-rate it is the longest of all. 



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. 209). But since the pressure in a vigorously- 

 beating heart may still become negative, though not to the same 

 extent as before, 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 distorted, auriculo- 

 ventricular rings, and of elastic structures in the walls of the ventricles, 

 exerts of itself a certain feeble suction upon the blood. 



The Pulse. At each contraction of the heart a quantity of 

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

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

 bloodvessels were rigid, it is evident (p. 74) 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 capillary tract, with its rela- 

 tively great surface, the chief resistance to the blood-flow 

 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 



