COURSE OF CIRCULATION IN MAMMALS 151 



beginning of the upstroke of the impulse-curve to the beginning of 

 the upstroke of the carotid pulse-curve. The first sound of the heart 

 is synchronous with the upstroke of the impulse-curve. A small cor- 

 rection must be made for the delay in the transmission of the pulse 

 to the carotid artery. The period of systolic output extends from the 

 beginning of the upstroke of the pulse-curve to the dicrotic notch, 

 and the diastolic period from this notch to the beginning of the next 

 upstroke. The relation of the auricular systole to the ventricular can 

 be determined by simultaneously recording the pulse in the jugular vein 

 and carotid artery. The c wave in the jugular pulse is synchronous 

 with the upstroke in the carotid pulse, and the a wave, which precedes 

 the c wave, marks the auricular systole (see Fig. 116). 



The intraventricular pressure rises or falls during the output 

 period according to the state of the peripheral resistance. The 

 maximal systolic pressure exerted by the heart varies with the internal 

 tension that is, with the degree of diastolic filling and obstruction 



SUP:VEN:CAV 



ART:ME.SOCARD. 

 PERICARD 

 SINUS TRANSV: 



VENOUS MESOCARD 

 INF:VEN:CAV: 



l"i :. .~>S. HEART PULLED FORWARDS TO SHOW ITS ATTACHMENT BY ARTESIAL (dd) 

 AND VENOUS (ce) MESCCARDIA. (Keith.) 



to outflow. The heart-muscle responds to increased tension by a 

 greater output of energy, and this it does with little loss in rapidity 

 of action. By its reserve power the heart may throw out during 

 hard exercise ten times the volume of the normal output per minute, 

 and may maintain its output when the aortic pressure is even twice 

 its normal value. 



The Movements of the Heart in Situ. The normal fulcra for the 

 movements of the heart in the closed thorax are afforded through the 

 pericardium (Fig. 58). The pericardium is reflected on to the wall of the 

 heart at the point where the vena cava and aorta leave the pericardial 

 sac. This part of the pericardium gives a fixation point to the auricles, 

 being attached to the roots of the lungs, and thereby to the thoracic 

 wall, to the diaphragm, and to the structures at the root of the neck. 

 On opening the chest, the normal fulcra for the movements of the 

 auricles are lost, and this renders it difficult to record the exact move- 

 ments of the heart. The longitudinal an.i circular muscle fibres of 



