THE CIRCULATION OF THE BLOOD. 281 



In accordance with the foregoing: the ventricular systole may be sub- 

 divided into two periods: 



1. The period of rising tension, from the beginning of the systole and the 



closure of the auriculo-ventricular valves to the opening of the semi- 

 lunar valves, the pre-sphygmic period, occupying from 0.02 to 0.04 

 second. 



2. The period of ejection, the sphygmic-period, from the opening of the 



semilunar valves to the end of the systole, occupying about 0.2 second. 

 The ventricular diastole may also be divided into two periods: 



1. The period of falling tension or relaxation, the post-sphygmic period, 



from the end of the systole and the closure of the semilunar valves to 

 the opening of the auriculo-ventricular valves, occupying about 0.05 

 second. 



2. The period of filling, from the opening of the auriculo-ventricular valves 



to the beginning of the succeeding auricular systole. 



Negative Pressure. As shown by the ventricular pressure curve there 

 is a moment when the pressure falls below atmospheric pressure, becoming 

 negative to it. The extent to which this takes place, its duration and fre- 

 quency, have never been satisfactorily determined. The cause of the 

 negative pressure, its influence on the opening of the auriculo-ventricular 

 valves, and on the entrance of blood into the ventricles are equally unknown. 

 A probable cause is an expansion of the base of the ventricles due to the 

 enlargement of the aorta and pulmonary artery. That it is not due to the 

 expansion of the thorax is evident from the fact that it occurs when the 

 thorax is open and the heart exposed. 



The Pulse Volume. The pulse volume or the systolic output or the 

 amount of blood discharged by the ventricle at each systole has long been a 

 subject of investigation, but by reason of the inherent difficulties of the 

 problem the results that have been obtained have varied within wide limits, 

 viz.: from 180 c.c. to 50 c.c. The methods that have been employed for 

 the determination of this volume are complicated and need not be detailed 

 here. Suffice it to say that the results of the more recent experiments 

 would indicate that the volume varies from 80 c.c. to 100 c.c. If the pulse 

 volume be assumed to weigh 100 grams and the total volume of blood in a 

 man weighing 70 kilograms to weigh 3864 grams then the pulse volume will 

 be about one-thirty-eighth of the total amount of blood. In 38 heart beats 

 therefore the entire amount of blood will have passed through the heart. 



The Intra-auricular Pressure. During the auricular systole the 

 pressure within the auricle undergoes variations as shown by direct examina- 

 tion by means of a cannula inserted into the auricular cavity and connected 

 externally with a recording tambour, or by indirect examination by means 

 of an exploratory tambour placed over the right jugular vein in close relation 

 to the clavicle. The pressure variations in the jugular vein which are thus 

 recorded by means of a tambour provided with a writing lever are believed 

 to be caused by, closely follow and reproduce the pressure variations in the 

 auricle. 



Among the most important of the direct examinations of the auricular 

 pressure are those of Porter, carried out by the insertion of a large cannula in 

 the auricular appendix, or in a pulmonary vein close to the auricle and con- 



