- THE CIRCULATION OF THE BLOOD 347 



the highest systolic pressure (b) takes place quickly, occupying about o.i 

 second ; and that the change from the highest systolic pressure to the succeed- 

 ing lowest diastolic pressure takes place slowly, in about 0.7 second, and that 

 the line of descent is interrupted by a secondary rise and fall of pressure be- 

 fore the former diastolic level is reached. l If a horizontal line is drawn across 

 the record parallel to a line uniting the lowest diastolic levels, and to a line 

 uniting the highest systolic levels and in the position of the arithmetic mean, 

 the triangular record of the pressure changes will be divided into two portions 

 of which the upper has a smaller area than the lower portion from which 

 it is apparent that the pressure is low for a longer period that it is high 

 and hence the mean pressure cannot be the arithmetic mean between the 

 diastolic and systolic pressures. The mean pressure, however, can for a 

 given period at least be experimentally determined. Thus, if at some one 

 point between the artery and the manometer, the lumen of the connecting 

 tube be largely obliterated by a constriction, the variations in the pressure 

 following the systole and diastole of the heart will be largely, if not entirely 

 excluded, and the mercury, instead of rising rapidly ^ ^ 



in the manometer and fluctuating with each heart- r\ pr~ 



beat, will rise slowly to a certain level and then I \ r / \ 

 remain at rest. The number of millimeters of 

 mercury thus supported represents the mean or ab- 

 solute pressure. The same result can be obtained 

 by employing the compensatory manometer of 

 Marey which presents a constriction of this char- FIG. 155. Sphygmogram or 

 acter. From many experiments made by Dawson p e curve - 



it has been learned that the mean pressure lies nearer to the diastolic than to 

 the systolic pressure and may be expressed numerically by the statement that 

 it is equal in millimeters of mercury to the diastolic pressure plus one-third 

 of the pulse pressure. In a tracing in which the respiratory undulations are 

 present the mean pressure can be calculated. The method by which this 

 is done, however, is rather complicated and need not be detailed here. In 

 a general way the mean pressure in such a tracing may be represented by a 

 line drawn horizontally across the tracing midway between the apex and 

 trough of the undulation. 



Estimates of the Mean Arterial Pressure. Because of the difficulty 

 in obtaining the pressure in small arteries, the experimental determinations 

 have for the most part been confined to large arteries such as the carotid, 

 brachial, and femoral, and hence the results which have been obtained have 

 reference to the lateral pressure in the aorta or in the large vessels which 

 immediately arise from it. The pressure obtained in the usual way at the 

 central end of a divided carotid is generally known as the "end pressure" 

 and represents the mean lateral pressure in the aorta or in the innominate 

 artery. Among the results thus obtained in different experiments from the 

 carotid artery of different animals are the following: In the horse, from 

 122 to 214 mm. Hg,; in the dog, from 140 to 160 mm.; in the cat, 150 mm.; 

 in the rabbit, from 90 to 100 mm.; in the sheep, 170 mm.; in the calf, from 



1 It must be remembered, however, that the cardiac systole endures though with diminishing 

 energy, even though the systolic pressure hi the artery is falling, for 0.32 second, when a rapid 

 dilatation sets in attended by the closure of the semilunar valves, an event indicated in the tracing 

 by the notch preceding the second rise of pressure at c. 



