VELOCITY AND PRESSURE OF BLOOD-FLOW. 495 



and the manometer one may place a maximum and a minimum valve so ar- 

 ranged that the blood-pressure and heart beat may be transmitted through 

 either valve. As is shown by the figure, if the connection is maintained 

 through the maximum valve for a certain time the highest pressure reached 

 during that period will be recorded, while, when the minimum valve is used 

 the lowest pressure reached will be indicated. 



Such valves, of course, act slowly and can not be used to determine the 

 maximum and minimum pressure in the artery during a single heart beat; 

 they record the highest and lowest point reached during a certain given 

 interval. 



Actual Data as to the Mean Pressure in Arteries, Veins, 

 and Capillaries.— The mean value of the pressure in the aorta 

 has been determined for many mammals. It is found that the actual 

 fio-ures vary with the conditions under which the results have been 

 obtained. Such values as the following may be quoted:* 



Horse 321 mms. to 150 mms. Hg. 



Dog 172 " " 104 " " 



Sheep 206 " "156 " " 



Cat 150 " 



Rabbit 108 " " 90 " '"' 



Man (probable, Tigerstedt) .... 150 " 



It appears from these figures that there is no proportion between 

 the size of an animal and the amount of mean arterial pressure. It 

 is probable that there may be a general relationship between the 

 size of the animal — that is, the size of the heart — and the amount 

 of pulse pressure or the oscillation of pressure with each heart beat, 

 but sufficient data are not at hand to determine this point. As 

 we pass from the aorta to the smaller arteries the mean pressure 

 decreases somewhat, although not very rapidly, while the pulse 

 pressure decreases also and to a more noticeable extent. 



This fact is illustrated in Fig. 199, which gives a graphic 

 representation of a number of experimental determinations f of 

 systolic and diastolic pressures in the large arteries of the dog. 



If we turn to the other end of the vascular system, the veins, 

 we find that the lowest pressure is in the venae cavce and that it 

 increases gradually as we go toward the capillary area. Accord- 

 ing to one observer,! the fall in pressure from periphery toward 

 the heart is at the rate of 1 mm. Hg for every 35 mms. of distance. 

 We have such figures as the following: 



Dog (Opitz). Sheep. 



Superior vena cava (near Jugular vein 0.2 mm. Hg. 



auricle) = —2.96 mms. Hg. Facial vein 3.0 mms. " 



Superior vena cava more Branch of brachial . . . 9.0 " " 



distal = —1.38 " " Crural 11.4 " " 



External jugular (left) . .= 0.52 mm. " 



Right brachial = 3.90 mms. " 



Left facial = 5.12 " 



Left femoral ^- 6.39 " 



Left saphenous = 7.42 " 



* See Volkmann. "Die Haemodynamik," 1850. 



t Dawson, "American Journal of Physiology," 9, 198, 1903. 



X Burton-Opitz, "American Journal of Physiology," 9, 198, 1903. 



