346 



TEXT-BOOK OF PHYSIOLOGY 



max>valve 



to manometer 



mm valve 



the curves following the cardiac systole. The relation of these two com- 

 ponents varies in different animals and in the same animal at different tunes. 

 If the diastolic pressure is low, the systolic increase may be considerable; if 

 the former is high, the latter may be slight in extent. 



There are good reasons for believing, however, that this record does not 

 represent either the true diastolic or the true systolic pressure but that the 

 limits between the two are far more widely apart than here represented. 

 For, owing to its inertia, the mercury is not capable of following the rapid 

 variations of the pressure throughout their extent, that occur with each 

 heart-beat. The employment of one of the various forms of the quickly 

 responsive spring manometers such as are used in determining the rapid 

 variations of intra-cardiac pressure will show a much greater difference 

 between the diastolic and systolic pressures, often amounting to as much as 

 40 millimeters. 



For the purpose of obtaining the maximum systolic and the minimum 

 diastolic pressures, it is best, however, to insert between the cannula and the 



manometer a maximum and a minimum 

 valve similar in principle to that shown 

 in Fig. 154. By permitting the blood to 

 exert its pressure first through the maxi- 

 mum valve and then permitting the mer- 

 curial column to exert its pressure through 

 the minimum valve in the reverse direction 

 for a certain length of time, the maximum 

 systolic and the minimum diastolic pres- 

 sures will be recorded. By this method 

 Dawson found an average maximum sys- 

 tolic pressure in the carotid artery of the 

 dog of 162, and a minimum diastolic pres- 

 sure of 103 mm. of mercury, a difference of 

 59 mm. Hg. The difference between these 

 two pressures is known as the pulse pres- 

 sure. (A diagram showing the relation of 

 these different pressures one to another will 

 be found on page 349.) 

 In a series of experiments it will be found that the blood-pressure in the 

 arteries, recorded with the mercurial manometer, though rising and falling a 

 certain number of millimeters, yet retains a fairly constant general average, 

 the result of an adjustment between the number of heart-beats per minute 

 and the amount of the resistance offered to the escape of blood into the capil- 

 laries and veins. Though the tracing fails to record accurately the diastolic 

 and systolic pressures it approximates a certain average or mean of the pres- 

 sure thus recorded, which represents the power driving the blood through 

 the vessels. It is frequently stated that hi a tracing in which the respiratory 

 undulations are absent, the mean pressure is the arithmetic mean of 

 the systolic and diastolic pressures. This is, however, not strictly correct, 

 for if the pressure is recorded by means of a spring manometer or a sphygmo- 

 graph applied over the artery of man, a record much different in appearance 

 and similar to that shown in Fig. 155 will be obtained. In such a record 

 it will be observed that the passage from the lowest diastolic pressure (a) to 



1 'to heart 



FIG. 154. -v. FRANK'S VALVE. 

 This is placed in the course of the 

 tube between heart and manometer, 

 so that the latter may be used as a 

 maximum, minimum, or ordinary 

 manometer according to the tap 

 which is left open. (Starling.) 



