172 VARIATIONS OF THE BLOOD-PRESSURE. 



whereby the blood-pressure must be increased. The reverse effect 

 occurs during expiration (Schweinburg). [Section of both phrenic 

 nerves and opening of the abdominal cavity cause the respiratory 

 undulations almost entirely to disappear. The respiratory undulations, 

 therefore, depend in great part upon the changes of the abdominal 

 pressure and the effect of these changes on the amount of blood in the 

 abdominal vessels. When making a blood-pressure experiment, pres- 

 sure upon the abdomen of the animal with the hand, causes the 

 blood-pressure to rise rapidly.] 



(g.) Variations with each Pulse-beat. The mean arterial pressure 

 undergoes a variation with each heart-beat or pulse-beat, causing the so- 

 called pulsatory undulations (Fig. 77, c). The mass of blood forced into 

 the arteries with each ventricular systole causes a positive wave and an 

 increase of the pressure corresponding with it, which of course corre- 

 sponds in its development and in its form with the pulse-curve. 



In the large arteries Volkmann found the increase during the heart-beat to be 

 = t^ (horse) and f r (dog) of the total pressure. 



None of the apparatus described in 84 gives an exact representation of the pulse- 

 curve. They all show simply a rise and fall, a simple curve. The sphygmograph 

 alone gives a true expression of the undulations in the blood-pressure which are 

 due to the heart-beat. 



(A.) If the heart's action be arrested or interrupted by continued 

 stimulation of the vagus (Brunner, 1855), or by a high positive 

 respiratory-pressure (Einbrodt), the arterial blood-pressure falls enor- 

 mously, while it rises in the veins as the blood flows into them 

 from the arteries to equilibrate the difference of pressure in the two 

 sets of vessels. This experiment shows, that even when the difference 

 of pressure is almost entirely set aside, the passive blood presses upon 

 the arterial walls i.e., on account of the overfilling of the blood- 

 vessels, a slight pressure is exerted upon the walls, even when there 

 is no circulation (Brunner). [As already stated, the arterial pressure 

 depends on the condition of the central organ the heart and on the 

 peripheral organs the small arteries. If the action of the heart be 

 arrested, then the blood-pressure rapidly falls. Fig. 78 shows the 

 effect on the blood-pressure, of arresting the action of the heart, by 

 stimulation of the peripheral end of the vagus. There is a sudden 

 fall of the arterial pressure, as shown by the rapid fall of the curve 

 from a]. 



For the effects of the nervous system upon the blood-pressure, see " Vaso-motor 

 Centre" (vol. ii.) 



PatholOgicaLIn persons suffering from granular or contracted kidney and 

 sclerosis of the arteries, in lead poisoning, and after the injection of ergotin, which 

 causes contraction of the small arteries, it is found, on employing the method of 



