126 TKAUBE-HERING CURVES. 



muscular fibres of the arteries. This experiment shows that it is the vaso-motor 

 centre which is specially acted upon by the venous blood.] 



[Traube-Hering Curves. The following experiment proves that the varying 

 activity of the vaso-motor centre suffices to produce undulations in the blood- 

 pressure tracing. Take a dog, curarise it, expose both vagi and establish artificial 

 respiration ; then estimate the blood-pressure in the carotid. After section of the 

 vagi, the heart will continue to beat more rapidly, but it will be undisturbed by 

 the cardio-inhibitory centre. Thus the central factor in the causation of the blood- 

 pressure remains constant. Suddenly interrupt the respiration, and, as already 

 stated, the blood-pressure will rise steadily and uniformly, owing to the stimulation 

 of the vaso-motor centre by the venous blood. In this case the peripheral factor, 

 or state of tension of the small arteries throughout the body, is influenced by the 

 condition of the nerve-centre, which controls their action. After a time, the blood- 

 pressure tracing shows a series of bold curves higher than the original tracing. 

 These can only be due to an alteration in the state of the small arteries, brought 

 about by a condition of rhythmical activity of the vaso-motor centre. These 

 curves were described and figured by Traube, and are called the Traube or Traube- 

 Hering curves. As in other conditions, stimulation causes exhaustion, and soon the 

 venous blood paralyses the vaso-motor centre and the small arteries relax, blood 

 flows freely out of the larger arteries, and the blood-pressure rapidly sinks. 

 Variations in the blood-pressure have been observed after a mechanical pump has 

 been substituted for the heart, i.e., after all respiratory movements have been set 

 aside, so that the only factor which would account for the phenomena of the Traube- 

 Hering curves is the variation in the peripheral resistance in the small arteries, 

 determined by the condition of the vaso-motor centre.] 



Variations. The respiratory undulations of the blood-pressure become more pronounced the 

 greater the force of the respirations, which produce greater variations of the intra-thoracic 

 pressure. In man, the diminution of the pressure within the trachea is 1 mm. Hg. during 

 tranquil inspiration ; while during forced respiration, when the respiratory passage is closed, it 

 may be 57 mm. Conversely, during ordinary expiration, the pressure is increased within the 

 trachea 2-3 mm. Hg. , while during forced cxpiratiou, owing to the compression of the abdominal 

 muscles, it may reach 87 mm. Hg. 



Other Factors. The increase of the blood-pressure during inspiration, as well as the fall 

 during expiration, must in part depend upon the pressure within the abdomen. As the dia- 

 phragm descends during inspiration, it presses upon the abdominal contents, including the 

 abdominal vessels, whereby the blood-pressure must be increased. The reverse effect occurs 

 during expiration (Schivchiburg). [Section of both phrenic nerves and opening of the abdominal 

 cavity cause the respiratory undulations almost entirely to disappear. The respiratory undula- 

 tions, 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, pressure 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-bent, causing the so-called pulsatory 

 undulations (fig. 107). The mass of blood forced into the arteries with each 

 ventricular systole causes a positive wave and an increase of the pressure corre- 

 sponding with it, which of course corresponds 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 = I V (horse) 

 and T V (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. 



(h) Arrest of the Heart's Action. If the heart's action be arrested or interrupted 

 by continued stimulation of the vagus, or by high positive respiratory pressure, the 

 arterial blood-pressure falls enormously, while it rises in the veins as the blood 

 flows into them from the arteries to equilibrate the difference of pressure in the two 



